Influence of surface quality on corrosion resistance of stainless steel and aluminum alloy butt welds after innovative finishing

Abstract

The maintenance properties of machine parts rely on a system of surface quality parameters, including geometrical, mechanical properties, and surface structure. This study examines how corrosion properties – a critical aspect of maintenance – depend on key surface quality parameters of butt-welded joints post-finishing. The research compares traditional grinding with an innovative post-weld finishing method. Corrosion results from material thermodynamic instability during manufacturing; over time, materials tend toward a more stable oxidized state. Welded joints pose significant corrosion fatigue challenges due to their structural complexity, material heterogeneity, and stress concentration, underscoring the study’s importance. Test samples of AISI 304 L, AISI 316 L, EN AW-5058 H321, and EN AW-7075 T651 were butt-welded and finished with both methods. Surface morphology and roughness were characterized using a Zeiss Smart Zoom 5 digital microscope and an Alicona Infinite Focus G6 optical profilometer, enabling both 2D and 3D topographic analysis. Residual stresses were measured using the ProtoiXRD Combo X-ray diffractometer, and microhardness was determined with a NEXUS 4303 Vickers tester. Metallographic investigations were carried out using light and scanning electron microscopy. Electrochemical corrosion testing was conducted in 3.5% NaCl solution at 20 °C using a three-electrode system and an Autolab PGSTAT 302 N potentiostat. Tafel curve extrapolation, conducted via NOVA2 software, was used to calculate corrosion potential (E_corr) and current density (I_corr). Results showed that the innovative finishing method improves surface uniformity and reduces tensile residual stresses, leading to enhanced corrosion resistance, particularly in stainless steel specimens. The findings demonstrate that post-weld surface condition plays a key role in controlling corrosion behavior and confirm the effectiveness of the proposed finishing method in improving the integrity and longevity of welded joints.

Introduction

The primary cause of corrosion in metals is their inherent thermodynamic instability, which arises during the manufacturing process. Metals and alloys, when exposed to operating conditions, naturally tend to transition towards a more stable oxidized or ionic state, driven by the second law of thermodynamics. This process, termed corrosion, is essentially a physicochemical reaction in which the metal undergoes a spontaneous transformation from an energetically unstable state to a more stable, oxidized form. The rate and nature of this corrosion process are governed by a complex interplay of factors, which can be classified into external and internal categories¹.

External factors influencing corrosion include the chemical composition of the operational environment, such as the presence of corrosive agents (e.g., chloride ions, sulfur compounds, or acidic substances) that significantly accelerate degradation. Temperature within the working area also plays a critical role, as increased temperatures can enhance the rate of corrosion reactions. Additionally, environmental dynamics, such as fluid flow or mechanical agitation, contribute to corrosion by continually replenishing the corrosive medium at the metal surface, promoting localized attack and erosion-corrosion processes .

Internal factors relate to the intrinsic properties of the metal or alloy itself, including its chemical composition, microstructure, and the specific processing methods employed during manufacturing. The alloy composition determines its susceptibility to various types of corrosion, such as pitting, intergranular, or stress corrosion cracking. Moreover, metallurgical treatments influence the grain structure, phase distribution, and homogeneity of the metal, which are critical in defining its corrosion resistance. Surface processing techniques further contribute by modifying surface roughness, which can act as a barrier or, conversely, a site for localized corrosion initiation depending on the topography created. As the height of microroughness increases, the contact area between the part and the surrounding aggressive environment expands, consequently accelerating corrosion processes. Furthermore, the small radii of curvature in microprofile depressions act as energy-intensive sites (stress concentrators), where corrosion centers are most likely to initiate^2,3.

The degree of surface hardening and the presence of residual stresses within the surface layer are also critical; these factors can either enhance resistance to specific types of corrosion or, conversely, introduce zones of mechanical vulnerability prone to stress-corrosion interactions. Strain hardening exhibits a complex and sometimes contradictory influence on corrosion resistance. While it is generally observed that strain hardening can significantly reduce corrosion resistance, certain hardening methods may improve it. For instance, surface rolling may enhance corrosion resistance by creating a hardened surface layer with a more positive electron potential than the base metal, offering a protective advantage. This effect is further supported by the high thermodynamic stability of the hardened layer, which results from the formation of a fine-grained and highly dispersed microstructure. Such a dual nature of strain hardening’s impact on corrosion resistance is likely due to both the specific material composition and the hardening technique used. Additionally, the hardening process increases oxygen diffusion from the air into the surface layer, promoting the formation of stable chemical compounds such as FeO, Fe₂O₃, and Fe₃O₄, which provide protective properties by forming a passive layer on the metal surface^4,5.

It is generally accepted that residual stresses of any sign, as a primary factor contributing to the thermodynamic instability of metals, reduce corrosion resistance. However, it is also known that the increased resistance to corrosion fatigue (under simultaneous exposure to cyclic stresses and a corrosive environment) in rolled samples is due to the presence of compressive residual stresses. Tensile residual stresses, on the other hand, promote stress corrosion cracking (or static corrosion fatigue), typically occurring at stress concentrators, especially in corrosive environments⁶. Compressive residual stresses tend to slow down the process of corrosion cracking, likely because the fatigue factor predominates over the corrosion factor in these scenarios. This interplay between stress type and environmental influence highlights the critical role that residual stress orientation and intensity play in dictating the corrosion, especially under complex service conditions.

This article presents an in-depth study on the dependence between corrosion resistance – a crucial aspect of maintenance properties – and the primary surface quality parameters of butt-welded joints after final finishing treatments. The research investigates how different surface characteristics impact corrosion performance, with a focus on comparing the effects of traditional grinding versus innovative post-weld finishing technique. Traditional grinding, commonly used to smooth welds, is evaluated against advanced methods designed to refine the surface profile and potentially enhance corrosion resistance. The study aims to identify which finishing approach yields optimal corrosion protection, considering factors such as surface roughness, residual stresses, microstructural changes, and microhardness. By quantifying these effects, this research provides insights into selecting finishing techniques that improve the durability and longevity of welded joints in corrosive environments for four material grades: AISI 304 L, AISI 316 L, EN AW-5083 H321, and EN AW-7075 T651⁷.

The stainless steels AISI 304 L and AISI 316 L are widely used in industries where corrosion resistance, hygiene, and mechanical integrity are critical. AISI 304 L, with its low carbon content, is commonly employed in the food processing industry, kitchen equipment, storage tanks, and architectural structures. AISI 316 L, enriched with molybdenum, offers superior resistance to pitting and crevice corrosion, making it particularly suitable for more aggressive environments such as marine applications, chemical and petrochemical plants, offshore oil and gas platforms, and pharmaceutical equipment. Both steels are often selected for their excellent weldability and compatibility with surface finishing processes that enhance durability and cleanliness.

The aluminum alloys EN AW-5083 H321 and EN AW-7075 T651 serve essential roles in sectors where high strength-to-weight ratios and corrosion resistance are required. EN AW-5083 is known for its exceptional performance in marine environments and is commonly used in shipbuilding, cryogenic tanks, and pressure vessels. Its resistance to seawater corrosion and good weldability make it ideal for structural applications. EN AW-7075, a heat-treatable alloy with very high strength, is widely used in the aerospace and automotive industries, particularly in load-bearing components such as aircraft frames, wing spars, and suspension parts. In both cases, surface preparation is a critical step before coating or painting to ensure adhesion, durability, and long-term corrosion protection.

The novelty and originality of the proposed innovative post-weld finishing technology lie in its comprehensive approach to addressing challenges inherent in traditional methods such as grinding. Current solutions are limited by numerous drawbacks that impact the quality and reliability of welded structures, increase production costs, and pose environmental and human health risks. Grinding with abrasive tools often introduces surface defects, including structural notches, tensile residual stresses, and microcracks, leading to material fatigue and reducing both its fatigue strength and corrosion resistance – a phenomenon widely documented by researchers. Moreover, grinding is labor-intensive and often requires manual work. In light of current advancements in knowledge and technology, the proposed method represents a significant step forward^8,9.

This pioneering technology employs a specialized cutting tool designed for uniform removal of weld reinforcement in a single pass, ensuring a high-quality surface finish. The tooth geometry of the tool is optimized through strength calculations and precisely adapted to the specific material and operational conditions, significantly enhancing its resistance to impact loads. Preliminary tests have shown that the cutting edges demonstrate excellent durability and service life, which is essential for minimizing production costs. The tool’s low wear rate further ensures consistent processing quality over an extended period of use, without the need for frequent replacement or sharpening.

The innovative technology is also environmentally friendly, an increasingly critical consideration given rising environmental protection standards in industrial processes. By minimizing pollutants such as dust, waste, and cooling-lubricant fluids, this approach aligns with the modern industry’s commitment to sustainable development^10,11.

The results of comprehensive studies on the relationship between corrosion properties and treatment quality parameters will enable a detailed assessment of the practical value of this innovative technology, highlighting its potential for broad application across various industries, including construction, power engineering, machinery manufacturing, shipbuilding, aerospace, and military defense.

Materials and methods

To investigate the impact of weld finishing on corrosion properties, four distinct material grades were chosen, representing metals commonly utilized across multiple sectors of mechanical engineering. These materials were specifically selected due to their prevalent use and varying corrosion behaviors, providing a broad basis for evaluating the effects of different weld finishing techniques. This selection allows for a comprehensive analysis of how finishing processes influence corrosion resistance and durability, essential factors for mechanical engineering applications where material longevity and performance are critical. This selection includes two popular austenitic stainless steel grades, AISI 304 L and AISI 316 L, as well as two aluminum alloys, EN AW-5083 H321 and EN AW-7075 T651. Extensive research in contemporary scientific literature highlights these materials due to their diverse welding characteristics and corrosion properties. The chemical composition and mechanical properties of these material, as per the inspection certificate EN 10204-3.1 (mill test certificate), are detailed in Tables 1 and 2.

Table 1.

The chemical composition and mechanical properties of selected stainless steels.

Table 2.

The chemical composition and mechanical properties of aluminum alloys.

AISI 304 L stainless steel is a standard austenitic chromium-nickel alloy known for its general corrosion resistance in natural environments, but less suitable for saline or chloride environments due to the risk of intergranular corrosion at high temperatures. AISI 316 L, an acid resistant stainless steel, has improved corrosion resistance due to its molybdenum content, which provides better resistance to organic and inorganic acids and chlorides, making it well suited to demanding conditions^12,13.

The study also examines aluminium alloys. EN AW-5083 H321 has excellent corrosion and seawater resistance, ideal for marine applications^6,14, with moderate strength and high fatigue resistance but limited machinability^15,16. EN AW-7075 T651, an aluminium-zinc alloy, achieves tensile strengths up to 600 MPa, with high fatigue resistance but lower corrosion resistance due to the addition of copper, which improves strength but increases susceptibility to atmospheric corrosion. This diverse range of materials provides a comprehensive basis for evaluating how innovative post-weld finishing method affect corrosion resistance in comparison with conventional grinding^16,17.

Once the materials for the study had been selected, the specimens were fabricated as butt joints. This type of joint, one of the simplest and most widely used, consists of two flat metal pieces positioned parallel to each other. Its simplicity, effectiveness and cost efficiency make it a preferred choice for joining various components. For this study, the specimens had a V-groove weld produced using the TIG (Tungsten Inert Gas) welding process at a controlled current (I_w). The welding current was set individually for each material to ensure optimum weld quality: AISI 304 L at I_w = 140 A, AISI 316 L at I_w = 140 A, EN AW-5083 H321 at I_w = 160 A, and EN AW-7075 T651 at I_w = 170 A¹⁸. After welding, the test specimens, manufactured to the dimensions shown in Fig. 1, were finished using two different methods: conventional grinding and an innovative post-weld finishing technique.

Fig. 1.

Dimensions of the test sample.

In industrial practice, removing weld reinforcement is often necessary to meet various structural and aesthetic requirements. This process may be required for the installation of different types of structural joints, the placement of sealing elements, or simply to improve the visual quality of the weld seam. There are several established methods for removing weld reinforcement, each offering specific benefits depending on the material and requirements of the structure. Traditional techniques include cutting, grinding, plasma cutting, and milling, each suited to different operational demands and structural needs.

An innovative approach to post-weld surface finishing, as described in recent studies, involves the use of specialized tools designed to enhance the control, consistency, and quality of the weld surface finish¹⁹. This method, often referred to as innovative seam finishing, provides a level of precision and efficiency that traditional methods may not achieve, especially for applications where weld aesthetics and corrosion resistance are crucial.

The choice of technique for weld reinforcement removal depends on several factors, including the thickness and composition of the reinforcement, the material of the welded structure, and the specific quality standards required for the final welded joint. Each method has inherent advantages and limitations, making it essential to select the appropriate technique based on the specific structural and operational requirements. The article presents a comparative analysis of conventional grinding and the innovative post-weld finishing method to evaluate their impact on the corrosion properties of the welded joints under study. Both finishing processes effectively remove the weld reinforcement – the height of the weld above the base metal surface – thereby achieving a smooth, flush finish as detailed in Table 3.

Table 3.

Preparation of test samples.

Condition of the weld bead surface	Material under investigation
	AISI 304 L	AISI 316 L	ENAW-5083 H321	ENAW-7075 T651
Weld bead before finishing
Weld bead after grinding
Weld bead after innovative finishing technique

The grinding process was performed using a hand grinder. In contrast, the innovative post-weld finishing technique was executed with the BM25 hydraulic vertical boring machine from Norgesa. This setup included a specially designed fixing kit to securely hold the weld specimens in place and guide the cutting tool, which featured a unique design tailored for precise post-weld finishing (Fig. 2). This combination of non-standard tool design and stable specimen positioning allowed for consistent and controlled material removal, contributing to a higher-quality surface finish compared to traditional grinding methods^10,18.

Fig. 2.

Instruments were used in the innovative post-weld finishing process: 1 – BM25 hydraulic vertical boring machine from Norgesa, 2 – design of the weld bead finishing process, 3 – photograph of fixing kit, 4 – photograph of non-standard cutting tool.

After the samples were fabricated, a detailed research methodology was developed, incorporating a series of critical steps arranged in a precise, predefined sequence, as illustrated in the accompanying Fig. 3. This methodology was designed to ensure consistency and reliability in data collection, allowing for a thorough investigation of the samples’ properties under controlled conditions. Each step in the sequence was carefully planned to build upon the previous one, maximizing the accuracy and reproducibility of the experimental results.

Fig. 3.

Research methodology scheme.

In order to examine the resulting surface morphology and to identify areas with characteristic roughness features, in the first stage of this study images were taken at a magnification of 32 using a Zeiss Smart Zoom 5 digital industrial microscope (Carl Zeiss AG, Oberkochen, Germany). In the second stage, non-contact optical methods – widely validated in the recent scientific literature^20,21 – were used to measure surface roughness and evaluate the influence of the innovative material removal technique and conventional grinding on key geometric surface parameters. Detailed surface roughness and topography measurements^22,23 were taken for specific areas using the Alicona Infinite Focus G6 system (Alicona Imaging GmbH, Vienna, Austria). This high-precision optical measurement tool enabled accurate acquisition of both 2D profile roughness parameters – such as Ra (average roughness), Rz (mean peak-to-valley height), Rt (total profile height), Rv (maximum valley depth) and Rz (ten-point height) – and stereometric 3D roughness features, including Sq (root mean square roughness), Sp (maximum peak height), Sv (maximum valley depth), Sz (maximum height) and Sa (average roughness).

In the subsequent third stage of analysis, X-ray diffraction was used to quantify residual stress values in selected regions of the welded specimens, both parallel and perpendicular to the weld line. Residual stress analysis was carried out using a Proto Manufacturing ProtoiXRD Combo diffractometer, with measurements based on the sin²Ψ method. CrKα radiation with a beam diameter of 2 mm was used under controlled conditions of 20 kV anode voltage and 4 mA current.

Metallographic analysis was carried out at stages 4, 5 and 6 according to the research methodology (Fig. 3) to assess microstructural changes^24,25, microhardness²⁶ and the degree of hardening²⁷ within the surface layer following both the innovative post-weld finishing and traditional grinding techniques^27,28. Standard specimen preparation methods were used, using specialised equipment such as a specimen cutter, automatic grinder-polisher and inlay press. In addition, reference specimens taken immediately after welding were included for comparison. Microstructural observations were made on etched specimens using Marble reagent No. 25 according to ASTM E407 for steel and Keller reagent No. 4 according to ASTM E407 for aluminium alloys^29,30. Metallographic observations were made by light microscopy (LM) using a Leica DMI-3000X microscope (Leica Microsystems GmbH, Wetzlar, Germany) and by scanning electron microscopy (SEM) using a Hitachi S-3400 N (Hitachi High-Tech Corporation, Tokyo, Japan) equipped with a backscattered electron (BSE) detector set at an accelerating voltage of 20 kV. Microhardness measurements were performed according to ASTM E384 using the Vickers method with a NEXUS 4303 microhardness tester (INNOVATEST Europe BV, Maastricht, The Netherlands). A load of 0.25 N was applied for aluminium alloys and 0.5 N for steel samples. The degree of hardening (U) of the weld surface was calculated as the relative increase in hardness, expressed as a percentage, using the following formula:

1

where: HV_p – the hardness of the weld after finishing, HV_s – the hardness of weld bead before finishing.

The final stages 7–9 (Fig. 3) of the study were the corrosion resistance tests. Corrosion testing, focusing on the weld face, was evaluated both before and after finishing for selected aluminium alloys and stainless steels. Testing was conducted under laboratory conditions using an aqueous solution of 3.5% NaCl (simulated seawater) at a controlled temperature of 20 °C, in accordance with the standards specified in PN/EN ISO 17,475, ASTM D 2776, and ASTM G39. Electrochemical tests to determine the current (I_corr), the corrosion potential (E_corr) and the corrosion rate (V_corr), were carried out on the welded test specimens. In this study, the three-electrode system (Fig. 4) consisted of the test sample, a saturated calomel electrode EK-101 (Hg | Hg₂Cl₂(s) | KCl) as the reference electrode (RE), and a silver/silver chloride electrode (Ag | AgCl(s) | KCl) as the counter electrode (CE).

Fig. 4.

Design of a three-electrode electrochemical cell for corrosion testing: 1 – electrochemical cell base, 2 – test sample connector, 3 – upper cell gasket, 4 – saturated calomel reference electrode (SCE), 5 – stopper, 6 – stopper for silver/silver chloride counter electrode, 7 – cell casing, 8 – silver/silver chloride counter electrode, 9 – aqueous 3.5% NaCl solution, 10 – clamping screws, 11 – test sample exposed to the solution over an area of S = 1 cm².

To assess the corrosion resistance, specifically the corrosion rate, of the tested welded joints in both the weld (W) and heat-affected zone (HAZ), an electrochemical method was employed by analyzing Tafel curves. These curves were obtained using the three-electrode system depicted above, allowing precise measurements of electrochemical behavior across these critical weld regions. The corrosion rate was calculated from the steady-state corrosion potential at a controlled ambient temperature of 20 °C. For this purpose, the modular Autolab PGSTAT 302 N potentiostat from nLab was utilized, operated through NOVA2 software to ensure accurate data acquisition and analysis, as illustrated schematically in Fig. 5. This setup enabled a detailed evaluation of corrosion susceptibility across different zones of the welded specimens, contributing to a comprehensive understanding of how welding and subsequent finishing processes affect material performance in corrosive environments.

Fig. 5.

Schematic view of the Autolab PGSTAT 302 N electrochemical test bench: 1 – electrochemical cell with the sample connected to the MUX, 2 – potentiostat, 3 – computer set controlling the Nova 2 software.

The test method involved polarizing the welded joints by applying a fixed series of potential values. The potential of the electrode under test was continuously monitored by a potentiostat, which regulated the current between the saturated calomel reference electrode (RE) and the counter electrode (CE). This potentiometric technique measures the electrode potential, which directly relates to the chemical composition of the solution, exploiting the principle that the electrochemical potential (E) of a carefully selected electrode is dependent on the chemical environment of the solution in which the sample is immersed³¹.

During polarization, the sample alternates between functioning as an anode and a cathode, allowing the generation of anodic and cathodic polarization curves over time at a constant current density. Each measurement in this method is based on determining the electromotive force (EMF) of a cell consisting of two electrodes: a measuring electrode (E_p) whose potential varies with the ion concentration (or activity) of interest, and a reference electrode (E_o), which maintains a stable and reproducible potential. This setup ensures precise control and high reproducibility in assessing the corrosion behavior of the test specimens, guaranteeing that the measured potentials accurately reflect the sample’s interaction with the corrosive environment. To determine the corrosion resistance (corrosion rate) of the welded joints, Tafel curves were generated using the electrochemical method. Tafel extrapolation, a mathematical technique, allows estimation of the corrosion current (I_corr) or corrosion potential (E_corr) within an electrochemical cell, thereby enabling the calculation of the corrosion rate. This extrapolation involves extending the linear portions of the anodic and cathodic curves until they intersect; this can be performed either manually or with computer software. In this study, the NOVA2 software was employed to process the measurement results.

The Tafel diagram provides a graphical representation of the relationship between the current generated in an electrochemical cell and the electrode potential. It consists of two divergent logarithmic curves that represent the cathodic and anodic currents. An example of a Tafel diagram is shown in Fig. 6.

Fig. 6.

Example Tafel diagram.

To apply the Tafel equation, measurements must be made within ± 0.25 V of the corrosion potential. The general Tafel equation can be written as³² :

2

where: – overpotential the difference between the actual electrode potential and its equilibrium potential (η = E – E⁰ ), – a constant that, for η = 0, determines the value of the exchange current density, – Tafel’s constant, the directional coefficient of the polarization line, j – current density [A/cm²] — the electric current per unit area of the electrode.

In the case of the anode region, this equation can be written as follows³²:

3

where: – anodic overpotential, – charge transfer coefficient (0.3 ÷ 0.7), z – number of electrons, F – Faraday constant (F ≈ 96485 C/mol), R – universal gas constant = 8.314 [J/(mol·K)], T – temperature in Kelvin [K], j₀ – exchange current density [A/cm²] – characteristic of the electrode-electrolyte system.

Conversely, for the cathode area:

4

where: – cathodic overpotential.

Equation (4) illustrates the linear relationship between the overpotential and the logarithm of the current density. By extrapolating the line for zero value of the overpotential, we obtain logj₀. The electron transfer coefficient can be determined by the angle of inclination of the obtained straight lines. This equation is applicable for the range of 50 ÷ 70 mV of overpotential.

Tafel’s equation for the transition reaction, which is irreversible, the value of its E⁰ and the overpotential are unknown, is the following:

5

Using the Stern-Geary equation, the value of the corrosion current can be determined indirectly³²:

6

where: – slope of the anodic extrapolation branch [Vdec^− 1],slope of the cathodic extrapolation branch [Vdec^− 1], 2.3 – conversion factor to switch from natural logarithm to base-10 logarithm (since Tafel slopes are typically expressed in base-10), – polarization resistance at corrosion potential [Ωm²].

When determining Tafel curves, it is important to use high overpotentials in the polarization process. However, this destroys the surface of the test material and introduces anomalies in the corrosion rate results for different samples. To avoid such undesirable effects, it is necessary to polarize the test material in a small range of overpotentials to obtain the value of the polarization resistance Rp. This value is equal to the inverse of the slope coefficient of the rectilinear section of the relationship j = f(η) or j = f(E), recorded at a very small value of the overpotential (± 20 mV from the corrosion potential)³²:

7

If the values of the Tafel coefficients are not known or it is impossible to determine them from tests, the approximate value of the corrosion current can be estimated using the Stern-Geary equation, where the values of and are assumed to be 0.12 Vdec^− 132:

8

The intensity of the corrosion process can be determined by the mass loss of metal over an area of 1 m² in one day, which is referred to as the corrosion rate r_corr. Gravimetric measurements are used to calculate this value. The formula for the mass corrosion rate can be written as follows³²:

9

where: – corrosion rate [g/cm²·s] or [mm/year], ∆m – mass loss of the corroding material [g], S – surface area of the sample [cm²] or [m²], t – time of exposure or corrosion test duration [s].

Alternatively, the intensity of the corrosion process can be expressed as the average wear rate of the section r_p, i.e. the reduction in the cross-sectional dimension of the material by 1 mm in one year³²:

10

where: r_p – corrosion rate [mm/year], – corrosion rate [g/(cm²·s)], d – material density [g/cm³].

For electrochemical corrosion processes, the mass corrosion rate is related to the corrosion current density, assuming that the anodic reaction for a given sample is described by the equation:

11

where: M_e – metallic atom (neutral state), z – number of electrons released or accepted per atom during the electrochemical reaction.

This equation relates the corrosion current to the mass loss of the sample , which is consistent with Faraday’s Law I, which states that the mass of a substance released at the anode or cathode is directly proportional to the duration of electrolysis and the current³²:

12

where: M – molar mass of the metal [g/mol].

A proportional relationship between the corrosion current density j_corr and the corrosion mass loss r_corr is obtained by applying Eq. (12):

13

Results

The results of the study of 2D and 3D surface roughness parameters after the removal of the weld face by the innovative method and traditional grinding are presented in the visual form – surface topography (Figs. 7 and 8) and in the resulting form – roughness measured values (Table 4).

Fig. 7.

Isometric images of surfaces after the removal of the weld face by the innovative methods: a) AISI 304 L, b) AISI 316 L, c) EN AW-5083 H321, d) EN AW-7075.

Fig. 8.

Isometric images of surfaces after the removal of the weld face by the traditional grinding: (a) AISI 304 L, (b) AISI 316 L, (c) EN AW-5083 H321, (d) EN AW-7075 T651.

Table 4.

The 2D and 3D surface measurement results.

Roughness parameter [µm] (innovative method/grinding)	Material
	AISI 304 L	AISI 316 L	EN AW-5083 H321	EN AW-7075 T651
Rq	1.65/0.70	0.50/0.50	2.36/1.27	1.95/0.94
Rt	13.88/7.41	6.11/5.17	17.64/12.96	20.05/8.70
Rz	6.89/4.26	2.56/2.85	9.27/6.91	8.36/5.21
Ra	1.16/0.53	0.30/0.36	1.64/0.94	1.18/0,70
Rc	4.28/2.04	1.52/1.52	5.93/3.55	6.57/2.65
Sq	1.96/1.02	0.56/1.17	2.38/1.76	1.94/1.76
Sp	25.63/6.76	10.04/6.18	8.70/17.67	5.94/8.35
Sv	16.39/10.69	13.19/14,69	15.79/14.02	41.89/14.55
Sz	42.03/17.45	23.22/20.87	24.49/31.69	47.83/22.89
Sa	1.34/ 0.78	0.32/8.88	1.64/1.33	1.18/1.37

Following the established methodology, the subsequent phase of the research involved determining the residual stresses within the weld beads, both with the weld face intact and after treatment using the innovative finishing method as well as traditional grinding. The measured residual stress values for the tested welds are summarized in Table 5, providing insights into the effects of each finishing technique on the stress distribution across the weld surface.

Table 5.

Residual stress values.

Material	Nr codes	Finishing	Residual stress [MPa]
			x-axis – across the weld axis		y-axis – along the weld axis
			σx	Deviation	σy	Deviation
AISI 304 L	1	as-welded	-31	31	-225	49
	2	after grinding	-323	8	6	12
	3	after innovative finishing	-67	12	-238	16
AISI 316 L	1	as-welded	8	36	-41	34
	2	after grinding	-508	14	-205	19
	3	after innovative finishing	-74	26	-177	19
EN AW-5083 H321	1	as-welded	-22	11	-37	9
	2	after grinding	-54	6	49	6
	3	after innovative finishing	-49	7	-52	4
EN AW-7075 T651	1	as-welded	-11	5	-48	4
	2	after grinding	-56	7	112	5
	3	after innovative finishing	-73	6	-154	5

A comparative analysis of the microstructure of welds from various material grades, both untreated and following weld bead removal using the two finishing methods, is presented in Fig. 9. This comparison highlights the structural differences induced by each finishing technique across the different grades, offering insights into their effects on weld integrity and material properties.

Fig. 9.

Butt-weld microstructure: 1 – as-welded, 2 – after grinding, 3 – after innovative finishing.

Hardness measurements, similar to microstructural observations, were performed on samples taken from weld cross-sections. The measurements were conducted from the surface inward, in a direction perpendicular to the weld face. A summary of the measurement results is presented in Fig. 10 in the form of curves showing the dependence of the weld microhardness on the distance from the weld surface. The microhardness measurements validated the findings from the microstructural observations, demonstrating that post-weld finishing induced more significant changes in steel specimens compared to aluminum alloy specimens. This conclusion is further corroborated by the hardening degree (U) values presented in Table 6, which highlight the differential impact of the finishing methods on the microhardness profiles of each material type.

Fig. 10.

Curves of microhardness dependence on the distance from the weld surface (after grinding and after innovative method): (a) AISI 304 L, (b) AISI 316 L, (c) EN AW-5083 H321, (d) EN AW-7075 T651.

Table 6.

Microhardness values and calculated degree of work hardening.

Material	Finishing method	Deformation zone depth* h [µm]	Microhardness		Hardening degree U [%]
			after finishing HVp	before finishing HVs
AISI 304 L	after grinding	10.6 (1.5)	341	203	68
	after innovative	3.1 (1.0)	480	203	136
AISI 316 L	after grinding	9.4 (1.3)	323	200	61
	after innovative	6.0 (2.3)	457	200	129
EN AW-5083 H321	after grinding	4.5 (1.8)	109	85	28
	after innovative	2.7 (0.8)	125	85	47
EN AW-7075 T651	after grinding	3.1 (0.7)	166	104	60
	after innovative	2.3 (0.7)	163	104	57

* Standard deviation in brackets.

To evaluate the corrosion resistance before and after finishing weld bead using electrochemical methods, Tafel curves were generated. By extrapolating the linear portions of the anodic and cathodic branches, estimates for the corrosion current (I_corr) and corrosion potential (E_corr) can be obtained (Fig. 11). This technique involves extending the linear segments of the Tafel curves until they intersect, providing key indicators of corrosion behavior. The Tafel curves visually represents the relationship between current and electrode potential within an electrochemical cell, serving as a critical tool for assessing material performance in corrosive environments.

Fig. 11.

Comparison of Tafel curves for selected materials ( Blue squarebox– as-welded, Navy blue squarebox– after grinding, Purple squarebox– after innovative finishing): a) AISI 304 L, b) AISI 316 L, c) EN AW-5083 H321, d) EN AW-7075 T651.

1. b)

To analyze the impact of key surface quality parameters on the corrosion resistance of the welds, the test results have been consolidated into Table 7.

Table 7.

Test results consolidated table.

Material indication	Finishing	Corrosion parameters		Residual stress			Microhardness		Surface topography parameters
		Corrosion current Icorr [A]	Corrosion potential Ecorr [V]	σx [MPa]		σy [MPa]	Deformation zone depth h [µm]	Hardening degree U [%]	Sa [µm]	Sz [µm]	Sp [µm]	Sv [µm]
304	as-welded	4.84E-07	-0.25634	-31		-225	–
	after grinding	1.11E-07	-0.18708	-323		6	10.6	68	0.78	17.45	6.76	10.69
	after innovative finishing	7.08E-07	-0.26030	-67		-238	3.1	136	1.34	42.03	25.63	16.34
316	as-welded	2.56E-06	-0.22436	8		-41	–
	after grinding	1.49E-07	-0.20117	-508		-205	9.4	61	8.88	20.87	6.18	14.69
	after innovative finishing	3.03E-09	-0.14938	-74		-177	6.0	129	0.32	23.22	10.04	13.90
5083	as-welded	5.03E-07	-0.78064	-22		-37	–
	after grinding	6.33E-06	-0.88723	-54		49	4.5	28	1.33	31.69	17.67	14.02
	after innovative finishing	6.77E-07	-0.79940	-49		-52	2.7	47	1.64	24.49	8.70	15.79
7075	as-welded	6.42E-08	-0.26057	-11		-48	–
	after grinding	9.64E-06	-0.95801	-56		112	3.1	60	1.37	22.89	8.35	14.55
	after innovative finishing	3.38E-07	-0.22159	-73		-154	2.3	57	1.18	47.83	5.9	41.89

Discussion

In-depth analysis of the relationship between weld surface finishing and corrosion behavior was conducted by examining the variation in corrosion potential (E_corr) as a function of residual stresses introduced during mechanical processing, as well as by evaluating the influence of surface stereometry (topographic features) on corrosion current density (I_corr), as summarized in Table 7. The residual stress component aligned with the direction of tool movement along the weld axis (σ_y) was selected as the key mechanical variable for correlation with electrochemical parameters, due to its dominant contribution to surface stress state after machining. To quantitatively assess these dependencies, Pearson linear correlation coefficients were calculated between the measured residual stresses and corrosion potential values for all investigated material grades. The resulting coefficient of r = – 0.79, with a statistical significance level of p < 0.05, indicates a strong inverse correlation. This suggests that increased residual stress in the surface layer, particularly tensile stress, tends to lower the corrosion potential and thus accelerates the anodic activity of the surface. This trend implies that finishing processes, while improving geometry and appearance, can adversely affect electrochemical stability by introducing surface energy states that favor corrosion initiation.

However, an exception was noted for AISI 316 L stainless steel, where higher residual stresses were associated with a slightly increased corrosion potential. This deviation may be attributed to the inherently high corrosion resistance and passivation behavior of 316 L, whose protective oxide film likely dominates over the stress effect under the test conditions. The observed deviation partially explains the slightly lower overall correlation strength in the aggregated data set.

For the aluminum alloys examined, it was observed that all mechanical finishing methods led to a reduction in corrosion potential (E_corr) values. Specifically, aluminum alloys subjected to abrasive treatments demonstrated lower E_corr values compared to those finished with innovative techniques. Among the alloys, aluminum EN AW-7075 T651 exhibited higher corrosion potential values than alloy EN AW-5083 H321, suggesting greater resistance to anodic reactions under identical conditions.

In analyzing corrosion potential for AISI 304 L stainless steel, results showed that grinding contributed to an increase in E_corr values, indicating improved resistance to anodic dissolution. The innovative finishing method further raised, though by a modest 4%. For AISI 316 L stainless steel, potentiodynamic studies revealed an increase in E_corr after both types of mechanical finishing, indicating improved corrosion resistance following treatment.

Polarization studies additionally confirmed that corrosion current density (I_corr) – a key indicator of uniform corrosion rate – varied from 3.03 × 10⁻⁹ to 2.56 × 10⁻⁶ A/cm² across different material grades and finishing processes. Specifically, the I_corr for EN AW-5083 H321 alloy weld surfaces was recorded at 5.03 × 10⁻⁷ A/cm², while for corrosion-resistant AISI 304 L stainless steel, it measured 4.84 × 10⁻⁷ A/cm². These findings illustrate the nuanced impact of finishing techniques on corrosion behavior, underscoring the importance of selecting appropriate methods to optimize long-term durability and resistance in diverse environmental conditions.

The correlation values between the corrosion current density measurements for the different finishing methods and materials are presented in Table 8, providing a comparative analysis of how each finishing technique influences corrosion rates across the various alloys. The correlation analysis was conducted using the Statistica software package (StatSoft Inc.), and the correlation coefficient r was derived directly from the numerical data presented in Table 7 using the built-in PEARSON function.

Table 8.

Values of the linear correlation coefficients r between the values of the corrosion current density I_corr and the analysed parameters.

Parameter	Sa	Sz	Sp	Sv	h	U	σy	Finishing*
r	-0.17	-0.1	0.11	-0.2	-0.34	-0.4	0.78	-0.53

* To assess the general effect of surface treatment on corrosion behavior, a categorical variable was introduced by assigning numerical codes to each condition: 1 – as-welded, 2 – after grinding, 3 – after innovative finishing.

This data offers insights into the relationship between surface treatment type and material response in corrosive environments, enabling a clearer understanding of which finishing methods may optimize corrosion resistance in specific material applications.

A weak correlation (r = – 0.2 to 0.11) was found between corrosion current density and parameters characterizing the actual surface structure of the weld faces, meaning that surface roughness after finishing treatment did not significantly impact the electrochemical corrosion rate of the materials analyzed. However, a strong correlation (r = 0.78) was observed between corrosion current density and residual stress values present in the tested metal alloys.

The correlation coefficient r = – 0.79 refers specifically to the relationship between corrosion potential and residual stress in the weld surface layer across all tested material grades. This value is not included in Table 7 or Table 8, as it does not relate to the full set of surface parameters presented there, but only to one variable. It is discussed in the text to highlight the strong inverse relationship between residual stress and corrosion potential.

The corrosion resistance of the materials studied in this article is primarily attributed to the presence of passive layers on their surfaces. As previously discussed, increasing residual stress results in a decrease in the corrosion potential of the materials in simulated seawater conditions. This correlation may be due to the lower integrity (or compactness) of the passive layer, which can also be compromised by the chloride ions present in the corrosive environment. When the passive layer loses continuity, allowing direct contact between the electrolyte and the unprotected metal surface, the corrosion rate of the mechanically treated welds increases.

Other researchers have similarly concluded that increased internal stress contributes to higher corrosion rates. Sokolov et al.³³ suggest that the relationship between corrosion and internal stresses arising from plastic deformation following mechanical treatments is due to an increase in structural defects within the crystal lattice. These changes occur as dislocations move along multiple slip systems characteristic of the observed crystal structure, proceeding along crystallographic planes and directions with densely packed atoms, where the shear resistance is minimized. Plastic deformation mobilizes dislocations, increasing the likelihood of annihilation upon encountering dislocations of opposite signs.

Vasil’ev et al.³⁴ hypothesizes that internal stresses promote corrosion in regions of high stress concentration. In the absence of conditions that support passivation, materials tend to dissolve rapidly and selectively, depending on the distribution of internal stresses within the material. Bai et al.³⁵ believes that residual stresses can simultaneously reduce activation energy and surface atomic density, thereby decreasing the corrosion resistance of materials. Ying³⁶ asserts that stress plays a role in enhancing both anodic and cathodic reactions during corrosion, negatively shifting the corrosion potential of P110 steel. The corrosion rate of P110 steel increases as the barrier effect of corrosion products is weakened by applied stress. Tang et al.³⁷ studied the impact of stress on local anodic dissolution using microelectrochemistry and found that applied stresses accelerate anodic dissolution. These studies suggest that stress concentrations at metal defects can lead to stress corrosion cracking, while localized corrosion can produce surface defects on the metal³⁸. Such defects from electrochemical corrosion may be the origin of microcracks, making the material more susceptible to further corrosion.

It was also observed that the type of weld surface treatment applied influences the corrosion rate as indicated by the corrosion current density values. Innovative finishing has a less negative effect on corrosion rate than grinding, as indicated by a linear correlation coefficient of r = – 0.5. Theoretically, grinding should have less effect on internal stresses than innovative finishing due to the shallow depths of cut in abrasive finishing. However, in industrial practice and in this study, weld surfaces are often finished using manual electric grinders where the depth of cut is not strictly controlled.

Conclusions

1. A strong negative correlation (r = − 0.78) was found between residual stress and corrosion potential, indicating that increased residual stresses significantly reduce corrosion resistance in most of the materials studied.

2. Surface roughness parameters showed weak correlation with corrosion current density, suggesting that residual stress plays a more dominant role in corrosion susceptibility than topographic features alone.

3. The innovative finishing method resulted in lower tensile residual stresses and higher microhardness near the weld surface compared to traditional grinding, especially in stainless steels.

4. Electrochemical tests revealed that samples treated with the innovative method generally exhibited higher corrosion potentials and lower corrosion current densities, confirming improved corrosion resistance.

5. Among aluminum alloys, EN AW-7075 T651 showed better corrosion performance than EN AW-5083 H321 under the same finishing conditions.

6. The grinding process, due to its manual and uncontrolled nature, introduced higher variability and potentially deeper plastic deformation, leading to greater corrosion activity.

7. The results confirm that the proposed innovative post-weld finishing method enhances the corrosion resistance of welded joints and can be considered a more effective alternative to traditional grinding, especially in applications requiring long-term performance in aggressive environments.

Author contributions

Conceptualization, O.Ł.; methodology, O.Ł.; software, W.J.; validation, O.Ł., R.S.; formal analysis, O.Ł. and R.S.; investigation, O.Ł.; resources, O.Ł.; data curation, W.J.; writing—original draft preparation, O.Ł.; writing—review and editing, O.Ł. and R.S.; visualization, O.Ł. and W.J.; supervision, O.Ł., R.S.; project administration, O.Ł.; funding acquisition, O.Ł.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.