Es besteht eine wachsende Nachfrage nach Komponenten aus Leichtmetalllegierungen in der Luft- und Raumfahrt und im Automobilbereich, vor allem um Treibstoffkosten zu sparen. Magnesiumlegierungen versprechen als Leichtbauwerkstoffe ein großes Potenzial für verschiedene Anwendungen. Dieser Vorteil ergibt sich aus ihren niedrigen Dichten und ihrem hohen spezifischen Festigkeits-/Gewichtsverhältnis. Weitere Vorteile sind gute elektrische und thermische Leitfähigkeit, gute Schlagzähigkeit, Fähigkeit zur Dämpfung von Stoßwellen, leichte Umformbarkeit bei Raumtemperatur, Schweißbarkeit, Knickfestigkeit, Duktilität und Druckdichtigkeit. Obwohl Magnesium wie das Traummetall der Konstrukteure klingt, hat es zwei gravierende Nachteile, die seine weit verbreitete Anwendung einschränken: schlechte Korrosionsbeständigkeit und die relativ schlechten mechanischen Oberflächeneigenschaften. Aufgrund der außergewöhnlichen technischen Eigenschaften von Magnesiumlegierungen wurden in den letzten Jahrzehnten zahlreiche Versuche unternommen, geeignete Oberflächenschutztechniken zu entwickeln. In diesem Artikel werden die Fortschritte der chemischen Umwandlungsschichten auf den Magnesiumlegierungen diskutiert.
3.1.4 Chrome-manganese Conversion Coating
Chrome-manganese conversion coating is a modified chromate coating process which provides the moderate corrosion resistance by itself without application of subsequent paint. The coating is suitable for the treatment of all types of alloys. It provides dark brown to black coloured film and is used when good protection is required with negligible dimensional change.
The treatment is usually carried out after solvent degreasing, alkaline cleaning and acid pickling. Instead of acid pickling, a mild acid cleaning in chromic acid is recommended for components of close dimension tolerance. The following bath formulation is used for chrome-manganese conversion coating:
|Sodium or Potassium dichromate, Na2Cr2O7 · 2H2O or K2Cr2O7||80–100 g/L|
|Manganous sulphate, MnSO4 · H2O||40–50 g/L|
|Magnesium sulphate, MgSO4 · 7H2O||40–50 g/L|
|Temperature||Room temperature to boiling|
|Tank material||SS, PP|
|Time||15 minutes to 2 hours as shown below depending on the operating bath temperatures|
|Room temperature||2 hours|
|50–60 °C||45 minutes|
|60–80 °C||30 minutes|
|Coating thickness||7–11 µm|
Film microhardness improves when baths are operated at higher temperatures. As the solution is used, its pH is increased from 4.0 to 5.0. The solution has then to be replenished with the addition of 50 g/L of manganese sulphate and the pH is adjusted back to 4.0 with the addition of sulphuric and chromic acid in equal quantities. Addition of 1 ml/L of the following mixture, reduces the pH of bath by 0.1.
|Chromium trioxide, CrO3||250 g/L|
|Sulphuric acid, H2SO4, 98 % (SG 1.84)||250 g/L|
A process of black chromate conversion coating on magnesium-lithium (LA141A and MLA 9, Al 1.25 %, Li 10 %, by weight, balance Mg) [74, 75] and magnesium-aluminium alloys (such as AZ91, AZ80, and AM60 ) was described by Sharma. The process involves the following sequence:
- Ultrasonic solvent degreasing in isopropanol for 3–5 minutes
- Alkaline cleaning in sodium hydroxide 55 g/L, trisodium orthophosphate 10 g/L and potassium fluoride 1 g/L operating at 85–90 °C for 5–6 minutes, followed by water rinsing
- Acid cleaning in 50 % chromic acid for 4–7 minutes for Mg-Li alloys. For Mg-Al alloys a solution of chromic acid 120 g/L and nitric 70 % 110 ml/L for 30–60 seconds was used
- Chemical brightening in a solution containing 8–10 % orthophosphoric acid (V/V) in isopropanol for 3–5 minutes, followed by rinse in isopropanol, dip in hot water and dry for Mg-Li alloys. For Mg-Al alloys a fluoride activation in 40 % HF 400 ml/L for 1 minute was carried out
- Chromate conversion coating
|Potassium dichromate, K2Cr2O7||90 g/L|
|Manganous sulphate, MnSO4 · H2O||40 g/L|
|Magnesium sulphate, MgSO4 · 7H2O||40 g/L|
|Potassium fluoride, KF||1–2 g/L|
|Time||2–3 hours depending on the bath temperature, followed by water rinsing|
|Tank material SS 304 with immersion heaters Coating thickness||8–11 µm|
- Sealing for magnesium-lithium alloys only in a solution containing 2 % potassium dichromate, K2Cr2O7 and 10 % ammonium bifluoride, (NH4)HF2 at room temperature for 3 minutes, hold the job over an open vessel of hot water (70–75 °C) for 2–3 hours
- Heat treatment by placing the job over glass or aluminium plate in an oven operating at 70–75 °C for 1–2 hours for magnesium-aluminium alloys and for 6 hours for magnesium-lithium alloys
- Rinse in hot water for 2–3 minutes, polish gently with soft cloth to remove the boom, dip in hot isopropanol and dry.
The black chromate film described herein provided good microhardness of the order of 130 VHN after heat treatment. A high solar absorptance (0.91) and infrared emittance (0.90) makes these coatings suitable for spacecraft thermal control application. The coatings were exposed to relative humidity of 95 ± 5 % at 50 ± 1 °C for 48 hours to evaluate their corrosion resistance for pre-launch conditions of spacecraft. To evaluate the effect of post-launch space condition a thermal cycling test was performed. The test specimens were subjected to 1000 cycles of –45 °C to + 80 °C. A cycle consists of lowering the temperature to –45 °C, a dwell of five minutes and raising the temperature to 80 °C with a dwell of five minutes. The first six cycles were of 1 hour each to evaluate the thermal soak and the rest cycles were of 5 minutes duration (thermal shock). The coatings have passed these tests without any visual degradation and no change in their optical properties was reported after the test.
3.1.5 Phosphate Treatment
Phosphating is the widely used metal pre-treatment process. Due to its economy, ease of operation and ability to afford moderate corrosion resistance, wear resistance, adhesion and lubricative properties, it plays a significant role in the automobile, process and appliance industries. To keep pace with the rapid changing need of the finishing systems, many modifications have been put forth in their development – both in the processing sequence as well as in the phosphating formulations
This phosphating treatment on magnesium alloys has been used primarily for brush or spray touch-up of small areas of previously treated surfaces that have been damaged prior to painting. It has replaced the chrome-pickle as a repair treatment primarily because of better reproducibility under production shop condition and the fact is that it is less corrosive if entrapped between faying surfaces or in pocketed areas of any assembly. A solution of following composition is used:
|Ammonium acid phosphate, (NH4)2H2PO4||40 g/L|
|Ammonium sulphite, (NH4)2SO3 · H2O||2–7 g/L|
|Denatured ethanol||75–100 ml/L|
|Temperature||20–30 °C (room temperature)|
|Tank material||SS, PP or PE|
The solution is brushed or sprayed for 1 minute or until a continuous grey coating is formed. Alternatively, parts may be immersed in the solution for 1–2 minutes or until gassing stops. The treatment is followed by cold water rinsing, hot water should not be used.
Zhang et al.  described a chrome-free conversion coating process on magnesium-lithium alloy from a phosphate–permanganate solution. The following bath formulation and operating conditions were used:
|Potassium permanganate, KMnO4||40 g/L|
|Potassium dihydrogen phosphate, KH2PO4||50 g/L|
|Temperature||50 ± 5 °C|
The morphology, the composition and the corrosion resistance of this coating were examined. A thin and non-penetrating cracked morphology was reported. The main elements of the conversion coating were Mg, O, K, P and Mn. The results of the electrochemical measurements and the immersion tests demonstrated that the corrosion resistance of the magnesium-lithium alloy has been improved by the phosphate–permanganate conversion treatment.
3.1.6 Galvanic Black Anodizing
Galvanic black anodising on magnesium alloys is an improved version of chromate conversion coating. Processes of galvanic black anodising were investigated in details by Sharma et al. [78–84] on magnesium alloys AZ31B, ZM21 and magnesium lithium alloys. The studies were primarily carried out for the suitability of these coatings for thermal control of spacecraft. The testing and evaluation of galvanic coatings found them suitable for ground as well as space applications. Following sequence of operations was used for different magnesium alloys:
- Solvent degreasing in isopropyl alcohol for 5–10 minutes
- Alkaline cleaning in a solution containing sodium hydroxide 50 g/L and trisodium orthophosphate 10 g/L; operating at 60 ± 5°C for 5–10 minutes, followed by water rinsing
- Acid pickling in a solution containing:
For AZ31B alloy: Chromium trioxide 180 g/L, ferric nitrate 40 g/L and potassium fluoride 3.75 g/L for 2 minutes, followed by water rinsing.
For ZK 60A alloy: (a) 50 % Chromium trioxide for 1–2 minutes, water rinsing. (b) Chemical brightening in a solution containing chromium trioxide 180 g/L, ferric nitrate 40 g/L and potassium fluoride 3.75 g/L for 10–15 seconds, followed by water rinsing. This acid pickling process provides effective cleaning without significant attack on base metal and hence is recommended for close dimensional tolerance components.
For Mg-Li alloys: (a) Chromium trioxide 500 g/L, ferric nitrate 1 g/L and potassium fluoride 0.5–1.0 g/L for 3–5 minutes with a post treatment of water rinse. (b) Fluoride activation by dipping in 40 % hydrofluoric acid (50 ml/L) for 10 minutes, followed by water rinsing.
- Black anodizing in the following solution and operating conditions:
|Potassium dichromate, K2Cr2O7||25 g/L|
|Ammonium sulphate, (NH4)2SO4||25 g/L|
|pH||5.5, (5.8 for ZM21)|
|Temperature||25 ± 2 °C (room temperature)|
|Cathode||anodizing tank (SS)|
|Anode to cathode area ratio||1:5 to 1:10|
|Galvanic current/voltage||0.8–2.4 mAcm–2/ 1.2–3.6 mVcm–2|
|Coating thickness||12–15 µm|
|Post treatment, water rinse|
- Heat treatment at 70 °C for 2 hours in an electric oven with clean hot air circulation.
The process can be represented by the following equations.
Electrolytic reaction (ionisation):
K2Cr2O7 → 2K+ + Cr2O72–
(NH4)2SO4 → 2NH4+ + SO42–
H2O → 2H+ + OH–
Mg0 → Mg2+ + 2e–
Mg2+ + Cr2O72– + 6OH–/3SO42– → 2MgCrO4 + 2Cr(OH)3 + Cr2(SO4)3 + 3O2 + 6e–
K+ + OH– → KOH
(NH4+ + 4OH– → NH4OH
2H+ + 2e– → H2↑
The word galvanic implies that the process does not require any current from outside for electrolysis. When the job is immersed in the anodising solution and is connected to the stainless-steel anodising tank, due to the dissolution of the outer surface of the alloy, a flow of current starts from job (anode) to tank (cathode), which sustains the reaction. Thus, the galvanic coating is the product of a controlled reduction-oxidation process.
The process is very simple and economical. Bath operates at the room temperature and no external current is required. The deposits were characterized by optical and scanning electron microscopy (SEM), adhesion tests, corrosion studies, thickness measurement and microhardness evaluation. The space worthiness of the coatings was evaluated by the humidity, the thermal cycling and the thermovacuum tests and measurement of optical properties.
The microstructure of the galvanic black anodized film was examined using optical and scanning electron microscopy. Coatings of this type are usually considered to have a gel-like structure as formed, but after drying, they harden and crack to give a microcracked 'mud-crack’ pattern. After heat treatment these microcracks became wider and edge curling of the blocks of the coating occurred. This probably resulted due to the dehydration and shrinkage of the film. The term micro-crack refers to a crack that does not extend from the base metal to the surface of the deposit. The hardness of the anodic coating was found to increase after sealing and by post deposition heat treatment (~ 100 VHN).
The influence of various operating parameters, namely galvanic current density, voltage, pH, electrolyte concentration, operating temperature, anodizing time and heat treatment on the anodic film formation have been investigated to optimize the process.
The process yielded satisfactory coatings in a wide range of electrolyte pH (4.5–6.5) and concentration (20–30 g/L of each constituent). Further, as these coatings provided high solar absorptance and thermal emittance (> 0.90) values, these are extremely suitable for thermal control application.
Most of the chemical treatments described herein are based on chromate or phosphate conversion process. The coatings are formed because the metal surface dissolve to a small extent, causing a raise pH value at metal surface-solution interface . A thin film of metal chromate or phosphate gets precipitated. The film is soft and gelatinous when freshly formed, once dried it slowly attains enough hardness. The freshly treated works should therefore be handled very carefully.
The chromate or phosphate coating offers protection to metallic substrate through two mechanisms: (i) the chromate or phosphate film provides a non-reactive barrier to humidity and air, thus retarding the corrosion; (ii) the chromate or phosphate film retains a water absorbing characteristic as long as it remains in hydrated form. When scratched or mechanically damaged enough water is absorbed by the film to swell and mend at the damaged areas. While the heating below 75 °C is benefited in hastening the hardening process, prolong heating above these temperatures may result in excessive dehydration of film that may result in reduction of its protection value. The films after painting can withstand up to 85 °C, because the impervious paint layer seal the water of hydration in the film.
3.1.7 Black Molybdate Conversion Coating
Molybdates (MoO42–) have long been utilised as one of the green corrosion inhibitors and are analogous to chromates (CrO42–), molybdenum and chromium being from the same group (VI A) of the periodic table. A novel galvanic conversion coating was studied on magnesium alloy AZ31B by Parida et al.  for spacecraft thermal control application. The authors have used an electrolyte system consisting of ammonium molybdate and magnesium chloride. The coatings were characterised by energy dispersive X-ray, infrared spectral and scanning electron microscopy studies. The thermoanalytical investigations have been carried out using thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The space worthiness of the coating was evaluated by environmental tests, namely, humidity, thermal cycling, thermovacuum performance and thermal stability tests. Optical properties (solar absorptance and infrared emittance) were measured before and after each environmental test to ascertain the stability of the film.
The following sequence of operations was adopted for black molybdate coatings:
- Mechanical cleaning with 600 grade silicon carbide paper
- Solvent degreasing in isopropanol for 5–10 minutes
- Alkaline cleaning in a solution of sodium hydroxide 50 g/L and trisodium orthophosphate 10 g/L at 65 ± 5 °C for 5–6 minutes, followed by water rinsing
- Acid cleaning in chromic acid 180 g/L and ferric nitrate 40 g/L at 25 ± 5 °C for 5–10 minutes, water rinsing
- Molybdate conversion coating in a following bath:
|Ammonium molybdate,(NH4)6MO7O24 · 4H2O||20 g/L|
|Magnesium chloride, MgCl2 · 6H2O||1 g/L|
|Temperature||25±5 °C (room temperature)|
|Cathode||SS 304 anodizing tank|
|Galvanic current/voltage||0.8–2.4 mAcm–2 / 1.3–3.8 mVcm–2|
|Coating thickness||12–15 µm|
|Post treatment water rinse|
Jobs are connected to the SS 304 container to form a galvanic cell. The elemental composition, surface morphology, environmental and thermal stability of coatings were studied. Energy dispersive X-ray (EDX) and infrared spectral studies reveal that these coatings consist of hydroxides and oxides of molybdenum and water of hydration. Scanning electron microscopy studies revealed the river bed type appearance of the coating’s morphology. Thermal stability studies revealed two major changes that occur when black molybdate coatings are heated: (i) dehydration started at ~50 °C and continued until ~250 °C, and (ii) decomposition of molybdenum hydroxides thereafter. The black molybdate conversion coatings provide high emittance as well as high absorptance (~0.90), and excellent environment stability for stringent space conditions.
3.1.8 Stannate Treatment
Stannate conversion coatings have been engineered as a promising chrome-free alternative for the corrosion protection of magnesium alloys. The stannate coatings are based on a simple immersion of job in a diluted stannate solution. Stannate conversion coatings act as a barrier for corrosive ions and oxygen. Such coating decreases the corrosion rates of magnesium alloys and displays noticeable self-healing characteristics. A top coating such as epoxy, polyurethane, fluoropolymer, or polyaniline might be a candid choice.
Stannate coatings on Mg and its alloys are formed by initial dissolution of substrate Mg followed by deposition of coatings after concentrations of Mg2+ and stannate ions reach critical concentrations at the substrate/electrolyte interface. It was also found that the coatings are mainly composed of hydrated magnesium stannate particles, MgSnO3 · 3H2O. The process is suitable to all magnesium alloys, even those which contain dissimilar metal fasteners or inserts (other than aluminium). The coating provides adequate corrosion resistance and an excellent paint base. The coating is especially useful where electrical continuity, RF grounding, etc. are required, since it has low electrical resistance. The preferred coating sequence involves, solvent degreasing, alkaline soak cleaning, hydrofluoric acid pickling, stannate immersion treatment, neutralizing (if parts are to be painted) and suitable rinsing steps. The bath composition and operational limits of the stannate treatment are as follows:
|Sodium hydroxide, NaOH||10–12 g/L|
|Potassium stannate, K2SnO3 · 3H2O||40–50 g/L|
|Sodium acetate, CH3COONa · 3H2O||10–25 g/L|
|Tetra sodium pyrophosphate, Na4P2O7||40–50 g/L|
|Tank material||polypropylene (PP)|
The bath is prepared by first dissolving the sodium hydroxide in water followed by potassium stannate and then sodium acetate. The solution is then heated to 60–65 °C before dissolving the pyrophosphate using good agitation.
The solution can be sprayed, brushed or the work can be immersed with a contact time of 30 seconds to 2 minutes. A coating thickness of 4–5 µm is obtained on magnesium jobs by this process and a slightly heavier coating on steel surfaces. Approximately 8–10 square feet area may be treated per litter of solution before replenishment of chemicals. If parts are to be painted, they should be neutralized with a 50 g/L sodium acid fluoride solution. Furthermore, use of wash primer is mandatory while painting over stannate film.
3.1.9 Cerium Oxide-Hydroxide Coating
Rare earth elements based conversion coatings are actively researched as an environmentally benign alternative to conventionally used chromate based conversion coatings for magnesium alloys.
As discussed earlier the magnesium and its alloys are biodegradable and biocompatible materials but have poor corrosion resistance in physiological environments limiting their applications in the biomedical field. To improve the corrosion resistance, some environmentally friendly chemical treatments have been developed on magnesium alloys. Advances in coatings on biodegradable magnesium alloys have been recently reviewed by Yin et al. . Among various processes, the rare earth elements-based coatings, the cerium oxide-hydroxide system appear to be promising [87, 88]. Several researches demonstrated that treatments with cerium salts solutions inhibit the metal corrosion. The formation of cerium oxides and hydroxides on the metal surfaces is generally the reason of this inhibition process because it gives rise to a blocking effect and reduces the rate of reduction reactions [89, 90]. It is known that magnesium alloys oxidation is accompanied with the reduction of hydrogen ions as cathodic reaction. This hydrogen discharge promotes the reaction of Ce3+ and Ce4+ species with OH– to form insoluble salts of Ce(OH)3 and Ce(OH)4 due to the increase of the pH in the interface between the substrate and the electrolyte solution [91, 92]. On the other hand, it has been reported that dissolved oxygen can promote the oxidation of Ce3+ to Ce4+ species.
It has been demonstrated that the cerium precipitation reaction is accelerated in the presence of oxygen when the pH solution is in the proper range . Further, the presence of oxygen in the cerium solution promotes the anodic formation of CeO2 which is better for the formation of compact ceria films . The addition of hydrogen peroxide in the cerium conversion solution promotes the formation of a cerium hydroxide/oxide coating containing mainly Ce(IV) species which are associated with higher degrees of protection. Hydrogen peroxide acts as oxidant agent and when it is added to the conversion solution, Ce3+ ions oxidize to Ce4+. Several studies expose a model of the mechanism by which the cerium-based coatings are formed in the presence of H2O2 [95–98]. The deposition reaction involves both oxidation of cerium and reduction of H2O2. During the process, the increased pH value of the cerium salt solution led to the precipitation of cerium hydroxides/oxides. The chemical reactions can be represented by the following equations:
H2O2 + 2e– → 2OH–
Ce3+ + OH– + ½H2O2 → Ce(OH)22+
Ce(OH)22+ + 2OH– → Ce(OH)4
Ce(OH)4 → CeO2 + 2H2O
The following sequence of processes is commonly adopted for obtaining the cerium oxide-hydroxide coatings on magnesium and its alloys:
- Solvent degreasing in isopropyl alcohol using an ultrasonic bath for 10 minutes at room temperature
- Alkaline cleaning in a solution containing 50 g/L sodium hydroxide and 10 g/L tri sodium orthophosphate for 10 minutes at 55 ± 5 °C followed by water rinsing
- Acid cleaning in a solution containing chromic acid 180 g/L, ferric nitrate 40 g/L, and potassium fluoride 4.5 g/L for 2–3 minutes at room temperature followed by water rinsing
- Cerium oxide coatings are then obtained from a solution containing 5 g/L cerium sulphate and 40 ml/L hydrogen peroxide at room temperature, pH 2.0 for 3-4 minutes. In place of cerium sulphate other cerium (III) salts, like, chloride, nitrate, phosphate can also be used.
Conversion coatings with many other rare-earth elements (La, Pr, Nd, Sm, Gd, Y, etc.) have also been investigated. A comprehensive review on these coatings has been recently published by Saji .
3.1.10 Carbonate Conversion Film
A carbonate-containing conversion film was obtained on the surface of magnesium AZ31 alloy by Nam et al  and its corrosion resistance was evaluated. The coating was deposited by immersing the job in a CO2 saturated NaOH solution for 1–60 minutes. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were utilized to investigate the morphology, crystallinity, and composition of the surface, respectively. The corrosion resistance was investigated by using electrochemical analysis in a 5 wt% NaCl solution. The formed film consists of a carbonate containing magnesium complex having the chemical formula [Mg5(CO3)4(OH)2 · 5H2O]. This composition is strongly influenced by the immersion time, the carbonate contents on the surface of the samples gets increased with increasing the immersion treatment time. Processing the job for about 10 minutes immersion time provides the optimum results for corrosion resistance.
A non-immersion, novel ionothermal strategy for developing a chemical conversion film was reported by Gu et.al . This coating was obtained on magnesium AZ31B alloy by interaction with a deep eutectic solvent (DES). Upon heating the choline chloride/urea mixture-based DES reacted with the magnesium alloy surface to form a promising anti-corrosion conversion film. The chemical conversion film is mainly composed by MgH2 and MgCO3 phases. The as-grown conversion film exhibited a super hydrophobicity after surface modification in a stearic acid/ethanol solution. Electrochemical tests and shown the improvement.
 A.K. Sharma: Metal Finishing, 87, No. 2(1989) 73–74
 A.K. Sharma: A process of chromate coating on magnesium-lithium alloys, Indian Patent No. 170666 (1988)
 A.K. Sharma: A process of flat absorber black chromate conversion coating on Mg-Al alloys, Indian Patent No. 180666 (1992)
 H. Zhang; G. Yao; S. Wang; Y. Liu; H. Luo: Surf. Coat. Technol, 202(2008) 1825–1830
 A.K. Sharma; R. Uma Rani; H. Bhojaraj; H. Narayanamurthy: J. Appl. Electrochem, 23(1993), 500–507
 A. K. Sharma: Metal Finishing, 91, No. 6(1993) 57–63
 A.K. Sharma; R. Uma Rani; A. Malek; K.S.N. Acharya; M. Muddu; S. Kumar: Metal Finishing, 94, No.4(1996) 16–27
 A.K. Sharma: J. Spacecraft Technology, 7, No. 1(1997) 49–57
 A.K. Sharma; R. Uma Rani; S.M. Mayanna: Thermochimica Acta, 376(2001) 67–75
 A.K. Sharma: A process of integral black anodizing on magnesium alloys Indian Patent No. 179637 (1991)
 A.K. Sharma: A process of galvanic black anodizing on magnesium-lithium alloy substrates, Indian Patent No. 179671, (1991)
 J. Kinndle: Chromate conversion coatings, American Electroplaters Society, Inc, FL (1986)
 B. Parida; R. Uma Rani; A.K. Sharma: Surf. Eng, 26, No. 5(2010) 361–366
 Z.-Z. Yin; W.-C. Qi; R.-C. Zeng; X.-B. Chen; C.-D. Gu; S.-K. Guan; Y.-F. Zheng: J. Magnesium Alloys, 8, No. 1(2020) 42–65, https://doi.org/10.1016/j.jma.2019.09.008
 X. Cui; Y. Yang; E. Liu; G. Jin; J. Zhong; Q. Li: Appl. Surf. Sci, 257, No. 23(2011) 9703–9709
 M.F. Montemor; A.M. Simoes; M.G.S. Ferreira; M. J. Carmezim: Appl. Surf. Sci, 254, No. 6(2008) 1806–1814
 M. Dabalà; K. Brunellia; E. Napolitanib; M. Magrinia: Surf. Coat. Technol, 172, No. 2–3(2003) 227–232
 K.A. Yasakau; M.L. Zheludkevich; S.V. Lamaka; M.G.S. Ferreira: J. Phys. Chem. B, 110(2006) 5515–5528
 C. Castano; M. O’Keefe; W. Fahrenholtz: Cerium-based oxide coatings, Curr. Opin. Solid State Mater. Sci, 19(2015) 69–76
 P. Yu; S.A. Hayes; T.J. O’Keefe; M.J. O’Keefe; J.O. Stoffe: J. Electrochem. Soc, 153, No. 1(2006) C74–C79
 Y. Yang; Y. Yang; X. Du; Y. Chen; Z. Zhang; J. Zhang: Appl. Surf. Sci, 305(2014) 330–336
 M.F. Scholes; C. Soste; A.E. Hughes; S.G. Hardin; P.R. Curtis: Appl. Surf. Sci, 253, No. 4(2006) 1770–1780
 B. Valdez; S. Kiyota; M. Stoytcheva; R. Zlatev; J.M. Bastidas: Corros. Sci, 87(2014) 141–149
 M. Eslamia; M. Fedela; G. Speranzab; F. Defloriana; N. Anderssonc; C. Zanellac: Electrochim. Acta, 255(2017) 449–462
 S. Chen; S. Zhang; X. Ren; S. Xu; L. Yin: Int. J. Electrochem. Sci, 10(2015) 9073–9088
 V.S. Saji: J. Mater. Res. Technol, 8, No. 5(2019) 5012–5035
 D. Nam; D. Lim; S.-D. Kim; D. Seo; S.E. Shim; S.-H. Baeck: J. Alloys Compd, 737(2018) 597–602
 C.D. Gu; W. Yan; J.L. Zhang; J.P. Tu: Corr. Sci, 106(2016) 108–116
 A.K. Sharma: Thin Solid Films, 208(1992) 48–54
 A.K. Sharma; R. Uma Rani; K. Giri: Met. Finish, 95, No. 3(1997) 43–51