Goatings linic Pretreatment of Metals II: Analysis and Testinh

摘要

The quality of metal pretreatments can vaiy between different suppliers, different coating lines, and different days on a given line Suppliers know this and do their best to keep quality up and variability down However, it behooves users and paint suppliers to do their own periodic checks on pretreatments, particularly if adhesion or corrosion problems occur The traditional method of testing is to apply an organic coating to pretreated panels and then subject them to salt spray or, preferably, cyclic corrosion testing to measure the corrosion resistance If you are a paint supplier, this should be done on panels or parts pretreated at the customer, since these may be very different from purchased standard test panels There are other, more rapid ways of evaluating pretreatments that can be useful, particularly when different products or conditions are being screened. A common measure of pretreatment quality is the coating weight, which usually is measured gravimetrically This method involves taking a pretreated panel or sheet metal part, measuring the area covered, weighing the panel or part, stripping the pretreatment from the metal substrate with an alkali solution and weighing again The result is given as weight/unit area, usually in the peculiar units of mg/ft2 If the coating weight is too low, then corrosion resistance probably will be compromised, but an overly heavy pretreatment means wasted material and possible cohesive failure within the pretreatment layer The definitions of too low and too high depend on the exact pretreatment and the end use of the pietreated metal.