Corrosion Performance of Wire Arc Additive Manufacturing of Stainless Steel: A Brief Critical Assessment

Abstract

To enhance the products fabricated from wire arc additive manufacturing (WAAM) processes, it is very important to implement a critical assessment of the corrosion performance of additively manufactured stainless steel (SS) for the application of additive manufacturing parts widely used in industries. The common defects in metal additive manufacturing, which include porosity, poor surface finish, oxidation, environmental factor, residual stress, and microstructural defects, are known to significantly influence the corrosion behavior of WAAM-processed SS components prepared to be used under different corrosive and marine environments. This article reviews the recently published works on WAAM-processed SS and provides a critical overview method to improve the corrosion performance of SS components built with the WAAM processes. It also documents some significant factors that determine the corrosion resistance of WAAM-processed SS and identifies key areas for future work.

Introduction

Metal additive manufacturing (MAM), commonly known as metal 3D printing, is a process by which complex geometric shapes are produced in a layer-upon-layer fashion using 3D computer-aided design models.¹ Additive manufacturing (AM) has brought numerous advantages over conventional manufacturing techniques, this benefit includes efficient use of material, net-shape manufacturing, suitability to low-volume production run, the ability to explore alloy composition, high-dimensional accuracy, and others.^1–4 To fabricate 3D functional parts, several MAM-based technologies have recently been employed by many researchers to fabricate high-performance, customized, and complex metallic structures for aerospace,² oil and gas,³ automotive, military hardware, and medical industries.^4,5 Metallic components can be manufactured utilizing several AM methods (Fig. 1), including electron beam melting, direct metal deposition, selective laser melting (SLM), direct metal laser sintering, and wire arc additive manufacturing (WAAM).⁵

FIG. 1.

Classification of metal additive manufacturing.

Among the different AM technologies, WAAM technology is widely accepted by industry due to its potential to fabricate large-scale, different complex metallic structures as shown in Figure 2. WAAM technique is a combination of welding and AM technologies, and one of the potential future applications of this technology could be producing stainless steel (SS) parts for industrial applications. WAAM is a potential technique for fabricating SS components used in a diverse manufacturing industry. As reported in the previously reviewed literature, the application of metallic components (e.g., spare parts) produced from the WAAM process has been well applied in the industry for the smooth running of equipment and facility platforms.³ Despite the advantage of AM over other conventional manufacturing processes, there are some shortcomings and imperfections that have been discovered in the products fabricated from AM processes, which include surface defects, unmelting part of powder, nonhomogeneous mixing of two successive melting layers, surface roughness, poor mechanical properties, residual stresses, porosity, and others.^5–7

FIG. 2.

The WAAM platform used for the fabrication of metallic parts. Reprinted from Salahi et al.⁶ WAAM, wire arc additive manufacturing.

For practical application of WAAM-processed SS, it would be interesting to document the influence of these AM defect formation, and its limitations on the corrosion properties of additively manufactured SS. Some research efforts have been channeled by many researchers to mitigate the effect of this shortcoming found in the AM parts directly by optimizing process parameters during the manufacturing processes. To further reduce failure in AM parts, more research efforts need to be geared toward enhancing the corrosion performance of additively manufactured SS parts prepared to be used under different loading and aggressive environments.

Stainless steels are iron-based alloys that consist of at least 12% chromium that is resistant to corrosion, and they are varied by the differences in composition.⁷ Chemical composition of SS includes 16–18.5% Cr, 10–14% Ni, 2–3% Mo, <2% Mn, <1% Si, <0.045% P, <0.03% C, <0.03% S, and balance Fe. SS is broadly classified as austenitic, ferritic, duplex, martensitic, and precipitation-hardenable alloys. Among several types of SS, 316L SS has been widely used in a diversity of industrial applications, such as pharmaceutical, petrochemical, oil and gas, marine shipping, and water desalination.⁸ 316L SS is an austenitic SS, which provides good resistance to chlorine attacks in marine environments. In one study, Dutta⁹ studied different types and a new application of SS. This work reported some factors that could affect the corrosion resistance of SS as a breakdown of passivity film, resulting in localized attacks such as pitting, crevice, intergranular corrosion, and stress corrosion cracking (SCC). Sun et al¹⁰ investigated the effects of the scan speed and porosity on the corrosion of 316L SS to examine a tendency for the corrosion rate increases as porosity increases.

According to Zhang,¹¹ it was demonstrated in their report that corrosion resistance can be influenced by several conditions, such as porosity, oxidation, poor surface finish, environmental factor, residual stress, and microstructural defects. To enhance the corrosion properties of steel materials, the addition of alloying elements, such as chromium, nickel, carbon, molybdenum, copper, nitrogen, aluminum, sulfur, and selenium, have been reported by Ettefagh¹² as one of the important methods to develop, and improve the corrosion resistance, mechanical properties, ductility, printability of the metallic parts, and the stability of phases in SS alloys. To further buttress this point, Ishizaki et al¹³ investigated the solidification structure of SS formed during AM. In this work, AM samples fabricated with and without Zr were examined using the electrochemical corrosion technique to determine the influence of Zr-added SUS304L powder on the equiaxed crystallization of solidified structures. According to their result, it was reported that SUS304L-Zr-3DP produced a lower current density compared with the SUS316L-3DP samples. This showed that the corrosion resistance of SUS304L-Zr is higher compared with SUS316L-3DP.

In one study, Gulsoy et al¹⁴ studied the corrosion performance of SUS316L samples comprising Zr, Nb, or Ti using an electrochemical corrosion technique. The addition of Zr, Nb, or Ti in the printed samples indicated improved corrosion resistance. An investigation by Mabruri et al¹⁵ revealed the influence of Mo and Ni on 13Cr martensitic SS through the tempered condition. It was reported that the addition of 1% and 3% Mo will increase both the tensile strength and the elongation of the steels, but the addition of 3% Ni will decrease both the tensile strength and the elongation of the steels. Aygul et al¹⁶ investigated the effects of heat treatment and molybdenum doping on the corrosion behavior of cobalt–chrome alloys. In this work, Mo-doped was discovered to have enhanced the corrosion and microhardness values of the alloy. As mentioned by Jin et al¹⁷ the high content of austenite and the formation of secondary austenite can significantly lower the corrosion resistance and strength of SS. This implied that adjusting the kinetics of second-phase formation remains one of the key techniques for enhancing the corrosion performance of alloys.¹⁸

Popkova et al¹⁸ compared the products made of 316L steel fabricated by the SLM method, with the parts obtained by traditional metallurgical techniques. Their results indicated that alloy 316L obtained by the SLM method exhibited a lower corrosion rate compared with similar alloys obtained by the traditional method. Zhang et al¹⁹ revealed the effect of the Ni content on SS built by laser melting deposition. It was confirmed that phase formation of the as-built sample transformed from ferrite to austenite with the increase in the Ni content, resulting in a decrease in microhardness and wear resistance. This implied that the corrosion resistance of SS improved drastically with the increase in the Ni content. An investigation by Zietala et al²⁰ considered the corrosion resistance of 316L SS produced by laser-engineered net shaping. The as-deposited sample indicated a lower corrosion rate due to the significant increase in the Cr and Mo content, and a decrease in Ni in the grain boundaries. This review work documents recent studies conducted to date examining the corrosion performance of WAAM-processed SS from a general point of view.

This study critically reviewed some significant factors determining the corrosion resistance of additively manufactured SS. These factors include corrosion mechanism, SS structural conditions, common defects, surface energy, crystallographic orientation and texture, surface roughness, microstructure properties, and microhardness that can influence the corrosion resistance of WAAM-processed SS. The benefit of this work is to guide against the failure of components built with the WAAM process widely used in industries, and corrosive environments if AM parts are to find widespread application. To reduce the risk of premature failure in AM parts, and increase the corrosion performance and lifespan of the AM-fabricated parts in industrial applications, there is a need to implement a critical assessment of the corrosion performance of wire arc additively manufactured SS parts prepared to be put into usage under different loading and corrosive environments.

Structural Conditions of Corrosion-Resistant SS

For high corrosion resistance, SS parts produced from the WAAM process must possess a uniform and homogeneous structure.⁷ The corrosion-resistant steels are divided into four main groups by their structural conditions.

Ferritic stainless steel

Ferritic SSs contain between 12% and 25% of chromium and <0.1% of carbon. Ferritic steels with a proportion of 11% to 13% Cr are considered slow to rust because they have low corrosion resistance.^13,21 Ferritic steels are chosen for their resistance to SCC, which makes them a suitable alternative to austenitic SSs in applications where chloride-induced SCC is rampant.¹⁷

Martensitic stainless steel

Martensitic SSs contain between 12% and 18% of chromium, together with carbon content ranging from 0.1% to 1.5%. As a result of its chemical composition, it can be hardened and strengthened through heat and aging treatment.⁷ This makes it a good choice for the manufacturing of mechanical valves, turbine parts, and medical instruments.⁸

Austenitic stainless steel

Austenitic steels are alloyed with around 18% chromium and at least 10% nickel. Their key feature is their high rust and acid resistance, which can be further improved with increased Cr and Ni content.⁹

Duplex stainless steel

Duplex steels have an austenitic–ferritic structure and have greater strengths than austenitic steels.²² The mechanical properties and the corrosion stability of duplex SS have resulted in austenite being replaced by duplex steel in many applications. The composition of the high amount of Cr and N contents of duplex SSs than those of austenitic SSs increases the corrosion resistance to localized corrosion in chloride environments.^23,24

Corrosion Mechanisms in SS

Pitting corrosion

Pitting corrosion is usually caused by the passivation of metals and the local breakdown of passivity in corrosive environments. Once a pit has been initiated, it can lead to the creation of small holes in the metal.⁷ Pits typically penetrate from the surface downward in a vertical direction. Pitting corrosion is triggered by nonuniformities in the metal structure, chloride, bromide, and iodide.²⁵ In AM technology, microstructural defects that occurred due to the high level of heat input in the AM-fabricated process could influence the integrity of passive oxide film, and compromise the pitting corrosion resistance.²⁶ The mechanism of pitting corrosion is presented in Figure 3.

FIG. 3.

Mechanism of pitting corrosion.

Crevice corrosion

It is a localized form of attack, where there is a breakdown of the surface passive layer, in crevices or on shielded areas beneath surface deposits.²⁷ The corrosion developed in the crevice region due to the same level of anodic and cathodic reactions.²⁸ Crevice corrosion has the potential to cause structural failure, making it a major concern in most industries.²⁷

Intergranular corrosion

Intergranular corrosion is a selective attack in the vicinity of the grain boundaries of SS. It is a type of corrosion that develops at grain boundaries because of elemental segregation and the precipitation of the secondary phase.²⁸ It occurs as a result of chromium depletion, mainly due to the precipitation of chromium carbides in the grain boundaries.^29,30

Stress corrosion cracking

SCC is the growth of cracks due to the simultaneous action of stress and a reactive environment.³¹ It has been reported that SCC could occur in any alloy material due to the influence of stress, susceptible material, and a critical environment.³² Rusnaldy et al³¹ evaluated the influence of heat treatment on susceptibility to SCC of 13Cr martensitic SS in 3.5% NaCl. “They found out that 13Cr MSS was extremely to SCC in a solution of 3.5% NaCl at a constant rate of 80% of ultimate tensile strength in tempered conditions.” The SCC initiation and growth in WAAM-processed SS occurred as a result of the inherent residual stress and distortion, which are caused by the high thermal input from its heat sources and highly concentrated melting and solidification of the melt pools during the WAAM process.³²

Galvanic corrosion

Galvanic corrosion occurs when two dissimilar metallic materials are electrically connected in a corrosive environment. It promotes the normal corrosion of a metal in electrolyte conditions.²⁴ Galvanic corrosion has been widely found hidden in buried pipelines, seawater installations, and many other industrial installations until failure occurs.²³

Uniform corrosion

Uniform or general corrosion has been described as a type of corrosion attack that is more or less uniformly distributed over the entire exposed surface of a metal. Uniform corrosion of SS happens when there is a general failure of the protective passive film permitting the entire surface to experience strong acid or hot alkaline environments.³³

Factors That Determine the Corrosion Resistance of WAAW-Processed SS

The influence of microstructure properties on corrosion resistance of SS

The corrosion performance of SS simply relied upon the stability of phases and a passive film, which is an attribute of several conditions such as microstructure features, alloy composition, corrosive media, metallurgical properties, and others.¹² The effect of microstructure performance on corrosion of SS produced by the WAAM process is found to be below the requirements needed for the production of functional metallic parts for structural applications since this is mainly affected by structural defects such as porosity, poor surface finish, oxidation, residual stress, and other printing errors that accidentally appeared during the printing process.³¹ Sprouster et al³⁴ studied the dislocation microstructure and its influence on corrosion behavior in additively manufactured 316L SS. This work discussed the microstructural features (such as irregular surface chemistry, cellular dislocation, multiscale interfaces, surface roughness, and bulk chemical heterogeneities) in AM SS. The authors attributed these microstructure features as one of the factors that can influence the material's corrosion resistance in AM SSs.

In this case, postfabrication heat treatment techniques can be simply employed to improve and modify the microstructure properties of additively manufactured SS. Al-Maharma et al³⁵ suggested that the internal flaws generated in the additively manufactured parts can affect the structural integrity and microstructure properties of the WAAM-processed SS components.^36–39 The influence of postfabrication heat treatment on the microstructure properties of WAAM-processed SS with different heat-treated conditions is given in Table 1. As evidenced in Table 1, the effect of the postfabrication heat treatment procedure conducted at different temperature conditions indicated improved microstructure properties of parts produced by the WAAM process. Based on the collated data from Table 1, it shows obviously that the corrosion resistance properties of WAAM-processed SS are extremely hinged on the microstructure properties that are influenced by AM printing conditions and postfabrication treatment techniques.

Table 1.

The Influence of Postfabrication Heat Treatment on the Microstructure Properties of WAAM-Processed Stainless Steel with Different Heat-Treated Conditions

Materials	Heat treatment techniques	Microstructure properties	Ref.
17–4 PH stainless steel	Solution treatment	After solution treatment, the microstructure transforms to nearly 100% lath martensite	36
420 Martensitic stainless steel	Austenitizing and tempering treatment	The austenitizing treatment resulted in the removal of retained austenite and δ-ferrite, while the tempering process promotes the precipitation of a variety of carbide particles at different tempering temperatures.	37
420 Martensitic stainless steel	Annealing and Q&T	The annealed sample showed islands of spherical chromium carbides embedded in a ferritic matrix, while the Q&T sample indicated intergranular carbides precipitated.	6
Super martensitic stainless steel	Solution and aging treatment	Microstructural analysis revealed a finer martensitic structure with an effective grain size of 3.6 μm.	38
ER70S	Normalizing treatment	It revealed a uniform and homogeneous ferritic/pearlitic microstructure within the melt-poor center, fusion boundaries, and the heat-affected zone.	39

Q&T, quenching and tempering; WAAM, wire arc additive manufacturing.

Due to the influence of the postfabrication heat treatment technique on the as-fabricated parts, as shown in Table 1, it is glaringly shown that selecting a suitable heat treatment technique can promote the enhancement of microstructure properties, and corrosion performance of SS as indicated in Figure 4.

FIG. 4.

The relationship between microstructure and property and the role of controlling invisible defects using nondestructive inspection methods.

Surface roughness

Surface roughness refers to the irregularities, which are inherent in the manufacturing process on the surface of the fabricated object. Surface roughness determines the texture and appearance of the surface, which plays a significant role in the corrosion performance of WAAM-processed SS. Surface roughness is one of the key factors that determine the corrosion resistance of wire arc additively manufactured SS. Especially in the WAAM process, there are several factors (including porosity, stair-stepping effect, mixed layer formation, un-melted particle, spatter, humping effect, gas expansion, and others)⁵ that produce the formation of surface roughness in additively manufactured metallic parts.^5,40,41 In this case, the influence of these factors will greatly deteriorate the surface quality and affect the corrosion performance of WAAM fabricated parts. Parts fabricated with a rough surface will be susceptible to corrosion attack, resulting in premature failure by initiating cracking during fatigue loading.⁴¹ This implied that the parts produced with a rough surface will promote and increase the risk of corrosion attack.

Mitigating the effect of surface roughness in the AM parts of SS is one of the means of preventing materials' failure to enhance their functional properties.¹² In one study, Rauch and Hascoet⁴² worked on improving additive manufactured surface properties with postprocessing techniques. This work explored high-speed machining, waterjet machining, and laser polishing techniques to improve the surface quality and eliminates unmelted region on the component layers. Surface roughness in WAAM-processed parts is significantly caused by the inappropriate selection of suitable printing parameters employed during the printing process. The higher the layer, the greater the surface roughness of the fabricated parts. An investigation by Hong and Nagumo revealed that surface roughness influences the formation of pitting corrosion on 301 SS.⁴³ It was established that the metastable pitting potential value improved as the surface roughness increased in 0.5 M NaCl solution. One study proved that by increasing the torch angle from 5° to 15°, surface roughness becomes finer up to 59 nm.⁴⁴ Figure 5 presents an uneven surface profile of the component fabricated by gas metal arc welding-based AM.

FIG. 5.

Surface profile of thin-walled part produced in WAAM. Reprinted from Schmuki.⁴¹

Microhardness

Microhardness has been described by Han et al⁴⁵ as one of the important factors that determine the corrosion resistance of SSs in WAAM-processed parts. A recent study by Yang et al⁴⁶ showed the effect of nano-TiC particle content on the microstructure and microhardness properties of SS. It was reported that enhancing microhardness increases deposited layer strength significantly achieving finer grain structure, thereby improving the corrosion resistance of steel materials. This implied that materials with improved microhardness will resist the corrosion species from penetrating pristine surfaces. Hence, the addition of alloying elements to steel materials is known to enhance the surface microhardness and wear properties of WAAM-processed parts.⁴⁵

The phase compositions

Improvement in corrosion properties of WAAM-processed SS is significantly hinged on their chemical composition and microstructure features. It has been established that a composition of phases such as δ-ferrite, metallic, nonmetallic, and precipitates in the steel could significantly affect the corrosion resistance of WAAM-processed SS.^45,46 A recent work conducted by Yang et al⁴⁶ revealed that (Fe, Ni) phase, TiC phase, and ferrite result in a strengthening layer of the AM parts. In terms of structural conditions, ferrite SS is more vulnerable to corrosion attack than austenitic SS. During the solidification process, it has been identified that a faster cooling of WAAM-fabricated parts can result in a more austenite phase and reduced ferrite phase through which corrosion resistance can be significantly improved.

Crystallographic orientation and texture

Improvement in corrosion properties and microstructure of SS 316L has greatly relied on crystallographic orientation.^44,47 In additively manufactured parts, the grain orientation, texture, and direction significantly play an important role in determining the corrosion performance of AM-fabricated parts. In one study, Chakravarthy and Jerome⁴⁸ revealed that adjusting the inclination angle from 5° to 15° tends to produce austenite grain structure with an appreciable growth at {111} direction and improved corrosion resistance due to the influence of surface energy. For example, face-centered cubic materials have the lowest surface energy in grains that are in direction of {111}. While {001} direction textured grains indicate higher surface energy and get easily corroded.⁴⁸ To support this point, Trisnanto⁴⁷ examined the effects of crystallographic orientation on the corrosion behavior of SS 316L. The effects of different crystallographic textures, namely {100}, {110}, and {111} on both general and pitting corrosion were investigated. This work revealed that the {111} texture indicated the highest general corrosion resistance, followed by the {100} and {110} textures.

Surface energy

Surface energy is known as one of the influential factors that control corrosion species in manufacturing materials. According to previous results, it has been established that textures mostly flow along the {100} and {111} planes parallel to the building direction (BD), but some also flow along the {100}, {110}, {111} planes at an angle of 45° to the BD and normal direction.⁴⁴ To confirm the relevance of this texture direction to the surface energy, an investigation conducted by Chakravarthy and Jerome⁴⁸ revealed that the {111} plane significantly promotes the least surface energy than other planes. This implied that there was an increase in the corrosion resistance of the materials. Generally, the higher the surface energy, greater the corrosion tendency in the materials. For example, materials with more fine grain always have more grain boundaries, which can increase the surface energy and act as a corrosion nucleation site.

Stress and strain

Corrosion resistance in metallic materials is also significantly driven by the nature of strain and stresses in the materials. It has been established that strain and stress typically promote corrosion, and SCC in manufactured components.²¹ Research shows that improved stress and strain in the materials can be greatly enhanced by selecting more suitable materials and manufacturing techniques.⁵ High levels of residual stress, and cracking in the WAAM-processed metallic parts must be greatly avoided, particularly for parts exposed to corrosive environments. In one study, Mercelis and Kruth⁴⁹ studied the formation mechanism of residual stress in AM materials using a cool-down phase model. During the printing process, each deposition layer experiences rapid heating and cooling cycles, resulting in the formation of large residual thermal stress in the material. Generally, there are many significant factors affecting the corrosion resistance of as-fabricated components due to the rapid heating, and cooling formation of the WAAM process.

As a result of this, the magnitude of residual stress effect (including stress intensity factor, crack location, and propagation direction) in the as-fabricated material also has a detrimental effect on AM metal corrosion.

Influence of Defects in WAAM of SS on Corrosion Resistance

The impact of corrosion on AM metallic parts has become one of the major concerns that are needed to be controlled if the industrial experts need to prevent the machinery and equipment built with the AM metallic component from unexpected failure, and increase its service life under different loading and operating conditions. The corrosion performance of WAAM-processed SS has become one of the major concerns in the application of additively manufactured metallic structural materials.²⁵ Some studies have informed that corrosion resistance can be influenced by several factors, including porosity, oxidation, environmental factor, residual stress, and microstructural defects.^{5,10,11,21,50} In addition to this, the defects that were induced during the printing of metallic components can cause a major detrimental effect on the component performance, which could also affect the mechanical performance, and electrochemical properties of additively manufactured parts.

Hence, it is high time to find out how the manufacturing process parameters and postheat treatment techniques could be used to prevent the inherent defects generated from the WAAM process and to improve the microstructure and corrosion resistance of additively manufactured SS components. In one study, Ornek²¹ indicated that porosities can reduce the threshold stresses needed for pit-to-crack transition and further complicate the SCC state.

Loss of alloying elements induced by the rapid heating and cooling rates may introduce defects, such as cracks, residual stresses, and porosity, during the deposition of metallic parts.⁵ The corrosion behavior in steel materials is strongly dependent on the chemical composition and microstructure of the designed materials. Changes in alloy composition during the printing process can significantly affect solidification microstructure, corrosion resistance, and mechanical properties, and can as well be a serious threat to producing high-quality metallic parts (Fig. 6).¹

FIG. 6.

Various categories of forces that caused materials and equipment failure.

In another study, Sander et al⁵⁰ reported that porosity may significantly influence the pitting characteristics of alloys produced by AM techniques. Takakuwa and Soyama⁵¹ investigated the effect of residual stress on the corrosion behavior in austenitic SS 316L. It was revealed that residual stress can be formed into AM-built components through surface finishing and heat treatment, as this can significantly affect the fatigue life and resistance to SCC. This work attributed the introduction of compressive residual stress as it enhances not only the mechanical properties but also the corrosion resistance of austenitic SS. Improving the resistance of SS components produced by the WAAM process to corrosion is essential to improve its service life and reliability in a marine and corrosive environment. Hence, the influence of residual stress, porosity, surface roughness, and microstructure defects as it affects the corrosion characteristics of SS fabricated by the WAAM process needs further investigation.

The Corrosion Behavior of SS Additively Manufactured by the WAAM Process

The conventional fabrication techniques are being faced with the challenges of producing near-net-shape and complex geometrical SS parts as compared with the AM techniques. Conventionally, SSs are fabricated using traditional methods such as casting, machining, and forging, which are capital intensive, and time consuming. The metallic components (i.e., spare parts) produced from conventional manufacturing techniques have been always used in industries to keep their plant and equipment running smoothly before the advent of AM technology. In recent times, metallic AM has replaced some metallic components fabricated by conventional manufacturing methods. In industrial applications, some of the metallic spare parts that are commonly used require complex shapes, which cannot be produced easily using conventional manufacturing processes. In the manufacturing of SS components, AM technology has the advantage of producing functional metallic complex shapes (i.e., SS), which can be fully utilized in various industrial applications such as aerospace,² oil and gas,³ automotive, military hardware, and medical industries.^4,5

One of the major concerns for adopting AM metallic components in industries is the low quality of the products produced from the AM techniques due to the impact of inherent defects formed during the deposition of layer-by-layer metallic parts. These defects usually affect the geometric accuracy, delamination of layers during deposition of parts, and deterioration of fatigue performance of WAAM-processed parts.⁵ In AM technology, the defects generated during the printing process could always affect the quality of the metallic parts and degrade the surface properties (i.e., this could bring out mechanical wear, corrosion, and loss of usefulness of the components) of the products to the path of failure (Fig. 6). It is well known that the loss of alloying elements and cooling rate significantly affect the microstructure constituents (such as ferrite, austenite, and martensite) of WAAM-processed parts.⁵² Corrosion assessment of additively manufactured SS needs to be put into consideration before the AM-fabricated components are being put into use under the different working and corrosive environments to prevent the unexpected failure of the components in applications.

To fully produce complete defect-free metallic components for industrial applications, hence, need to document the influence of the corrosion behavior of SS parts additively manufactured by the WAAM process. In recent times, few research works have been conducted on the aspect of the corrosion behavior of WAAM-manufactured SS.^7,47,53,54 For industrial applications, SS components have been developed for use in corrosive and marine environments.⁵³ In one study, Ko et al⁷ considered in their studies that the AM process parameters, the corrosion environment, heat treatment, and the type of SS used in the production of AM metallic parts are the most relevant factor that affects the corrosion behavior of SS. In AM technology, the influence of process parameters during the deposition of the metallic layer process can provide variation in cooling rates and thermal gradients, which could, as a result, lead to differences in residual stresses, porosity, and grain size in AM-fabricated parts.⁴⁷ Interestingly, process parameters are one of the important conditions in AM techniques that are known to significantly influence microstructure evolution.

Ron et al⁵⁴ evaluated the stress corrosion behavior of additively manufactured austenitic SS produced by the WAAM process. In their studies, the authors concluded that the influence of localized corrosion attack in the printed metallic alloys was felt mainly at the interface between the austenitic matrix, and the secondary ferritic phase. The reduction in corrosion resistance of ferritic and austenitic phases of printed alloys can significantly influence the impact of microgalvanic corrosion in the printed metallic parts. Additionally, the ferritic phase is known to have a lower pitting corrosion resistance tendency as compared with the austenitic phase.⁵² As evidenced in Figure 7, the localized corrosion attack in the printed 316L alloys was found to be in the form of pitting corrosion equally dispersed on the surface.

FIG. 7.

(a, b) Close-up views of the corrosion attack at the surface of the printed alloy. (c, d) Close-up views of the corrosion attack at the surface of counterpart AISI alloy. Reprinted from Ron et al.⁵⁴

In another work, Pütz et al⁵⁵ studied the microstructure and corrosion behavior of functionally graded wire arc additive manufactured steel combinations. This work explained the benefit of making a chemical and microstructural gradient in steels to achieve optimum corrosion properties in the metallic components fabricated by using the WAAM process. It is well known that the influence of high heat input during the production of parts in the WAAM process can create diverse complex microstructures resulting in interior phases and grain structures in the heat-affected zones.^12,55 As a result of this, the phase distribution in the steel materials could have a major effect on the corrosion performance of additively manufactured SS. To improve the corrosion behavior of WAAM metallic parts, the influence of high heat input in the WAAM process as it impacts the microstructure defects in the WAAM-processed parts needs to be comprehensively examined. Collazo et al⁵⁶ considered the effect of the optimized process parameters as it affects the corrosion properties of 316L SS.

This study stressed the importance of adjusting the process parameters in the printing build-up process to obtain the best manufacturing conditions and to produce the optimal corrosion resistance properties of 316L SS. Revilla et al⁵⁷ studied the microstructure and corrosion behavior of 316L SS prepared using different AM techniques. This work suggested that residual stresses can also affect the material's vulnerability to SCC.⁵⁷ Sprouster et al³⁴ studied the dislocation microstructure and its influence on corrosion behavior in additively manufactured 316L SS. As reported, the high dislocation microstructure and the formation of chemical heterogeneities found in the built samples could be a major primary factor known to deteriorate the corrosion performance of additively manufactured 316L SS. Kozuh et al⁵⁸ in an investigation on 316L austenite SS resolved that the presence of δ-ferrite phases in the microstructure can enhance the uniform corrosion and pitting sensitivity. Kazemipour et al⁵⁹ studied the microstructure and corrosion properties of wire arc additively manufactured AISI 420 SS.

This work characterized the three different portions of the printed sample built across the different BDs. As reported, the upper region of the built sample indicated an improved corrosion resistance and low pitting susceptibility than the bottom layers. Xie et al⁶⁰ studied the morphology, microstructure, mechanical properties, and corrosion resistance of the as-deposited sample. Due to the high level of heat input accumulated in the deposited layer samples, the dislocation density was reduced, which promotes corrosion attacks and thus increased the pitting potential in the printed middle region. In the coming years, some manufacturing companies in the aerospace, construction, automotive, oil and gas, medical industries, and others would like to achieve industrial products with at least 70% functional components made by the WAAM process.⁶¹ This can only be accomplished if the corrosion properties of additively manufactured metallic components are significantly improved to resist the failure of the parts in service under different loading and corrosive environments.

One of the most important causes of damage to the industrial platform is the corrosion of metallic components impacted by corrosive and marine environments with a high content of salts, acids, and alkalis. Improving the corrosion performance of additive manufactured SS for industrial applications has become a common concern to industrial stakeholders. WAAM is an area where further study is needed in the aspect of enhancing the corrosion properties and mechanical properties. These research efforts will mitigate the influence of microstructural defects, porosity, and residual stress in the additively manufactured SS, which could advance the corrosion properties of WAAM-processed SS parts.

Improvement in Corrosion Resistance of WAAM-Processed SS Through Posttreatment Techniques

Several attempts have been made in the past to improve the corrosion behavior of SS by different methods.^25,62–76 The corrosion resistance of WAAM-processed SS components can be further improved in harsh corrosive and marine environments through surface modification technologies, including heat treatment, alloying, dip-coatings/electrodepositions, surface mechanical treatment, anodization, machining process, microarc oxidation, and others.^{8,25,61–64,68,77,78} Among all these methods, heat treatment has been widely recognized as an important technique for enhancing the microstructure and mechanical properties as well as improving the corrosion resistance of steel materials.^25,62 The effects of heat treatment on the corrosion resistance and microstructural properties of WAAM-processed SS have not been fully investigated. The heat treatment techniques can be well utilized in refining the metallurgical characteristic of the metallic components produced by the WAAM process.⁵ In one study, Chen et al²⁵ studied the effect of the postmanufacturing heat treatment (PMHT) on the pitting resistance of the WAAM 316L SS.

In this study, the authors reported an excellent pitting resistance after the PMHT of 1150°C/1 h. The influence of PMHT on as-built 316L SS minimizes residual stress, elimination of cellular structure, and increases the presence of large columnar grains. Hence, the treated samples showed improved corrosion resistance after applying the PMHT techniques to as-printed samples. Chen et al⁶² investigated the effect of heat treatment on microstructure, and mechanical and corrosion properties of austenitic SS 316L using arc additive manufacture. This work concluded that controlling the amount of both σ and δ phases through heat treatment techniques can further enhance the corrosion resistance of steel accordingly. To further improve the corrosion resistance of steel, the corrosion resistance of SS can be optimized by altering the chemical composition and postfabrication heat treatment process to obtain the functional products for industrial application as described in Figure 4.³¹ In one study, Chen et al⁶² reported that corrosion properties of WAAM-processed 316L can be further enhanced through optimization of volume fractions of σ and δ-ferrite phases through heat treatment techniques.

The improvement of corrosion properties of WAAM-processed SS after heat treatment is mainly attributed to the stability of phases in SS alloys and their metallurgical characteristics. In one study, Popkova et al¹⁸ reported that the effect of the annealing process on the printed parts can lead to a decrease in the number of defects formed in the additively manufactured SS, thereby promoting a decrease in the corrosion rate. Table 2 presented previous research work conducted through posttreatment methods for improving the corrosion resistance of different classes of SS.

Table 2.

Improvement in Corrosion Resistance of Stainless Steel Through Posttreatment Techniques

Materials	Corrosion improvement methods	Research output	Ref.
301 & 304 Stainless sheets of steel	Surface mechanical attrition treatment.	The treated samples showed a lower weight loss, which symbolizes a low corrosion rate, hence an improved corrosion resistance.	66
Stainless steel	Surface mechanical attrition treatment.	This work predicted that the corrosion fatigue strengths of stainless steel can be better enhanced by the collective effects of surface treatments and the associated compressive residual stresses.	68
Stainless steel	Surface coating	By applying MgZnCa coating on the steel substrate. The coated steel substrate showed an improved corrosion resistance as compared with the uncoated steel substrate.	69
Austenitic stainless steels	Glow-discharge nitriding treatment	This work reported that low-temperature glow-discharge nitriding treatment can enhance the corrosion resistance in chloride ion-rich solutions of austenitic stainless steels.	70
Low alloy steel	Multifunction cavitation in water	The MFC-processed samples showed an improvement in residual stress and surface modification resulting in high strength and corrosion resistance.	72
AISI 304L stainless steel	Deep rolling treatment under cryogenic cooling	Machined samples deep rolled at a speed of 25 m/min, without cooling, showed better corrosion resistance than those processed under cryogenic cooling.	73
Stainless steel X6CrNiMoTi17-12-2	Slide diamond burnishing.	This work reported improved corrosion resistance in the samples after burnishing.	74
316L Austenitic stainless steel	ZrO2 sol/gel coating in nitric acid solution.	Treated samples showed a considerable enhancement in the corrosion resistance of 316L stainless steel by an increase in corrosion potential and transpassive potential and a decrease in passive current density and corrosion current density.	75
Austenitic stainless steel	Combined surface treatment (electron beam cladding + gas nitrocarburizing)	The corrosion resistance of the cladding layer showed an improved corrosion performance compared with the base material.	76
AISI 321 metastable austenitic stainless steel	Thermomechanical processing	The results indicated an improved corrosion resistance of ultrafine-grained samples as a result of the formation of the most stable adsorbed passive film.	77
Mild steel	Rolling process	The treated sample indicated a corrosion potential of −0.118 V, reduced corrosion current density of 0.133 mA/cm2, higher impedance, and phase angle maximum, causing an increase in corrosion resistance.	78
316 Stainless steel	Surface mechanical attrition treatment	The corrosion resistance of the nanostructured 316 stainless steel improved significantly after surface modification by SMAT and low-temperature annealing treatment.	79
420 Martensitic stainless steel	Tempering treatment	The sample tempered at 400°C showed the highest corrosion resistance while tempering at 500°C indicated the worst corrosion performance	37
316 Stainless steel	Solution treatment	Postprocessed samples indicated improved corrosion resistance to as-fabricated ones.	81

MFC, multifunction cavitation; SMAT, surface mechanical attrition treatment.

Summary and Future Work

The WAAM technology has caught much attention in industries and academia, as this technology provides significantly enhanced large-scale metallic components with a high deposition rate (up to 10 kg/h), design flexibility, manufacturing cost reduction, high material utilization, reduced lead time, and near-net-shape components. For WAAM-processed SS to realize its full adoption in industries, more research studies are needed to be channeled on the aspect of improving the corrosion resistance of the SS fabricated from the WAAM techniques.

Corrosion has been reported by many researchers as the main influential factor that caused the failure of engineering components in industrial sectors. Mitigating the influence of defects in WAAM-processed SS will be a better advantage to increase the corrosion resistance of steel materials to prevent the failure of the AM parts put into service under different loading and corrosive environments. This review article has critically documented the corrosion behavior of SS components produced by the WAAM process, several significant factors that determine the corrosion resistance of SS, and the overview methods to improve the corrosion performance of WAAM-processed SS. Improving the corrosion properties of steel materials has been the vision of many corrosion experts and material scientists. To prevent the incessant failure of the WAAM-processed SS components in industry applications, further research studies are required to be comprehensively patterned in the area of corrosion properties of SS as follows:

• i. The influence of residual stress, porosity, cracking, distortion, poor surface finish, and microstructure defects as it affects the corrosion characteristics of SS fabricated by the WAAM process needs further investigation.
• ii. Further research studies to find out how the manufacturing process parameters and postheat treatment techniques could be used to prevent the inherent defects generated from the WAAM process and improve the microstructure and corrosion performance of additively manufactured SS components are essential.
• iii. To improve the corrosion behavior of WAAM-processed metallic parts, the impact of high heat input in the WAAM process as it influences the microstructure defects in the WAAM-processed parts needs to be fully examined and controlled.
• iv. The corrosion properties of WAAM-processed SS components have not been fully investigated, and more research efforts need to be explored by the researchers utilizing different posttreatment methods to enhance the corrosion properties of SS parts produced by the WAAM process.
• v. The influence of the galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, uniform corrosion, and SCC behavior of WAAM-processed duplex and austenitic SSs in chloride and acidic environment has not been fully investigated.
• vi. The influence of microstructure properties, surface roughness, microhardness, crystallographic orientation and texture, phase composition, surface energy, and stress and strain as they affect the corrosion resistance of SS needs to be comprehensively investigated.

Authors' Contributions

B.O.: Conceptualization, literature review, and article draft preparation. I.O.: Literature review, reviewing, and editing. T.O.: Reviewing and editing, and literature review. P.F.: Reviewing and editing, and literature review. All authors equally helped to write this article.

Author Disclosure Statement

No potential conflicts of interest were reported by the authors.

Funding Information

No funding was received for this article.