SW Dec 2015 - Page 50

PAINT & POWDER Pretreatment for zinc surfaces A lightweight zinc phosphate coating is recommended. Generally electrodeposited zinc coatings present no pretreatment problems but hot dipped galvanised coating can affect adhesion. Increasing degree of spangle decreases adhesion characteristics. Chromate conversion coatings The main conversion coating for aluminium and its alloys is a chromate coating which can be colourless or of the yellow chromium oxide or green chromic phosphate type. The coating weight recommended is 0.1-0.5g/m2. The five stage process normally consists of an alkali degrease, rinse, chromate conversion, followed by two rinses. Again the chromate coating should be of low film weight for maximum adhesion. For high quality applications it is usually necessary to employ a final rinse with de-mineralised water. The conductivity of the final rinse bath is then monitored to ensure its cleanliness. No-rinse systems One way of avoiding the need for this is to use a dry-in-place or no-rinse process. These are predominantly a form of chromate. It is arguable whether they are true conversion coatings or merely dried-on films with some reaction with the substrate but the advantages of needing no rinse are obvious. Heavy-metal free pretreatments The increasingly strict environmental standards in the developed world mean that there is a move away from heavy-metal containing pretreatment, particularly chromate. Early chromate-free pretreatments had poor performance but more recently standards have improved with the first approvals for use on architectural aluminium applications being awarded by the Qualicoat organisation in 1996. Effluent disposal Local authorities work to different standards in dealing with effluent discharge. However they are all becoming more stringent and cautious as to what effluent they will accept. Generally iron phosphate solutions can be passed to drain without treatment, zinc phosphate solutions usually have to be below a specified concentration level which can normally be achieved by diluting in ordinary water. Some final rinse solutions contain chromate, which requires special treatment because of its toxic effects on marine life. www.interpon.co.uk