ASTM D1732:03(R2018) pdf free download – Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting.
7.4.1 A; guides to the current consumptions with various alloys, some data are presented in Table I. It will be appreciated that rigid current consumpfions in the formation of the thin tafl coating cannot be spccilicd. since this coating can he varied in thickness considerably between say 0.(M)Ol in. (2.5 pm) and 0.0(N)5 in. (12.7 pm. and still he formed below the critical spark potential at which the dark-green vitreous coating begins to develop.
7.5 Soluilon Control—With depletion of the bath upon use. the final voltage required to impress the above currents Ofl the article will slowly rise, hut even Inc the lull green coating at least 20 ft2 (50 dm2) of surface can be treated per gallon tl.() 1) of solution before any change in the final voltage is experienced: and up to 40 ft (98 dm2)Igal (I L) can he treated before revivification of the solution becomes necessary.
7.5. I Prior to revivification. analyie for dichromate and phosphoric acid (see Sections 10 and 12). and make up differences from the original composition by additions of sodium dichromate and phosphoric acid.
7.5.2 A simple method for the determination of fluoride in the solution is not yet available. The fluoride concentration. howeser. is not critical, provided it always exceeds minimum concentration, below which pitting of the articles may ensue during their treatment. When revisilications are made by additions of sodium dichromate and phosphoric acid, make a concurrent addition of ammonium acid fluoride, equal to three times the weight of sodium dichromatc added. Secondly. if pitting of the articles should occur under treatment, it is an indication that the fluoride content of the solution is ton low. and about 5 0, (38 g)lgal (I L) of ammonium acid fluoride shall then be added in order to reliese the trouble.
7.6 Tank and Rack Maieriais— Mild steel is suitable ft)r tanks and heating coils.. Other metals, such as copper. aluminum, lead, tinc. Monel, and stainless steels are attacked by the electrolyte. When direct current is used the tank itself can be the cathode. but, when alternating current is used, at least Iwo magnesium parts are neces ry. of area ratios not more than 3 to I. each one serving as electrode. The articles shall not contact the tank. otherwise pitting will ensue.
The anodic coating consists of Iwo phases: The first-formed suhcoatin phase is a light tan in color, and causes a dimensional increase of aboutin. (5 ni) per side: the second and thicker phase. formed at higher voltages, is dark brown and causes a dimensional increase of from 0.001 in. to 0.0l2 in. 25 .im to 30 jiml per surface. The paint-base characteristics are similar, and excellent for both the aboc phases.