Table 3.
Overview of the selected conversion coating procedures reported in the literature.
| Treatment | Pre-Treatment | Bath Component/ Concentration/ Mechanism |
Initial Bulk pH | Duration, Bath T | Thickness, Surface Composition/ Morphology |
Performance | Testing Medium | Alloy | Reference/ Year |
Advantage/ Disadvantage |
|---|---|---|---|---|---|---|---|---|---|---|
| Cr(VI) | Alkaline decreasing Nitric acid pickling Chromic acid pickling HF activation |
K2Cr2O7 40 g/L K2SO4 20 g/L |
n/a | 1–14 min 75 °C with air bubbling |
11 µm Cr2O3, Cr(OH)3, K2CrO4, MgO |
Ecorr shift −1.61 to –1.3 V icorr decreased from 0.079 to 0.02 A/m2 for bare to CCC coated substrate |
1% NaCl |
EV31A | [80] 2014 |
Highly effective, with self-healing effect, abrasion resistance, commercially available, one step process, long bath life / carcinogenic, toxic, banned by EU regulations, urgent need for replacement |
| Cr(III) | Grinding to 1000, ultrasonication in acetone | (0.3 M CrCl3 0.05 M NH4H2PO2) in choline chloride:ethylene glycol (1:2) |
n/a | 30–60 min 30 ± 5 °C under ultrasonic treatment followed by methanol rinsing |
3 µm Cr2O3 Microcracks |
Ecorr shift −1.51 to –1.45 V icorr decreased from 609 µA/cm2 to 1.25 µA/cm2 for bare to Cr(III)CC coated substrate |
3.5% NaCl | AZ31 | [135] 2016 |
Commercially available, robust and easy bath maintenance and process control, one step process / moderate corrosion protection, weak self-healing properties, contains minor amount of carcinogenic Cr(VI) |
| Pickling/activation processes according to SAE-AMS-M-3171 | e.g., 1–5 g/L Cr2(SO4)3, 1–5 g/L K2ZrF6, 0–5 g/L MeBF4 0–5 g/L ZnSO4 0.5–1.5 g/L soluble cellulose 0–10 g/L surfactant |
3.7–4.0 | 5–15 min Ambient temp up to 50 °C |
Adhesion 2–2.5 times higher than for DOW 7 chromate treated process | n/a | n/a | AZ91C-T6 ZE41-T5 |
[136] 2010 |
||
| Phosphate | Alkali washing in 60 wt.% NaOH, grinding to 1200 grit, cleaning in pure water and ethanol |
35 g/L Mn(H2PO4)2, 0.5 g/L of NaF or C6H5Na3O7 or C6H8O7 |
2.5 | 1 s–20 min 95 °C |
Lamellar structure with block particles. Intermediate layer: Mg3(PO4)2, AlPO4, and Mg(OH)2. Outer layer: MnHPO4. |
Ecorr shift from −1.5 to −0.34, −0.468, and −1.37 V for the bare to phosphated substrate containing citric acid, NaF and Na-citrate, respectively. Icorr reduction from 460 µA/cm2 to 5 nA/cm2, 32 nA/cm2 and 5 µA/cm2 for the bare to phosphated substrate containing citric acid, NaF and Na-citrate, respectively. | 3.5 wt.% NaCl | AZ31 | [205] (2013) |
Commercially available, eco-benign, good adhesion with paint, / moderate corrosion protection effect, requires elevated temperatures, multi-step process, requires accurate operation, bath maintenance and control due to low stability of bath’s pH |
| Grinding to 1200 grits, cleaned with industrial alcohol in ultrasonic bath, degreased in NaOH, acid pickling in mixture of HF and C2H6O2 | 4–36 mL/L H3PO4, 40–90 g/L Ba(H2PO4)2 1–3 g/L NaF |
n/a | 10–30 min, 60–100 °C |
Mg, MgO, and some amorphous phases | Corrosion spots appear after 20 h of SST for the phosphate conversion coated sample, while white massive corrosion blocks after 8 h of SST covered the untreated sample | SST, damp heat test | AZ31 | [400] (2009) |
||
| Grinding to 4000 grits in ethanol, rinsed in ethanol |
0.1 M Mg(OH)2, 0.24 M H3PO4 |
3.2 | 20 min, 45 °C |
2.5 µm thick coating after 20 min of phosphating. Micro-cracks structure. Coating composed of MgO/Mg(OH)2 and Mg-PO4 compounds |
Ecorr shift from −1.61 V to −1.41 V for the bare to phosphated substrate. Icorr reduced from 223 µA/cm2 to 6.9 µA/cm2 for the bare to treated substrate. Pit initiation time was delayed from 10 min to 24 h for the bare to the phosphated substrate. |
0.1 M and 0.05 M NaCl |
AZ31 | [401] (2017) |
||
| Grinding to 2000 grits in ethanol, rinsed in ethanol |
Step 1: H3PO4, step 2: 0.05 M (NH4)2HPO4 |
Step1: 5 Step2: n/a |
Step 1: 30 min, 40 °C Step 2: 30–60 min, 80 °C |
Inner layer: MgHPO4.3H2O Outer layer: MgNH4PO4.6H2O |
Corrosion resistance of phosphated substrates is about 20 times better than untreated samples. Ecorr shift from −1.6 V to −1.53 V for the bare to 2-step treated substrate. Icorr reduction from 63 µA/cm2 to 3.7 µA/cm2for the bare to 2-step treated substrate. |
SBF | AZ31 | [402] (2015) |
||
| grinding to 2500 grits, degreased in absolute ethanol, acid pickling in HNO3 and then HF solution. Rinsed by distilled water between each step |
10 g/L Y(NO3)3 then in NH4H2PO4 bath with concentration of 1–2.5% | n/a | 30–180 s, 75–90 °C |
Y2O3, YOx/y, Mg3(PO4)2, AlPO4 and YPO4 | Ecorr shift positively about 180mV compared to the uncoated one at, Icorr reduced from 70.2 µA/cm2 to 7.7 µA/cm2 for bare to conversion coated sample |
3.5% NaCl | AZ91 | [403] (2016) |
||
| Phosphate-permanganate | Grinding to 1500 grit, polishing with 0.3 µm Al2O3 paste, pure water cleaning, alkaline degreasing with NaOH + Na3PO4, pure water cleaning, acid pickling with H3PO4, surface activation with HF |
20 g/L KMnO4, 60 g/L MnHPO4 |
n/a | 10 min, 50 °C |
Network-like cracks in coating containing metal oxides (Mg, Mn and Al), Hydroxide, phosphates and spinel for AZ series alloy | Equivalent or slightly better passive capability than the conventional Cr6+-based conversion treatment of AZ series alloys, but an inferior capability for the pure Mg | 5 wt.% NaCl | AZ61,AZ80, AZ91, and pure Mg | [187] (2002) |
|
| Grinding to 2000 grit, rinsed with DI water, cleaned in acetone, dried in a stream of hot air |
0.87 M NH4H2PO4, 0.063–0.51 M KMnO4 |
n/a | 10 min, 60 °C |
Three layer: 1- porous layer on substrate 2- compact intermediate layer 3: cellular overlay. Thickness in the range of 8–1 µm | Less than 10% corroded fraction after 24 h SST for the phosphate solution containing 0.51 M KMnO4, while more than 50% of bare AZ31 was corroded after 24 h | Solution of 0.05 M NaCl and 0.10 M Na2SO4. SST (ASTM B117) |
AZ31 | [189] (2013) |
||
| Degreasing with ethanol, acid pickling with H3PO4, tap water rinsing, NaOH activating, tap water rinsing | 100 g/L NH4H2PO4, 30 g/L KMnO4 |
3.5 | 40 °C | First layer: homogenous but with many cracks/ Second layer: nodules of Mn-rich oxides |
Reduction in corrosion rate from SST by phosphate-permanganate conversion coating. +200 mV shift to Ecorr and two orders of magnitude reduction in icorr, comparing untreated and phosphate-permanganate coated sample |
Salt spray ASTM B117, Electrochemical tests in solution containing Na2SO4, NaHCO3 and NaCl (pH 8.2) |
AZ91 and AM50 | [404] (2010) |
||
| Grinding to 1200 grits, DI water, air stream drying |
0.1 M KMnO4 0.025 M Mn(NO3)2 0.02 M KH2PO4 |
1.7 | 90 s 25 °C |
230 nm nearly crack-free |
Ecorr shift –1.56 V to –1.41 V icorr decreased from 20 µA/cm2 to 1.6 µA/cm2 for bare to PCC coated substrate compared to 0.4 µA/cm2 for DOW1 CCC Sufficient electrical conductivity, Poor crystallinity |
0.05 M NaCl + 0.1 M Na2SO4 |
AZ31 | [81] (2015) |
||
| Blasting (alumina F220-500), degreasing (e.g. NaOH), pickling (H3PO4), activating (or HF) |
0.2 M KMnO4 0.1 M Na3PO4 2 g/L Ca(NO3)2 2 g/L Y(NO3)3 |
2.5–5 H3PO4 |
n/a | n/a | CC treated uncoated samples withstood 168 h of SSF and 500 h of humidity test, CC treated samples coated with primer and resin withstood 2000 h of SSF, Good adhesion of organic coats |
SSF (ASTM B117 Sec. 8.1 and 10.1); Humidity tests; Cross-cut adhesion tests |
EV31A, AZ91, AM60 |
[405] 2015, 2017 |
||
|
Zinc
-phosphate |
Grinding to 1000 grits, degreased in absolute acetone, rinsed by DI water |
Primary bath: 2 g/L ZnO, 12 g/L H3PO4, 1g/L NaF, 4 g/L C4H4O6Na2, 6 g/L NaNO3, 0.5 g/L Na4P2O7 + 2 g/L nano-CeO2 or 2 g/L nano-ZnO or 2 g/L nano-ZrO2 |
n/a | 60 min, 60 °C |
n/a | Icorr reduction from 1.24 mA/cm2 to 0.06 mA/cm2 for original phosphate coated to the nano-CeO2 modified coating. Ecorr shifted from −1.42 V to −1.30 V for original phosphate coated to the nano-CeO2 modified coating. Significant reduction in crack ratio and size on the nano-CeO2 modified coating compared to the original phosphate coating |
3.5% NaCl | AZ91D | [406] (2017) |
|
| Grinding to 2000 grit, degreased in KOH, rinsed in distilled water |
1 M H3PO4, 0.004–0.068 M Zn(NO3)2.6H2O, 0.042 M NaNO2, 0.021 M NaNO3, 0.024 M NaF, 0.034 M Na2HPO4.12H2O |
2.1–4 | 50 °C | Outer porous hopeite crystal and inner dense amorphous compound | Icorr reduction to 50 time lower value from bare to the treated sample at pH of 3.07 | 0.5 M NaCl | AZ31 | [175] (2013) |
||
| Grinding to 3000 grits, alkaline degreasing, acid pickling |
50 g/L Zn(H2PO4)2, 20 g/L NaH2PO4, 30 g/L 50% Mn(NO3)2, 5 g/L C6H8O7, 0.2 g/L C18H29NaO3S |
1.8–2.6 | 15 min, 45 °C |
Homogeneous and ordered crystals containing Zn3(PO4)2 and MnHPO4. Some cracks |
Ecorr shift from −1.571 V to −0.370 V for bare to coated substrate in phosphate solution of pH 2. Icorr reduction from 129 µA/cm2 to 5 µA/cm2 for bare to coated substrate in phosphate solution of pH 2. |
3.5% NaCl | Mg-8.5Li | [204] (2014) |
||
| Heat treatment of samples for 0–24 h at 400 °C, grinding to 2000 grits, cleaned with distilled water, degreased in KOH, rinsed in distilled water |
12.4 g/L H3PO4 (85 wt.%), 5 g/L Zn(NO3)2.6.H2O, 20 g/L NaH2PO4.12H2O, 3 g/L NaNO2, 1.84 g/L NaNO3, 1 g/L NaF |
3–3.2 | 50 °C | Inner layer of MgZn2(PO4)2 and Mg3(PO4)2. Outer layer of hopeite (Zn3(PO4)2·4H2O) | the sample with 24 h heat treatment withstood 24 h in immersion test, While the bare sample withstood only 3 h in immersion test | 0.5 M NaCl | AZ91 | [407] (2013) |
||
| Grinding to 1000 grits, rinsed with DI water, degrease in alcohol |
2 g/L ZnO, 12 g/L H3PO4, 1 g/L NaF, 4 g/L C4H6O6Na2, 6 g/L NaNO3, + 0.5 g/L of Basic bath TSPP or ATMP or EDTA |
n/a | 20 min, 45 °C |
n/a | Ecorr shift from −1.45 V to −1.40 V, −1.43 V and −1.45 V for basic phosphate solution to solution containing TSPP, ATMP and EDTA, respectively. Icorr reduction from 30 µA/cm2 to 8.5 µA/cm2, 10 µA/cm2and 28 µA/cm2 in the presence of TSPP, ATMP and EDTA, respectively. |
Salt-water test (SWI), 3.5% NaCl | AZ91 | [203] (2014) |
||
| Calcium-phosphate | Grinding to 2000 grits, degreased in absolute acetone, rinsed by DI water, dried under atmospheric condition |
40 g/L Ca(NO3)2, 40 mL/L H3PO4 then in the 5 g/L NaF solution, then surface modification in a 0.05 M ethanol of stearic acid solution |
Phosphating 2.8; Fluoride 12 |
Phosphating: 20 min, 37 °C ± 2 °C fluoride bath: 2h, 80 °C, 15 h stearic acid |
Micro-protrusions, submicro-lumps and nano-grains with diameter of about 1–2 µm. Ca3(PO4)2, Ca(H2PO4), Ca10(PO4)6F2, and MgF2 |
Icorr reduction from 129 µA/cm2 to 1.3 µA/cm2 for bare to the substrate coated with phosphate, fluoride and stearic acid. Ecorr shift from −1.54 V to −1.36 V for bare to the substrate coated with phosphate, fluoride and stearic acid. |
3.5% NaCl | Mg-5Zn-1.5Ca | [199] (2017) |
|
| Grinding to 2000 grit, cleaning in acetone. No pre-treatment such as alkaline degreasing or acid pickling. |
12 g/L Ca(NO3)2.4H2O, 1.2 g/L CaO, 8 mL/L H3PO4 (85% v/v) |
2.4–3.2 | 5 s–40 min, 15 °C, 37 °C, 60 °C |
Ca9Mg(HPO4)(PO4)6, MgHPO4.3H2O. Thickest coating at bath pH of 3.2 |
Lowest icorr 2.9 µA/cm2 obtained at pH 3.0 | SBF solution | AZ60 | [145] (2016) |
||
| Grinding to 2000 grits, cleaned in DI water and ethanol and then dried in open air |
0.05 M Ca(NO3)2.4H2O, 0.03 M NaH2PO4.2H2O |
n/a | 48 h, room T |
CaHPO4.2H2O, Ca2P2O7 (after heat treatment) with thickness of 30 µm |
Ecorr shifted from −1.666 V to −1.566 V and −1.515 V fore bare, Ca-P coated and Ca-P coated followed with heat treatment. Icorr reduced from 35 µA/cm2 to 3.5 µA/cm2 and 1 µA/cm2 for bare, Ca-P coated and Ca-P coated followed with heat treatment. |
Hank solution | ZK60 | [146] (2012) |
||
| Grinding to 300 grits, alkaline cleaning (NaOH, Na3PO4), acid pickling (CH3COOH+NaNO3) etching, HF activation, DI water |
2 g/L Ce(NO3)3 2 g/L La(NO3)3 2 g/L KMnO4 |
4.0 | 5 min 40 °C |
15 µm La2O3, CeO2, Mn2O3, and MnO2 homogeneous with microcracks |
Ecorr and icorr decreased from 1.58 V/0.13 mA/cm2 to 1.44V/0.031 mA/cm2 for bare to RE coated substrate, compared to 1.11 V/0.056 mA/cm2 for Cr(VI) CC Excellent adhesion to substrate |
3.5% NaCl | Mg–Li | [246] (2009) |
||
|
Rare-earth element Rare-earth element |
Grinding to 600 grits, polishing by 1-µm diamond paste, acetone, distilled water, degreasing with NaOH and Na3PO4 at 80 °C, DI water |
0.02 M Ce(NO3)3 5 g/L H2O2 (30 wt.%) |
4 | 15 min 25–55 °C |
MgO, Mg(OH)2, CeO2, and Ce2O3, The highest uniformity and compactness of coating observed at 35 °C |
Ecorr and icorr decreased from 1.543 V/0.25 mA/cm2 to 1.504 V/3 µA/cm2 for bare to RE coated substrate |
3.5% NaCl | AZ91 | [241] (2015) |
Commercially available, high corrosion resistance / expensive, unsatisfactory long term stability |
| Grinding to 2500 grits, polishing by 3.5-µm diamond paste, degreased in ethanol, acid pickling by HNO3 (0.8%) and then 40% HF, rinsed with distilled water and subsequent drying before each step |
10 g/L Y(NO3)3 | n/a | 30 °C | Y2O3, YOx/y, Al2O3, and MgO | Improvement in corrosion resistance was not so significant, however, the post-treatment with the silica sol coating reduced the corrosion current density by two orders of magnitude, Ecorr shifted positively about 140 mV, The corrosion current density decreased about two orders of magnitude |
3.5% NaCl | AZ91 | [219] (2017) |
||
| Grinding to 180 grits, cleaned with isopropyl, rinsed with DI water, dried in room temperature, etched in HNO3, alkaline cleaning in Na2SiO3.5H2O |
4 wt.% CeCl3.7H2O, 6.7 wt.% H2O2, 0.25 wt.% organic gelatin |
n/a | 5–180 s in CeCC solution, followed by 5min at 85 °C immersion in 2.5wt.% NaH2PO4 | Three-layer coating: nanocrystalline MgO, nanocrystalline CeCC and outer amorphous CeCC layer | Best corrosion behavior for the thinner CeCC (100 nm) | NSST | AZ31 | [236] (2016) |
||
| Grinding to 1200 grits, rinsed with DI water, degreases with acetone, acid pickling in 0.15M HCl or 0.46 M HF, rinsed in DI water, dried in stream of air |
0.05 M Ce(NO3)3.6H2O, 0.254 M H2O2 |
2.9 | 180 s, room T |
200 nm thickness on HCl pickled samples with chemical composition of Mg(OH)2, Al(OH)3. 300 nm thickness HF-pickled samples contained MgF2, as well. CC contained Mg/Al hydroxide and CeO2 |
Adhesion grade was 1B, 3B, and 5B for the cerium coating on the as-polished AZ31, the HCl-pickled AZ31, and the HF-pickled AZ31, respectively. Corroded area after 24 h of SST was >80% for the cerium-coated as-polished AZ31, 20~25% for the cerium-coated HCL-pickled AZ31, and <1% for the cerium-coated HF-pickled AZ31. |
SST, 3.5% NaCl for electrochemical tests, adhesion test according to ASTM D3359-02 |
AZ31 | [408] (2012) |
||
| Grinding to 800 grits, Rinsed with DI water, dried in a stream of hot air, |
0.05 M Al (NO3)3, 0.001–0.05 M Ce(NO3)3 |
n/a | 2 min, 15–20 °C |
6 µm compact coating with some observed micro-cracks, Al(OH)3, Al2O3, Mg(OH)2, MgO, Ce2O and Ce2O3 |
The most positive Ecorr at Ce(NO3)3 concentration of 0.005 M, which also exhibited the lowest icorr of value 0.022 mA/cm2 | 5 wt.% NaCl | AZ91 | [409] (2013) |
||
| Grinding to 1500 grits, cleaning in acetone, degreased with NaOH+Na3PO4 | 6 g/L La(NO3)3, 3 g/L Na2MoO4 |
4 | 25 °C | 5–6 µm, Cracked layer with “dry-mud” morphology |
Ecorr shifted 500 mV to more positive values with respect to bare substrate. two orders of magnitude in icorr |
3.5 wt.% NaCl | AZ31 | [229] (2010) |
||
| Grinding to 1000 grit, degreased in acetone, washed with triply distilled water | 5–50 mM Ce(NO3−)3·6H2O in purified N2 gas saturated atmosphere. 1–20 mM H2O2, 1–10 mM ascorbic acid |
n/a | 50 °C | CeO, CeO2, Ce2O3, MgO, Mg(OH)2 | Small positive shift of Ecorr and 4-time reduction of icorr by addition of ascorbic acid to the bath. | Ringer solution | AZ91 | [221] (2016) |
||
| Vanadate | Grinding to 2400 grits, ultrasonication in acetone, stream air drying |
NaVO3 30 g/L Vanadium oxide precipitation |
8 | 10 min/ 80 °C |
0.1–1.6 µm Vanadium oxides microcracks |
Ecorr and icorr decreased from −1.63 V /0.1 mA/cm2 to −1.37 V/0.56 µA/cm2 for bare to vanadate coated alloy |
0.1% NaCl | AZ61 | [254] 2007 |
High corrosion protection ability for a number of mg alloys / toxic if swallowed, suspected of damaging fertility, toxic to aquatic life with long lasting effects [258]. not industrially feasible |
| Grinding to 800 grits, acetone, air drying |
NaVO3 50 g/L Vanadium oxide precipitation |
10 min/ RT |
1.5–2.5 µm Vanadium oxides microcracks |
n/a | 3.5% NaCl | AZ31 | [253] 2011 |
|||
| Molybdate | Grinding to 300 grits, alkaline decreasing 40 g/L NaOH, 10 g/L Na3PO4·12H2O Acid pickling (200 mL/L CH3COOH 50g/L NaNO3) |
25 g/L Na2MoO4·12H2O 4 g/L NaF (optional SiO2 nanoparticles) |
3 | 10 min/ 66 °C |
12 µm Multiple microcracks SiO2 addition decreases the number of microcracks |
For MoO4− −1.04 V/16.1 µA/cm2 For MoO4−+SiO2 −0.81 V/3.6 µA/cm2 |
3.5% NaCl | AZ31 | [260] 2013 |
Moderate corrosion protection / limited commercial availability |
| Stannate | Grinding to 1500 grits, air drying, acid pickling and activation 0.25%HF + 0.25%HCl |
0.25 M Na2SnO3, 0.073 M CH3COONa, 0.13 M Na3PO4, 0.05 M NaOH. |
alkaline | 1 h/ 40 °C potentiostatic conditions |
0.6–1.8 µm deposit composed of MgSnO3·3H2O |
Ecorr decreased from −1.77 V to −1.55 V for bare to stannate treated alloy |
Borate buffer (0.15 M H3BO3 and 0.05 M Na2B4O7, pH 8.5) |
AZ91 | [266] 2007 |
Commercially available, environmentally acceptable / moderate corrosion protection, long time treatment, typically requires elevated temperature |
| Grinding to 2400 grits, polishing with 3 and 1um diamond paste, ultrasonication in acetone, stream hot air drying, activated in 11.25% HF |
30–60 g/L K2SnO3·3H2O, 10 g/L CH3COONa·3H2O, 50 g/L Na2P2O7 2.5–15 g/L NaOH Nucleation and growth of round particles |
12.6–13.2 | 2–60 min/ 60–90 °C |
Few microns thick round agglomerates of submicron particles with remaining discontinuity in surface coverage |
SST with rating numbers varying from 8 (bare AZ61) to 4 (stannate treated AZ61) | 5% NaCl SST |
AZ61 | [410] 2006 |
||
| Grinding to 1000 grits, cold air stream drying, acid pickling and activation HF, HCl, HNO3 |
50 g/L K2SnO3, 10 g/L CH3COONa, 50 g/L Na2P2O7 5 g/L KOH |
12.4 | 1–10 min/ 82 °C |
Round agglomerated submicron particles of MgSnO3·3H2O | Ecorr and icorr decreased from -1.60 V /12 µA/cm2 to –1.44 V/0.67 µA/cm2 for bare to stannate treated alloy |
0.05 M NaCl + 0.1 M Na2SO4 |
AZ91 | [86] 2011 |
||
| Grinding to 800 grits, acetone, air drying |
25 g/L K2SnO3·3H2O + NaOH |
12.9 | 30 min/ RT |
Corrosion rate decreased by 1/3–1/2 | 3.5% NaCl | AZ91 | [411] 2013 |
|||
| Fluoride | Untreated | 7–28 M HF | Highly acidic | 1–24 h/ RT |
Up to 2 µm Mg(OH)xF2−x |
Ecorr and icorr decreased from −1.473 V/0.11 mA/cm2 to −1.468 V/0.017 mA/cm2 for bare to HF treated alloy |
3.5% NaCl | AZ31 | [99] 2010 |
Commercially available / moderate corrosion protection HF is highly toxic, fatal if swallowed, in contact with skin or if inhaled [293] |
|
Fluoro-metallates
Zr, Ti or Zr/Ti fluorides |
Grinding to 1200 grits | 0.01 M TiCl4, 0.01 M H2SiF6 5 mL/L HNO3 |
4 by NaOH |
0.5–10 min/ 40 °C |
0.2–0.5 µm micro-cracks Mg(OH)2, MgF2, Si(OH)4, Ti(OH)4 |
Ecorr and icorr decreased from −1.55 V/9.9 µA/cm2 to −1.48 V/0.48 µA/cm2 for bare to treated alloy |
0.05 M NaCl 0.1 M Na2SO4; SST |
AZ31 | [125] 2012 |
Commercially available, excellent paint adhesion, good corrosion resistance, Single step process, operable at room temperature, Well-compatible with pre- and post-treatment / requires accurate process operation, active R&D topic |
| Grinding to 2000 grits, ultrasonication in acetone, hot air drying, 20% HF 20 h/RT |
0.2 M Zr(NO3)4: methanol:AcAc (molar 1:4:8) aged for 48 h, concluded by alloy dipping |
Withdrawal speed: 6 m/h | Micron and submicron pores and cracks | Ecorr and icorr decreased from −1.614 V/12.9 µA/cm2 to −1.516 V/0.53 µA/cm2 for bare to treated alloy |
3.5% NaCl | AZ91 | [289] 2008 |
|||
| Grinding to 2000 grits, degreasing in NaOH (40 g/L) +Na2SiO3 (40 g/L) |
(a) H2TiF6 0.5 g/L and H2ZrF6 1.5 g/L (b) H2ZrF6 1.5 g/L + tannic acid 1.5 g/L (c) H2TiF6 0.5 g/L + tannic acid 1.5 g/L |
2.5 by NaOH |
3 min/ 25–30 °C |
5–6 μm Micro-cracks MgF2, Mg(OH)2, MgO, TiO2, ZrO2, Ti, and Zr metal–organic complex |
icorr decreased from 93.72 μA/cm2 to 1.047 μA/cm2 |
3.5 wt.% NaCl | AZ91 | [306] 2015 |
||
| Grinding for SST: alkaline (NaOH, Na2CO3, Na3PO4, soap) and acidic (NH4F, H3PO4) treatment followed by air drying |
0.03–0.1 M Ce(NO3)3 0.03–0.1 M ZrO(NO3)2 0.02–0.05 M NbxOyFz |
4 by NH4F |
24 h /RT | CeO2, Ce2O3, ZrO2, Nb2O5, MgO, MgF2, composition did not change after anodic/cathodic polarization | Ecorr and icorr decreased from –2.07 V/626 µA/cm2 to –1.76 V/13 A/cm2 for bare to 24 h treated alloy |
0.5 M Na2SO4, or SST |
AZ91 | [296] 2008 |
||
|
Phytic acid
(phytate) |
Grinding to 1400 grits, ultrasonication in acetone for 10 min | Phytic acid/0.5 g/L | 5 | 20 min/RT | 4–5 µm, Mg/Al phytate microcrackes |
Ecorr shifted from –1.906 V to –1.735 V icorr decreased from 429.4 to 373.0 mA/cm2 for bare to PA coated substrate excellent adhesion to substrate and epoxy coating |
5% NaCl | AZ61 | [318] |
Excellent adhesion to substrate and epoxy coating, environmentally benign / limited corrosion protection, relatively expensive, commercially unavailable |
| Grinding to 1200 grits, DI water | Phytic acid/0.5%/chemisorption | 5 | 10–30 min/ 29 °C |
14–20 µm magnesium phytate (Mg12−x HxPhy) |
icorr 37 µA/cm2 excellent adhesion |
Phosphate buffer solution. pH 7.4 37 °C |
AZ31 | [324] | ||
| Grinding to 1500 grits, alkaline degreasing and acid pickling | Phytic acid/ 20 g/L/ deposition |
6 | 10 min/ 35 °C |
7 µm Mg/Al phytate, macroscopic: smooth gray microscopic: flower-like cracked deposits |
Ecorr shifted from −1.645 V to −0.905 V icorr decreased from 1.1 mA/cm2 to 2.3 µA/cm2 for bare to PA coated substrate excellent adhesion |
3.5% NaCl | Mg-Li alloy Mg - 11 wt.% Li, 3 wt.% Al, 0.5 wt.% RE |
[322] | ||
| Grinding to 2000 grits, alkaline degreasing and acid pickling |
Phytic acid / 20 g/L/ deposition |
9–10 | 0.5–3 min 25 ± 5 °C, Then hot air drying |
transparent, microcracks |
Ecorr shifted from −1.46 V to –1.31 V | 0.05 M NaCl | AZ31 | [412] | ||
| Grinding to 2000 grits, washing in acetone and DI water, hot air drying |
Phytic acid/ 5 g/L/ deposition |
8 | 20 min RT |
0.34 µm Integrated and uniform |
icorr 6 orders of magnitude lower than bare alloy | RT 3.5% NaCl |
AZ91 | [323] | ||
| Grinding to 1200 grits, washing in acetone |
Phytic acid/ 50% heat post-treatment improves corrosion resistance |
n/a | 3 h RT |
2.2 µm | Ecorr shifted from −1.64 V to –1.50 V icorr decreased from 24 µA/cm2 to 1.2 µA/cm2 for bare to PA coated substrate excellent adhesion |
Phosphate buffer solution | Pure Mg | [413] | ||
| Grinding to 4000 grits, washing in acetone, ethanol and DI water, followed by alkaline degreasing and acid pickling, followed by acetone, alcohol and DI water and hot air drying. Then 3M NaOH for 12 h at 60 °C, washed in DI water and dried in vacuum oven | Phytic acid/ 5 g/L covalent immobilization |
5 | 20 min 60 °C |
n/a | Ecorr shifted from −1.44 V to –1.45 V icorr decreased from 0.27 mA/cm2 to 0.14 mA/cm2 for bare to PA coated substrate excellent adhesion |
Phosphate buffer solution 37 °C |
Pure Mg | [60] | ||
| Grinding to 2000 grits, ultrasonic treatment in ethanol, dried by warm air | Slurry prepared at 55 °C for 48 h 0.06 M Mg(NO3)2·6H2O, 0.03 M Al(NO3)3·9H2O +0.06 M Na2MoO4 and 0.2 M NaOH |
alkaline | AZ31 sample was kept in slurry for 36 h 100 °C in autoclave |
17 µm Typical LDH flakes, MgAl-LDH, (Mg6Al2(OH) 16MoO4·4H2O) |
Ecorr shifted from −1.54 V to –1.21 V icorr decreased from 31.7 µA/cm2 to 0.16 µA/cm2 for bare to LDH coated alloy |
3.5% NaCl | AZ31 | [384] 2014 |
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| LDH | Grinding to 2000 grits, ultrasonic treatment in ethanol, dried in air stream | Slurry prepared at 40 °C for 48 h + 12 h [Mg(NO3)2 Al(NO3)3 at molar ratio 3:1 + Na2CO3/NaOH |
alkaline | AZ31 sample was kept in slurry for 24–48 h 100 °C in autoclave |
7 µm typical LDH flakes, MgAl-LDH, (Mg6Al2(OH)16CO3·4H2O) |
Ecorr shifted from –1.56 V to –1.18 V icorr decreased from 30.4 µA/cm2 to 0.07 µA/cm2 for bare to LDH coated alloy |
3.5% NaCl | AZ31 | [377] 2014 |
High corrosion protective ability, environmentally benign, can be loaded with corrosion inhibitors for active corrosion protection, can be grown at RT and ambient pressure / at the early development stage, relatively expensive, active R&D topic |
| Grinding to 5000 grits, PEO treatment, Ultrasonic treatment in ethanol, dried in air stream | 0.1 M NaNO3 | 8 by NaOH |
12 h 100 °C in autoclave |
8 µm typical LDH flakes, LDH-MgAl-NO3 or LDH-MgAl-VO3 |
Ecorr shifted from –0.74 V to –0.47 V icorr decreased from 3.9 µA/cm2 to 0.95 µA/cm2 for PEO treated to PEO-LDH-NO3 coated alloy |
3.5% NaCl | AZ31 | [369] 2017 |
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| Grinding to 5000 grits, PEO treatment, Ultrasonic treatment in ethanol, dried in air stream | 0.05 M Al(NO3)3, 0.3 M NH4NO3 | 8.72–12.04 | 12 h 100 °C in autoclave |
Typical LDH flakes, MgAl-LDH, Mg(OH)2 |
Ecorr shifted from −1.51 V to −1.34 V, icorr shifted from 32.68 to 0.118 μA/cm2 for bath pH 8.72 to 11.72 |
3.5% NaCl | AZ31 | [370] 2017 |
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| Grinding to 1200 grits, DI water, dried in air | Al(NO3)3 EDTA, NTA |
8–12 | 15 min to 6 h at 95 °C and 48 h at 25 °C ambient pressure |
20–60 nm typical LDH flakes, MgAl-LDH |
n/a | n/a | AZ91 |
[385] 2018 |
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| PEO-treated AZ91 PEO electrolyte 1 g/L KOH, 8 g/L Na3PO4 and 12 g/L NaAlO2 |
0.05 M Al(NO3)3, 0.5 M NaNO3, 0.5 g of AZ91 flakes 0.05 M DTPA 0.003 M salicylate-Na |
10.0 | 0.5 to 8 h at 70 or 95 °C, ambient pressure | Typical LDH flakes were grown on top of PEO and inside PEO pores | n/a | n/a | AZ91 |
[373] 2020 |