Chromate treatments alloys

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

Magnesium alloys do not retain a high lustre like the aluminium alloys and cannot be electroplated with the same ease as the zinc alloys. Finishing is usually achieved by painting or lacquering. Chromate treatments are used to protect against corrosion, but have little decorative value. 

Aluminium alloys are widely used in aircraft construction, extrusions for building purposes, skins of caravans (aluminium coil) and beverage cans. In each case they are protected by organic surface coatings on interior and exterior surfaces. Prior to coating, the metal is pretreated to improve corrosion protection, normally with a chromate treatment. The most widely used of the well established chromating treatments are the amorphous chrome phosphate process. 

In the past, poor attention has been given to the effect of the pre-treatment of metal surfaces on the variation of adhesive strength. Traditional treatments such as chromate treatment uses health-hazardous materials, and the phosphatation method, used particularly on aluminum alloys in the automotive industry, has the disadvantage of generating high amounts of sludge in the treatment bath. 

BS 6536 1985 refers to four grades of carbon steel strip coated with Al-Si alloys containing 5-11 wt.% Si. The availability of the steel grades allows account to be taken of the type and degree of forming which is to be applied to the coated steel. Various coating masses are supplied in the range 40-180 g/m which may be additionally protected by a chromate passivation treatment. 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Figure 10.8 Scanned image of the surface of two alloy panels showing adhesion failure caused by the omission of O2 plasma treatment of the substrate prior to plasma film deposition and application of the primer (Deft 44-GN-72 MIL-P-85582 Type I Waterbased Chromated Control Primer), a) Panel after Skydrol LD4 fluid resistance test, which had the O2 plasma treatment prior to film deposition and primer application, b) Panel after scribed wet (24-h immersion in tap water) tape test, which had not been treated with the O2 plasma treatment prior to film deposition and primer application.

Table 31.1 Adhesion Test Results of Chromated Spray Primers [Deft 44-GN-36 (A) and Courtauld 519X303 (G)] to A1 Alloys Prepared with Chemical Cleanings and Plasma Surface Treatments...

Fe-, Cd-, and Sn-based alloys. Chromate solutions are also used to seal coatings formed by anodizing or hydrothermal treatment. CCCs are primarily used to improve adherence of subsequently applied organic coatings, or to impart corrosion resistance under atmospheric exposure conditions. Certain types of CCCs contain labile hexavalent chromium (Cr(VI)), which can be leached from the coating when it is contacted by an attacking solution. This Cr(VI) can be transported... 

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