Corrosion Protection of AZ91D Magnesium Alloy by Anodization Using Phosphate Electrolyte

Abstract

Mechanism of corrosion protection obtained by anodization for die-cast plates of ASTM AZ91D (Mg-9 mass%Al-0.7Zn) magnesium alloy has been studied. Anodization was conducted by conventional Dow17 which utilizes chromium oxide (VI), ammonium fluoride and phosphoric acid, and by environment-friendly Anomag whose electrolyte consists of phosphate and ammonium salt. The anodized surface obtained in Dow17 showed local corrosion in salt spray test (SST) after $500 ks to form corrosion products consisting of magnesium hydroxides. On the other hand, the surface anodized in Anomag was covered with amorphous film, showed only discoloring in SST and corrosion product was scarcely observed. When the anodized surfaces were trenched with ceramic knife to form locally exposed substrate, corrosion product was formed on the trench in the case of Dow17, but corrosion was well suppressed by formation of new type of protective film in the case of Anomag. Anodic polarization curves indicate that the surface anodized in Dow17 is protected by passive substances through which electrolyte can easily reach the substrate, and that in Anomag show sacrificial function where the anodized layer dissolves quite slowly into the electrolyte prior to the substrate. The excellent corrosion protectivity obtained in Anomag is considered to be based on the formation of a new type of protective film as well as sacrificial function of the original amorphous anodized layer.

Effects of Alloying Elements on Characteristics of Anodic Oxidized Coatings in Various Mg-Al-Zn Series Alloys

“…The coating on the AZ91D magnesium alloy retained anticorrosion performance for over 3600 ks and clearly showed superior anticorrosion performance in comparison with the other magnesium alloys. In previous research, 16) on the AZ91D magnesium alloy, when the anodized surface by this process was trenched with ceramic knife to form locally exposed substrate, corrosion was well suppressed by formation of new type of protective film. In addition, this coating showed the sacrificial function where the anodized layer dissolved quite slowly into the electrolyte prior to the substrate in anodic polarization curve.…”

Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ80 Magnesium Alloy in Alkaline Aluminate Solution

“…36) The incorporation of phosphate in anodic oxide films on magnesium is known, [37][38][39][40][41] but the enrichment in the inner part of the present coating is the subject of further study.…”

Effect of Environmentally Friendly Anodization on the Mechanical Properties and Microstructure of AZ91D Magnesium Alloy

“…Anodization was conducted by electrolysis using direct current (DC) or alternating current (AC) with a solution of phosphate and ammonium salts. [11][12][13][14][15][16][17][18] The counter electrodes were plates of stainless steel (JIS (Japanese Industrial Standards) SUS316L) which face both surfaces of the specimen, and the temperature of the electrolyte was 298 AE 5 K. The resulting thickness of the anodized layers were 1, 5, or 10 mm, which corresponded to the final bias used during electrolysis. Hereafter, a specimen anodized by DC electrolysis and a thickness of anodized layer of 1 mm is expressed as ''DC 1 mm''.…”

“…According to previous research, 18) increase in the thickness of the anodized layer is preferable for improving corrosion resistivity of the substrate, due to the sacrificial function of the layer. This effect is markedly evident, especially in the case of thicker anodized layers (Table 2) with corrosion potentials that are less noble and corrosion currents lower than that of the substrate.…”

“…Another protective treatment for magnesium alloys by anodization is performed using an electrolyte that consists of phosphate and ammonium salts. [11][12][13][14][15][16][17][18] This anodization, which is conducted at a higher voltage than that by HAE or Dow17, is accompanied by visible local discharge on the substrate and supposedly an increase of temperature at the surface. Considering that AZ91D magnesium alloy is known to show changes in mechanical properties according to heat treatment, 19) this anodization is thought to bring about the same effect as a heat treatment, depending on the electrolysis conditions.…”