Magnesium Finishing - Part 7 -

– Part 7 – Chemical conversion coatings from galvanic black anodising to ionic liquid-based conversion coatings / continued from Galvanotechnik 8/2024

In the earlier part on magnesium finishing, we have covered chrome pickle, dichromate treatment, dilute chromic acid treatment, chromate conversion coating, chrome-manganese conversion coating, and phosphate treatment. In this part some of the other important chemical conversion coatings, viz. galvanic black anodising, black molybdate, stannate, cerium oxide-hydro­xide, carbonate, and ionic liquid-based conversion coatings are discussed.

The interior parts of spacecraft are typically coated with a flat, absorptive black finish to aid in thermal coupling, which is essential for effective thermal control of spacecraft. Galvanic black anodised components are characterized by high solar absorptance and thermal emittance, are well-suited for such applications.

Galvanic black anodising

Galvanic black anodising on magnesium alloys is an improved version of chromate conversion coating.

Processes of galvanic black anodising were investigated in detail by Sharma et al. [1-7] on magnesium alloys AZ31B, ZM21 and Mg-Li alloys. These studies were primarily carried out for the applicability of these specific coatings for thermal control of spacecraft. Following sequence of operations was used for different magnesium alloys:

• Solvent degreasing in isopropyl alcohol for 5-10 min.
• 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.
• 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 min.; followed by water rinsing.

For ZK 60A alloy: (a) 50 % Chromium trioxide for 1-2 min.; 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 is recommended for close dimensional tolerance parts.

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 min. with a post treatment of water rinse. (b) Fluoride activation by dipping in 40 % hydrofluoric acid (50 ml/L) for 10 min., followed by water rinsing.

• Black anodising in the following bath 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

Time: 60 min.

Cathode: anodising tank (SS)

Anode to cathode area ratio: 1:5 to 1:10

Galvanic current/voltage: 0.8-2.4 mA/cm² / 1.2-3.6 mV/cm²

Coating thickness 12 -15 µm

Post treatment, water rinse, and drying in air.

• Heat treatment at 70 °C for 2 hr. in an electric oven with clean hot air circulation.

The coating formation reactions can be represented by the following equations.

Electrolytic reaction (ionisation):

K₂Cr₂O7 → 2 K⁺ + Cr₂O₇^2–

(NH₄)₂SO₄ → 2 NH₄⁺ + SO₄^2–

H₂O → 2 H⁺ + OH^–

Anodic reaction:

Mg⁰ → Mg²⁺ + 2 e-

Mg²⁺ + Cr₂O₇^2– + 6 OH^– / 3 SO₄^2– → 2 MgCrO₄ + 2 Cr(OH)₃ + Cr₂(SO₄)₃ + 3 O₂ + 6 e^–

Cathodic reaction:

K⁺ + OH^– → KOH

NH₄⁺ + 4 OH^– → NH₄OH

2 H⁺ + 2 e^– → 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 simple and economical. The space worthiness of the coatings was evaluated by the humidity, the thermal cycling and the thermo-vacuum tests and measurement of optical properties.

The chromate (or phosphate) conversion coatings are formed because the metal surface dissolves in solution to a small extent, causing a raise in pH value at metal surface-solution interface [8]. A thin film of metal chromate (or phosphate) gets precipitated. The film is soft and has a gel-like structure when freshly formed, but after drying, it hardens and micro cracks to give a 'mud-crack’ pattern (Figure 1). The freshly treated works should therefore be handled very carefully. After drying and heat treatment due to the dehydration and shrinkage of the film, the micro cracks became wider and edge curling of the blocks of the coating occurs. 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 film was found to increase after sealing and heat treatment (~ 100 VHN). The galvanic black anodic films exhibit high solar absorptance and thermal emittance (> 0.90) values, which showed their extreme suitability for thermal control application.

The chromate and phosphate coatings offer protection to metallic substrate through two mechanisms:

the film provides a non-reactive barrier to humidity and air, thus retarding the corrosion rate;

the coating 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 beneficial in hastening the hardening process, prolonged heating above these temperatures may result in excessive dehydration of film that may cause reduction in its protection value. The films after painting can, however, withstand up to 85 °C, because the impervious paint layer seals the water of hydration in the film.

Black molybdate conversion coating

Molybdates (MoO₄^2-) have long been utilised as one of the green corrosion inhibitors and are analogous to chromates (CrO₄^2-); 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. [9] for spacecraft thermal control applications. The following sequence of operations was adopted:

• Mechanical cleaning with 600 grit SiC paper.Fig. 2: SEM image of molybdene conversion coating
• Solvent degreasing in isopropanol for 5-10 min.
• Alkaline cleaning in a solution of sodium hydroxide 50 g/L and trisodium orthophosphate 10 g/L, operating at 65±5 °C for 5-6 min.
• Acid cleaning in chromic acid 180 g/L and ferric nitrate 40 g/L, operating at 25±5 °C for 5-10 min.
• Molybdate conversion coating (Figure 2):

Ammonium molybdate,

(NH₄)₆MO₇O24_. 4 H₂O - 20 g/L

Magnesium chloride,

MgCl_2. 6 H₂O - 1 g/L

pH - 5.5

Temperature - 25±5 °C

Time - 60 min.

Cathode - SS304 anodising tank

Galvanic current/voltage - 0.8-2.4 mA/cm² / 1.3-3.8 mV/cm²

Coating thickness - 12-15 µm

Post treatment - water rinse

Jobs are connected to the SS304 tank to form a galvanic cell. These coatings consist of hydroxides and oxides of molybdenum and water of hydration, and have the river bed surface morphology. Thermal stability studies re­vealed two major changes that occur when black molybdate coatings are heated: dehydration started at ~50 °C which continues until ~250 °C, and decomposition of molybdenum hydroxides thereafter.

The black molybdate conversion coatings provide high absorptance as well as high emittance (~0.90), and ex­cellent environment stability for stringent space condi­tions.

Stannate treatment

Stannate conversion coatings have emerged as a promis­ing chrome-free alternative for the corrosion protection of magnesium alloys. Stannate film decreases the corrosion rates of magnesium alloys by acting as a barrier for corrosive ions and oxygen. Such coating displays noticeable self-healing characteristics. A top coat of thin epoxy, poly­urethane, fluoropolymer, or polyaniline film might be a candid choice.

Stannate coatings on magnesium are formed by initial dissolution of substrate followed by deposition of coatings when Mg²⁺ and [SnO₃]^2- ions reach critical concentrations at the substrate/electrolyte interface. The coating is mainly composed of hydrated magnesium stannate particles, MgSnO_3. 3 H₂O. 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, K₂SnO_3. 3 H₂O - 40-50 g/L

Sodium acetate, CH₃COONa . 3 H₂O - 10-25 g/L

Tetra sodium pyrophosphate, Na₄P₂O_{7 -} 40–50 g/L

Temperature - 82 °C

Time - 20 min.

Tank material - (PP) polypropylene

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 stannate solution can be sprayed, brushed or the work can be immersed with a contact time of 30 seconds to 2 min. 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 (0.75-0.92 m²) area may be treated per liter 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.

Cerium oxide-hydroxide film

Rare earth elements-based conversion coatings are actively researched as an environmentally benign alternative to conventionally used chromate-based conversion coatings for magnesium alloys.

Magnesium alloys are biocompatible materials but have poor corrosion resistance in physiological environments which limits their applications in the biomedical field. Advances in coatings on biodegradable magnesium alloys have been recently reviewed by Yin et al. [10]. Some environmentally friendly chemical treatments have been developed on magnesium alloys to improve their corrosion resistance. Among them, the rare earth elements-based cerium oxide-hydroxide coating appears to be very promising [10,11]. The formation of cerium oxides and hydro­xides on the metal surfaces is found to inhibit the metal corrosion by the blocking effect and reduces the rate of reduction reactions [12,13]. 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 [14]. This led to the precipitation of cerium hydroxides/oxides forming compact ceria films. Hydrogen peroxide in the cerium conversion solution acts as an oxidant, the dissolved oxygen promotes the oxidation of Ce³⁺ to Ce⁴⁺ species.

H₂O₂ + 2 e^– → 2 OH^–

Ce³⁺ + OH^– + ½ H₂O₂ → Ce(OH)₂²⁺

Ce(OH)₂²⁺ + 2 OH^– → Ce(OH)₄

Ce(OH)₄ → CeO₂ + 2 H₂O

The following sequence of operations can be used to produce the cerium oxide-hydroxide coatings on magnesium and its alloys [15]:

Solvent degreasing in isopropyl alcohol using an ultrasonic bath for 10 min.

Alkaline cleaning: sodium hydroxide: 50 g/L and trisodium orthophosphate: 10 g/L at 55±5 ºC, 10 min.

Acid cleaning: chromic acid: 180 g/L, ferric nitrate: 40 g/L, and potassium fluoride: 4.5 g/L at room temperature for 2-3 min.

Cerium oxide coating: cerium sulphate: 5 g/L, and hydrogen peroxide: 40 ml/L at room temperature, pH 2.0 for 3-4 min. In place of cerium sulphate other cerium (III) salts, like chloride, nitrate, phosphate can 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 published by Saji [16].

Carbonate conversion film

A carbonate conversion film was obtained on magnesium AZ31 alloy by immersing the job in a CO₂ saturated NaOH solution for 1-60 min. by Nam et al. [17]. The film consists of a carbonate containing magnesium complex having the chemical formula [Mg₅(CO₃)₄(OH)_2· 5 H₂O]. This composition is strongly influenced by the immersion time, the carbonate contents on the surface of the job get increased with increasing the immersion time. Processing the job for about 10 min. immersion time provides the optimum results for corrosion resistance.

Conversion coating with ionic liquids

A non-immersion conversion coating on magnesium AZ31B alloy by interaction of deep eutectic solvent (DES) with substrate was reported by Gu et al. [18]. Upon heating the choline chloride / urea-based DES reacts with the mag­nesium alloy surface forming a chemical conversion film composed of MgH₂ and MgCO₃ phases. The as-grown film exhibited improvement in the corrosion resistance, and a super hydrophobicity after surface modification in a stearic acid / ethanol solution.

REFERENCES:
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