Magnesium Finishing – Part 8 – Anodizing

Anodizing is an electrolytic oxidation process to produce protective film on the metallic job, which acts as anode in the electrolytic cell. It is one of the most widely used surface treatments for Al, Mg, Ti and their alloys. The surface of metal gets converted into metal oxide film to improve one or more surface properties, chemical, mechanical, electrical or optical.

Anodic films can be thin transparent that add interference effects to reflected light or thick porous coatings that can absorb dyes for decorative and functional application. Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors.

Fig. 1: Schematic anodizing set-up

The anodizing of magnesium alloys provides much superior corrosion and abrasion resistance than chemical immersion coatings. Anodized surfaces also serve as an improved base for subsequent paints or organic dyestuffs than most of the immersion treatments. Many of the electrochemical treatments, which failed to offer any major benefits over the chemical immersion processes have now become obsolete. The currently available anodic treatments on magnesium alloys are based on caustic, modified acid fluoride or mild alkaline electrolytes [1]. The properties of anodic film depend on several factors, such as the nature of the substrate, applied voltage, electrolyte formulation and temperature. The process can be controlled either by voltage or current. A schematic set-up of an anodizing process is illustrated in Figure 1.

Aluminium versus magnesium anodizing

Magnesium alloys can be anodized but the film does not deliver the same level of protection as anodic film on aluminium. This is because the degree of coverage for MgO is 0.81 (i.e., the Pilling-Bedworth ratio is less than 1) [2]. The anodic oxide coating formed on magnesium is very rough, and highly porous with a very coarse surface texture. The geometric mismatching of structure of magnesium alloy and oxide layer results in compressive strain leading to cracking of layer. This permits corrosive substances to infiltrate down to the base metal [3]. The major differences between aluminium and magnesium anodizing, and oxide films are illustrated in Figure 2, and summarized in Table 1 [4].

Fig. 2: Schematic illustration of the formation of anodic film structure on (a) Mg, and (b) Al alloys

The porous anodic film, once applied, must be further passivated/sealed to enhance its abrasion and corrosion resistance. Though numerous sealing techniques have been developed, hot-water, steam, and sodium silicate, and dichromate treatments are most common. During sealing, the surface micropores and microcracks decrease with volume expansion. Images of some anodized magnesium products are shown in the picture on page 1297:

Caustic anodizing

Caustic anodizing process has long been used for magnesium alloy components that do not require close dimensional tolerance. The electrolyte is composed of NaOH: 240 g/L, (HOCH₂CH₂)₂O: 83 ml/L, and Na₂C₂O₄: 2.5 g/L, operating at 75-80 °C, for 15-25 min. using 6-24 V AC or 6 V DC. The formation of anodic film can be represented by following chemical equations:

Mg° → Mg²⁺ + 2 e–

Mg²⁺ + 2 OH− → Mg(OH)₂

Mg(OH)₂ → MgO + H₂O

The secondary passivation reaction can proceed through the direct oxidation of MgO on the electrode surface

Mg° + H₂O → MgO + 2 H⁺ + 2 e^–

The anodic film produced is immersed in solution containing sodium acid fluoride and sodium dichromate for further passivation.

Dow 17 process

In the mid-1940s, the Dow Chemical Company developed Dow 17, the first anodizing process for magnesium alloys. This anodic treatment is applicable to all forms and alloys of the magnesium, and can be applied with either alternating or direct current. While this process renders green surface finish with good protection against corrosion and abrasion, it is largely replaced by its modified form, termed as modified acid fluoride anodizing (MAFA), to further enhance the surface properties.

Modified Acid Fluoride Anodizing (MAFA) process is based on an aqueous acidic electrolyte containing a combination of fluoride, phosphate, chromate and sodium ions. Either alternating or direct current can be used, but alternating current is most commonly employed because of lower cost of electrical equipment and higher production rate (as the coating is obtained on both the electrodes). However, at a given current density, approximately 30 % less time is required for anodizing when direct current is used. Three specific types of anodic layers can be produced by this process depending on the terminal voltage employed. Parts to be processed are first degreased, pickled to remove gross surface contamination and then anodized in either of the following solutions [5]:

For preparing the above solutions, the chemicals are added in the order as shown above while stirring. After addition of all the chemicals, the solution is heated to 82 °C and stirring continues to ensure thorough mixing of chemicals. A steel tank with a heating coil is recommended. Stainless steel and lead are attacked. The tank can be lined with any synthetic rubber of vinyl type to prevent electrical grounding. Either an unlined tank itself or mild steel electrodes are used as cathodes with d-c, but the work itself is used as electrodes with a-c.

Operating temperature:

70-82 °C. Bath will not operate below 60 °C, but can work up to boiling.

Current density:

0.5-5.0 A/dm² or even more

Voltage:

a-c or d-c up to 95 V (110 V, where extremely heavy coatings are required)

Time:

1-30 min. depending on the requirement of the type of coating.

Clear coating:

A very thin coating is used as a base for subsequent clear lacquers or paints to produce a final appearance similar to clear anodizing on aluminium. Anodizing time is 1-2 min.

Low voltage thin coating:

The best combination of protection value and paint base properties is obtained by this process. Process time is 2-5 min. depending on the nature of alloy and applied current density with end voltage of 60-65 V. A coating thickness of 5-6 µm is typically produced, which is grey to pale green in appearance.

Regular high voltage coating:

It offers the best combination of abrasion resistance and protection value as well as good paint base characteristics. Anodizing time is 10-30 min. with a final voltage of 75-95V for producing a coating thickness of 20-40 µm. The coating is full medium green in colour. The process is accomplished preferably at constant current mode. The voltage is gradually increased for the first 15-20 sec. and then continuously raised to maintain the constant current density. As the coating thickness increases, the electrical resistance also increases which results in the fall of voltage. To maintain the constant current density, voltage has to be increased. The terminating voltage varies with magnesium alloy used and applied current density, but the number of ampere min. per unit area remains constant for any alloy.

Sealing:

After anodizing, the parts are rinsed in cold water. Where the anodized jobs are to be left unpainted, they are sealed in an aqueous solution of sodium silicate 50 g/L, operating at 93-100 °C for 15 min. After sealing the parts are rinsed in water and dried.

Sharma et al. [6] investigated a modified acid fluoride anodizing process (MAFA) on magnesium alloy ZM21. The process provides excellent corrosion resistance with high thermal emittance. The following are the optimum electrolyte composition and operating conditions:

(1) Solvent degreasing in isopropyl alcohol for 5-10 min.

(2) 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 min. Followed by water rinsing.

(3) Acid pickling in a solution of chromium trioxide 180 g/L, ferric nitrate 40 g/L and potassium fluoride 3.75 g/L for 2 min.; water rinsing.

(4) Anodizing in the following bath solution and operating conditions:

Ammonium bifluoride, (NH₄)HF₂: 240 g/L

Sodium dichromate, Na₂Cr₂O₇ .2 H₂O: 100 g/L

Ortho phosphoric acid, 85 % H₃PO₄ (V/V): 90 ml/L

Temperature: 70-80 °C

Agitation: Intermittent by Teflon rod

Current density: 1 A/dm²

Voltage: 60-110 V (AC)

Time: 45 ± 5 min.

Coating thickness: 45 ± 5 µm

Tank material: vinyl lined steel, PP or PE

Post treatment: water rinse

(5) Sealing in a solution of sodium silicate, 50-55 g/L, operating at 93-100 °C for 15 min.

The anodic film is formed by a chemical reaction between the magnesium alloy surface and hexavalent chromium. Magnesium is oxidized by the hexavalent chromium, which is reduced to the trivalent state [6]. Alternating current enables replenishment of the reactant concentrations at the metal electrolyte interface on one electrode, while producing a coating on the other. The coating is composed of hexavalent and trivalent chromium and the substrate metal (in the form of chromate, phosphate, hydroxide, and bifluoride). In addition, some traces of sodium silicate due to sealing in silicate solution are also present. The Energy Dispersive X-Ray Analysis (EDX) analysis of the anodic film showed the presence of sodium, magnesium, phosphorus, silicon and chromium.

Process optimization was carried out by investigating the influence of operating parameters, electrolyte temperature, applied current density, and post anodizing sealing on the physico-optical properties of the anodic coating. At higher coating thickness (> 50 µm), sparking was observed due to higher terminating bath voltages. This results in very rough, spalled, and fibrous deposits. The process yielded a satisfactory coating over a wide range of electrolyte temperatures (60-90 °C) and applied current densities (1-5 A/dm²). As anodic film provides high solar absorptance (0.84) and IR emittance (0.88), it is found suitable for spacecraft thermal control applications. The space worthiness of film was evaluated by humidity (RH 95 %, 50±1 °C, 48 hr.), baking (300 °C, 48 hr.), thermal cycling (1,000 cycles, -50 °C, -200 °C with a dwell of 5 min.), and thermo vacuum (10 cycles, -50°C, -200 °C with a dwell of 2 hr.) tests. No degradation in the physico-optical properties of the coatings was observed.

Modified Caustic Anodizing: HAE Process

In 1950, Harry A. Evangelides at Frankford Arsenal, Philadelphia, USA, developed a modification in caustic type anodizing processes. This process subsequently became commercially popular as HAE anodic treatment. The HAE process provides improved corrosion protection and abrasion resistance among all the anodizing processes on magnesium and its alloys. The following solution is used:

Potassium hydroxide, KOH 168 g/L
Aluminium hydroxide, Al(OH)₃ 35 g/L
Potassium fluoride, KF 35 g/L
Trisodium phosphate, Na₃PO₄ . 12 H₂O 35 g/L
Potassium manganate, K₂MnO₄ or
Potassium permanganate, KMnO₄ 20 g/L

The chemicals are dissolved in water in the order as given above. If potassium permanganate is used, it should be completely dissolved in hot water before adding to the anodizing tank. A steel tank with heating / cooling coils is used. Since direct current is not satisfactory, alternating current is used in the HAE process where jobs work as electrodes. There are three types of coatings that can be produced from the same electrolyte with the variation in operating conditions.

Low voltage soft coating process:

A soft smooth olive-green colour coating of 5-10 µm is produced by using ~ 9 V a-c at a current density of 3-4 A/dm2 for 15-20 min. The bath temperature is maintained at 60-65 °C.

High voltage light coating process:

This coating is also soft and smooth, but tan in colour. The bath temperature is maintained at 20-25 °C and cooling is usually required. A current density of 1.8-2.2 A/dm2 is applied, the terminating voltage of ~60 V and a possessing time of 45-50 min. is used.

High voltage hard coating process:

The hard coating is obtained by anodizing to a terminating voltage of 85 V. A current density of 1.8-2.2 A/dm2 is used for 60-65 min. The bath is operated at a temperature of 20-25 °C and cooling of solution is required to maintain it. The thick dark brown 50-80 µm HAE coating is produced, which is very rough, hard, and brittle. The HAE treatment provides good abrasion resistance. However, it may adversely affect the fatigue strength.

After anodizing the parts are thoroughly rinsed in cold water and then a post treatment dip for 1-2 min. at room temperature is given in a solution containing 100 g/L of ammonium acid fluoride and 20 g/L of sodium dichromate. The parts are then rinsed in cold and hot water and then dried. This post treatment is necessary to neutralize the alkali retained in the coating. The neutralizing treatment enhances the protective value of the coating and improves the paint adhesion properties.

There are some proprietary anodizing processes available commercially:

Tagnite treatment:

Fig. 3: SEM of anodic layer formed by Dow 17, HAE and Tagnite process (Source: www.tagnite.com)

This treatment was developed by Technology Applications Group (TAG) provides a hard, smooth oxide layer with pores up to 10x smaller than DOW17 process. The process is carried out in two steps. In the first step, job is immersed in an aqueous solution of magnesium fluoride and/or magnesium ammonium fluoride. The second step involves anodic plasma treatment in hydroxides, fluorides, fluorosilicates and silicates solution, pH>12.5. This process provides a superior coating with higher abrasion and corrosion resistance compared with the HAE and Dow 17 processes. Scanning electron micrographs (SEM) of anodic layers formed by Dow 17, HAE and Tagnite process are shown in Figure 3.

Anomag process:

This process was developed by Magnesium Technologies Licensing (Auckland, New Zealand), the electrolyte consists of ammonia and sodium ammonium hydrogen phosphate. Three classes of layers (3-8 μm, 10-15 μm and 20-25 μm) can be produced with a gentle spark-free plasma discharge on the substrate. The coatings produced are characterized by good corrosion resistance, wear resistance and fatigue. Keronite acquired the Anomag magnesium business in 2007.

Photos: A. Sharma

References:

[1] A.K. Sharma: Chemical Conversion Coatings on Magnesium Alloys (Part 3), Galvanotechnik,111, no. 8 (2020) 1086-1089
[2] C. Xu; W. Gao: Pilling-Bedworth ratio for oxidation of alloys, Mat. Res. Innovat., 3, no. 4 (2000) 231-235. doi: 10.1007/s100190050008
[3] P. Kurze: Corrosion and surface protection (Chapter 7), Magnesium Technology, H.E. Friedrich, B.L. Mordike (Editors), Berlin (Heidelberg): Springer-Verlag, (2006) 431-468
[4] SA Salman; M. Okido: Anodization of magnesium (Mg) alloys to improve corrosion resistance (Chapter 8), Corrosion Prevention of Magnesium Alloys, Guang-Ling Song (Editors), Woodhead Publishing, (2013) 197-231. doi: 10.1533/9780857098962.2.197
[5] H.K. DeLong: Anodizing and surface conversion treatments for magnesium in Electroplating engineering handbook, L.J. Durney (Editor), Van Nostrand Reinhold, New York, (1984, reprinted 2000) 410-419
[6] A.K. Sharma; R. Uma Rani; K. Giri: Studies on anodization of magnesium alloys for thermal control applications, Met. Finish., 95, no. 3 (1997) 43-51. doi: 10.1016/S0026-0576(97)86772-4