Magnesium Finishing – Part 6 –

– Part 6 – Chemical conversion coatings from chrome pickle to phosphate treatment / continued from Galvanotechnik 7/2024

To prevent the corrosion, a barrier coating between the magnesium substrate and the surrounding environment is applied. A good barrier layer should be uniform, and adherent. It should possess self-healing characteristics, and minimal porosity, and macrocracks. Chemical conversion coatings are one of the easiest and widely used surface modification techniques for corrosion protection. The term conversion coating implies that during the process, the metal surface gets converted into the coating via a chemical and/or electrochemical reaction. The coatings thus formed are composed of chemically inert inorganic compounds that provide corrosion inhibition. In this part of the article, chrome pickle, dichromate, dilute chromic acid treatments, chromate, chrome-manganese, and phosphate conversion coating are discussed.

Chemical immersion coatings on magnesium are primarily intended for a short storage or shipment or to provide a good base for subsequent protective coating or paint [1,2]. In some cases, these treatments may, however, provide adequate corrosion protection by themselves e.g., interior parts of the engine that are normally covered with oil which adds to protection. Chrome pickling, dichromate treatment, phosphate coating, chrome-manganese conversion coating, and galvanic black anodizing on magnesium alloys have been used for corrosion protection in mild corrosive environments [2]. The corrosion resistance of these coatings can be further enhanced by application of subsequent coats of paint, lacquer or sol-gel coatings. In earlier days, hexavalent chromium compounds were wide­ly used for producing conversion coatings on magnesium alloys because of their excellent corrosion-inhibition properties. However, in recent years, efforts are made to reduce the use of Cr6+ as it is found as carcinogenic. Some alternative methods, viz., rare earth metal conversion coatings, stannate coating, molybdate conversion coating, are explored as the eco-friendly chemical conversion techniques [3,4].

Chrome pickle

Chrome pickle is the commonly used chemical treatment for magnesium alloys. The treatment can be carried out by dipping, spraying or by application of brush. As this treatment removes up to 15 µm surface per min., it is not re­commended for close tolerance parts. Composition A is recommended for wrought products while composition B (modified chrome pickle) for castings:

A standard 1-min. immersion time is used for all the alloys except for die castings where the time is reduced to 20-30 sec. The transfer time between treatment and cold-water rinsing to be kept minimum (5-15 sec.) to avoid the formation of loose powdery coatings. The colour of the coating is matte grey to yellow-red iridescent with a degree of fine surface etching which is good for paint adhesion. Large articles or repair areas can be treated by brush applications, but the colour obtained is not as uniform as attained by dip or spray processes.

To improve the corrosion resistance on wrought products the chrome-pickle treatment should be immediately fol­lowed by the sealing in boiling dichromate-fluoride solution (as described below under dichromate treatment) for 30 sec. Sealing of the old chrome-pickled film is not recommended since the aging of film prevents proper sealing.

Dichromate treatment

Dichromate treatment is the most common dip chemical process for magnesium alloys used today and it provides good corrosion protection. This treatment has no significant effect on dimensions and is normally applied on machined parts prior to painting. The appearance varies from light to dark brown depending on the alloy composition. The process sequence for dichromate treatment involves solvent degreasing, alkaline cleaning, fluoride treatment and dichromate coating.

Fluoride treatment is carried out by immersing the work in 280 ml/L hydrofluoric acid (40 % V/V) solution at room temperature. All magnesium alloys except aluminium zinc (AZ) are immersed for 5 min. A short dip for 30 sec. is used for AZ alloys, otherwise the passive fluoride film produced tends to retard the formation of subsequent dichromate coating. An alternative fluoride treatment using 50 g/L of sodium, potassium or ammonium acid fluoride or mixture of these salts may be used. The parts are immersed in this solution for 15 min. and then rinsed in cold water. This solution does not attack aluminium inserts, rivets, etc. that are present in magnesium jobs.

After fluoride treatment the dichromate treatment is applied by immersing the parts in the following solution:

The presence of calcium or magnesium fluoride assists in the chromate film formation. As these salts are only slightly soluble, their concentration need not be controlled. They can be conveniently suspended in the solution in cloth bags or the excess can be added in the tank so that the bath remains saturated with them. However, it must be noted that the high free fluoride concentration renders the bath inoperable, it must not exceed 0.2 %. After dichromate treatment, the parts are rinsed thoroughly in cold water and then in hot water to facilitate drying.

Dilute chromic acid treatment

This is the least expensive treatment on magnesium alloys. It can be applied by brush, spray or dip methods. The process affords better corrosion protection than phosphate treatment although the paint base properties are similar. It is less critical to apply than the chrome-pickle for touch up repair work on previously coated surfaces and is not corrosive if entrapped between faying areas. The following composition is employed:

The chemicals are added in water in the above order and stirred vigorously for about 15 min. to ensure the saturation of solution with calcium sulphate. For optimum results the surface is kept wet with solution until a brassy iridescent film is formed. Unlike chrome pickle the time be­tween pickling and cold-water rinsing is not critical. When water rinsing is not possible sponge drying of the drain off liquid at the edges of works is sufficient.

For machined AZ31B alloy components, a very dilute solution of the following composition can be effectively used:

Chromate conversion coating

Chromate conversion coating is characterized by its ability to produce a very thin, uniform coating with high productivity. Chromate coatings are formed via a complex oxidation-reduction process, magnesium is oxidized and chromium is partially reduced to its trivalent state. The film comprises a mixture of magnesium chromates, chromium chromate and hydroxide.

3 Mg + Cr₂O₇²⁻+ 14 H⁺→ 3 Mg²⁺+ 2 Cr³⁺+ 7 H₂O
2 Mg2+ + Cr₂O₇²⁻ + 2 H₂O → 2 MgCrO₄ + 2 H⁺
2 Cr3+ + Cr₂O₇²⁻ + 3 H₂O → 2 Cr(OH)CrO₄ + 4 H⁺

This process is widely used because it provides good corrosion protection through a barrier film as well as through the presence of corrosion-inhibiting hexavalent chromium.

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 coatings:

Film microhardness improves when baths are operated at higher temperatures. As the solution is used, its pH increases from 4 to 5. 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 with the addition of sulphuric and chromic acid in equal quantities. Addition of 1 ml/L of the following mixture, reduces the pH of the bath by 0.1.

A process of black chromate conversion coating on magnesium-lithium (LA141A and MLA 9, Al 1.25 %, Li 10 %, by weight, balance Mg) [5,6] and magnesium-aluminium alloys (such as AZ91, AZ80, and AM60 [7]) was described by Sharma. The process involves the following sequence:

1. Ultrasonic solvent degreasing in isopropanol for 3-5 min.;
2. 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 min., followed by water rinsing;
3. Acid cleaning in 50 % chromic acid for 4-7 min. 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 sec. was used;
4. Chemical brightening in a solution containing 8-10 % orthophosphoric acid (V/V) in isopropanol for 3-5 min., followed by rinse in isopropanol, dip in hot water and drying in air for Mg-Li alloys. For Mg-Al alloys a fluoride activation in 40 % HF 400 ml/L for 1 min. was carried out;
5. Chromate conversion coating with 90 g/L potassium dichromate (K₂Cr₂O₇), 40 g/L manganous sulphate (MnSO₄.H₂O), 40 g/L magnesium sulphate (MgSO₄.7H₂O), 1-2 g/L potassium fluoride (KF), at 55-90 °C for 2-3 hr. depending on the bath temperature, fol­lowed by water rinsing, tank material: SS 304 with immersion heaters, coating thickness: 8-11 µm
6. Sealing only for magnesium-lithium alloys, in a solution containing 2 % potassium dichromate, K₂Cr₂O₇ and 10 % ammonium bifluoride, (NH₄)HF₂ at room temperature for 3 min., hold over an open vessel of hot water (70-75 °C) for 2-3 hr.;
7. Heat treatment by placing the job over glass or aluminium plate in an oven operating at 70-75 °C for 1-2 hr. for magnesium-aluminium alloys and for 6 hr. for magnesium-lithium alloys;
8. Rinse in hot water for 2-3 min., polish gently with soft cloth to remove the bloom, 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 hr. to assess 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 1,000 cycles of -45 °C to + 80 °C. A cycle consists of lowering the temperature to -45 °C, a dwell of five min. and raising the temperature to 80 °C with a dwell of five min. The first six cycles were of 1 hr. each to evaluate the thermal soak and the duration of rest cycles was 5 min. each. The coatings have passed these tests without any visual degradation and no change in their optical properties was reported after the test.

The images of conventional yellow and black chromate conversion coated magnesium parts are shown in the cover picture of this article.

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, 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:

The solution is brushed or sprayed for 1 min. or until a continuous grey coating is formed. Alternatively, parts may be immersed in the solution for 1-2 min. or until gassing stops. The treatment is followed by cold water rinsing, hot water should not be used.

Phosphate-permanganate treatment

Zhang et al. [8] 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:

A thin and non-penetrating cracked morphology was reported. The main elements of the conversion coating were Mg, O, K, P and Mn. The electrochemical measurements demonstrated the improved corrosion resistance of the magnesium-lithium alloy by the phosphate-permanganate treatment.

References:

[1] S. Abela: Protective Coatings for Magnesium Alloys (Chapter 10), Magnesium Alloys: Corrosion and Surface Treatments, F. Czerwinski (Editor), IntechOpen, London, (2011) 195-220. doi: 10.5772/14100
[2] J. Gray; B. Luan: Protective coatings on magnesium and its alloys – a critical review, J. Alloys Compd., 336, no. 1-2 (2002) 88-113. doi: 10.1016/S0925-8388(01)01899-0
[3] L. Anicai; R. Masi; M. Santamaria; F. Di Quarto: A photoelectrochemical investigation of conversion coatings on Mg substrates, Corros. Sci, 47, no. 12 (2005) 2883-2990. doi: 10.1016/j.corsci.2005.05.033
[4] A.R. Shashikala; R. Umarani; S.M. Mayanna; A.K. Sharma: Chemical conversion coatings on magnesium alloys – A comparative study, Int. J. Electrochem. Sci., 3, no. 9 (2008) 993-1004. http://electrochemsci.org/papers/vol3/3090993.pdf
[5] A.K. Sharma, Chromate conversion coatings for magnesium-lithium alloys, Met. Finish., 87, no. 2 (1989) 73-74
[6] A.K. Sharma: A process of chromate coating on magnesium-lithium alloys, Indian Patent 170,666 (1988.07.05)
[7] A.K. Sharma: A process of flat absorber black chromate conversion coating on Mg-Al alloys, Indian Patent 180,666 (1992.06.11)
[8] H. Zhang; G. Yao; S. Wang; Y. Liu; H. Luo: A chrome-free conversion coating for magnesium-lithium alloy by a phosphate-permanganate solution, Surf. Coat. Technol., 202, no. 9 (2008) 1825-1830. doi: 10.1016/j.surfcoat.2007.07.094