Magnesium Finishing - Part 10 -

– Part 10 – Comparative Studies / Continued from Galvanotechnik 11/2024

A large number of surface treatments are developed for magnesium alloys, but only few have actually achieved commercial importance. In earlier parts of magnesium finishing, different types of chemical conversion coatings, anodizing, and micro arc oxidation processes were briefly described. In this article the comparative studies on the appearances, utilities, corrosion behaviour, and other characteristics of the important surface treatment procedures on magnesium alloys are outlined for ready reference of users to make a right choice for specific functional requirements.

A summary of the commonly used surface treatment processes on magnesium alloys along with their appearance, characteristics and applications is presented in Table 1 [1].

Comparative studies on the corrosion behaviour (polari­zation and electrochemical impedance studies, and salt spray test) of some of the important chemical conversion coatings viz, stannate, cerium oxide, chromate, and galvanic black anodizing on magnesium alloy AZ31B have been carried out by Shashikala et al. [2]. The different chemical conversion coatings were obtained with the following process sequence:

1. Solvent degreasing in isopropanol using an ultrasonic bath for 10 min.
2. Alkaline cleaning: sodium hydroxide: 50 g/L and tri sodium orthophosphate: 10 g/L for 10 min. at 55 ± 5 °C.
3. Acid cleaning: chromic acid: 180 g/L, ferric nitrate: 40 g/L and potassium fluoride: 4.5 g/L for 2-3 min.

An additional acid pickling step (280 ml/L 40 % HF for 10 min.) was used for stannate coating.

4. Different chemical conversion coatings were then obtained by using the following bath formulations and operating conditions:

1. Stannate conversion coating: 10-12 g/L sodium hydroxide, 40-50 g/L potassium stannate, 10-25 g/L sodium acetate and 40-50 g/L tetra sodium pyrophosphate operating at 82 °C, pH 11.6, for 20 min.
2. Cerium oxide coating: 5 g/L cerium sulphate and 40 ml/L hydrogen peroxide, pH 2.0 for 3-4 min.
3. Chromate conversion coating: 10 g/L chromic acid and 7.5 g/L calcium sulphate, pH 1.2 for 30-60 sec.
4. Galvanic black anodizing: 25 g/L potassium dichromate; 25 g/L ammonium sulphate; pH 5.8; temperature 28 ± 1°C; anode to cathode ratio 1:10; for 15 min., galvanic current 0.8-2.4 mA/cm² in an anodizing stainless-steel tank.

5. Water rinse and air dry.

From polarization studies the corrosion current (I_corr) of different conversion coatings was found in the following sequence: galvanic black anodizing < chromate coating < cerium oxide coating < stannate coating. The electrochemical impedance (Nyquist diagram) showed that the semicircle diameter decreases in the following order: galvanic black anodizing < chromate < cerium oxide < stannate. The salt spray studies were also in agreement with polari­zation and impedance data and concluded that galvanic black anodizing offers good corrosion resistance com­pared to other coatings. Based on all these investigations, it was concluded that the corrosion resistance of the coatings examined was in the following order: Galvanic black anodizing > chromating > cerium oxide coating > stannate coating.

The space worthiness of the coatings was examined by the environmental tests viz., humidity, thermal cycling and thermos-vacuum performance and evaluation of their optical properties. The optical properties (solar absorptance and infrared emittance) of the coatings were measured before and after environmental tests. No degradation was noticed in case of galvanic black anodizing and chromate conversion coatings. A small change in optical properties was observed in case of the cerium oxide coating while large scale degradation was observed with the stannate coatings.

Corrosion performance of phosphate-permanganate, GBA, dichromate treatment, MAO and MAFA coatings on magnesium alloy AZ31B was investigated by Umarani et al. [3]. Before final conversion coatings, the samples were degreased in isopropyl alcohol, alkaline cleaned (50 g/L sodium hydroxide and 10 g/L, tri sodium orthophosphate for 10 min. at 55 ± 5 °C), acid cleaned (180 g/L chromic acid, 40 g/L ferric nitrate, 4.5 g/L potassium fluoride for 2-3 min). For dichromate conversion coating, in place of acid cleaning, a fluoride treatment was carried out in a solution containing 280 ml/L of hydrofluoric acid (40 %, V/V) for 30 sec. MAO coatings were obtained on samples which were just degreased in isopropyl alcohol for 10 min.

Different chemical conversion coatings and anodizing processes were then obtained as follows. The details of electrolytic solutions and operating conditions are de­scribed in Table 2.

The scanning electron micrographs of different coatings are shown in Figure 1 [3]. Fig. 1: SEM of a) phosphate-permanganate, b) GBA, c) dichromate, d) MAO-silicate, e) MAO-phosphate, and f) MAFA on AZ31B alloy

The phosphate-permanganate, GBA and dichromate coatings showed networks of gel-like structure distributed all over the surface of the coatings, which, after drying, har­dened to give a microcracked pattern. The surfaces of the MAO and MAFA coatings exhibited porous morphology with pores distribution all over the coating.

Scanning electron microscope (SEM) observations re­vealed that the MAO silicate coating had pores of 2.76 ± 1.41 µm of irregular size. However, the phosphate-based MAO coating mainly had large size circular pores (7.31 ± 3.41 µm diameter). MAFA coating showed an irregular porous layer with pores diameters in the range 0.5–5 µm uniformly covering the surface. The elemental composi­tion (wt%) of various chemical conversion coatings studied by Energy Dispersive X-Ray Analysis (EDX) are presented in Table 3.

Fig. 2: Nyquist and Bode phase angle plots for bare AZ31B, phosphate-permanganate, GBA, MAO, and MAFA coatings

The impedance spectra of the coatings in 5 wt% NaCl solution were recorded as Nyquist and Bode plots (Figure 2). The Nyquist plot showed a single semicircle for all the samples. At the high frequencies, the interception of the real axis in the Nyquist plot is ascribed to the solution resistance (R_s), and at the lower frequencies, the interception with the real axis is attributed to the charge transfer resistance (R_ct). Since the diameter of the capacitive semicircle represents the resistance of the coatings, it can be said that the resistance decreases significantly with the decrease in diameter. Higher R_ct values represent better corrosion resistance. The modified acid fluoride anodic coating exhibited higher R_ct values than the other coatings. From these plots, the corrosion resistance of the coatings was found in the following order:

modified acid fluoride > micro arc oxidation-phosphate > micro arc oxidation-silicate > dichromate > Galvanic black anodizing > phosphate-permanganate > bare magnesium

The impedance data and phase angle maximum in the high frequency region decreases in the order of MAFA> MAO phosphate > MAO silicate > dichromate > GBA > phosphate-permanganate coating. This indicates a decreasing order of the protection value of the coating. The poor corrosion resistance of phosphate-permanganate, GBA and dichromate coatings can be attributed to electrolyte penetration through cracks and its reaction with the underlying magnesium substrate leading to the formation of magnesium hydroxide corrosion products which destabilize the coating by increasing its porosity [2]. MAFA exhibits a higher impedance value compared to other coatings. This may be attributed to the presence of a higher percentage of fluoride (10.21 wt%) in the form of MgF₂ in the coating. All other coatings contain only 2-4 wt% of fluo­ride. The presence of fluoride element increases the anti-corrosion stability of the anodized layer because of incorporation of the insoluble MgF₂ in the anodic matrix [9]. The impedance data of different conversion coatings on AZ31B alloy in 5 % NaCl solution are shown in Table 4.

The electrochemical linear polarization data, corrosion potential (E_corr), corrosion current density (I_corr), and polarization resistance (R_p) were deduced from the Tafel plot also showed that the MAFA offered higher corrosion resistance than other coatings. The linear polarization results were observed more or less in accordance with the electrochemical impedance data. From these studies, the corro­sion resistance of the coatings was found to be in the following order: MAFA > MAO phosphate > MAO silicate > Dichromate > GBA > Phosphate-permanganate > bare magnesium.

Conclusions

In general, the chemical immersion processes provide relatively poor corrosion protection and abrasion resistance than the electrolytic processes. Micro arc oxidation (MAO) and modified acid fluoride anodizing (MAFA) processes offer far superior corrosion resistance for long term service application. Of late, there has been increasing interest in the development of micro arc oxidation coatings with many additives to improve the surface mechanical properties. The hybrid composite coatings in addition to superior corrosion protection deliver other requisite functional properties, viz, hydrophobicity, thermo-optical, nanomechanical properties in addition to superior corrosion and abrasion resistance.

REFERENCES:
[1] A.K. Sharma: Chemical conversion coatings on magnesium alloys (Part 5), Galvanotechnik, 111, no. 10 (2020) 1462-1467
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