Coatings Clinic: pretreatment of metals II: analysis and testing.

The quality of metal pretreatments can vary 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 preueatment 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 pretreated metal.

Coating weight is not the only criterion, however. An irregular or sporadic pretreatment may have a high coating weight, yet give poor performance when painted. It is very useful to be able to observe the surface morphology of the pretreatment, especially those that form crystals, such as zinc phosphates. The usual means of doing this is scanning electron microscopy (SEM). A light microscope is of little use because of the small size of most crystals and the fact that diey are transparent to visible light. Light microscopes that provide dark field or interference images with reflected light have been used effectively, however. The SEM enables the investigator to see crystal size, shape, and orientation as well as the uniformity of the coating.

Two x-ray methods, energy dispersive x-ray and x-ray diffraction, are useful for analyzing pretreatments. SEM/energy dispersive x-ray combinations are used routinely to examine pretreatments and to determine their qualitative elemental compositions, but coating weight data can be obtained by comparing x-ray intensities of test specimens to the x-ray intensities of prepared standards. With this method, a specimen is subjected to the electron beam of the SEM, which results in the emission of x-ray photons from the specimen surface (zinc, phosphorous, iron, and, possibly, nickel and manganese in the case of zinc phosphates on steel; aluminum, chromium, and phosphorous for chromium phosphates on aluminum). X-ray diffraction equipment is less apt to be found in the analytical lab of a paint supplier or user, but it commonly is used to identify the different crystalline phosphates produced by the zinc phosphating process, such as hopeite ([Zn.sub.3][([P0.sub.4]).sub.2] * 4[H.sub.2]0) and phosphophyllite ([Zn.sub.2]Fe[([P0.sub.4]).sub.2] * 4[H.sub.2]0). X-ray diffraction also can be used for quantitative measurements of the ratios of different crystal types and the total amount of each phosphate.

Another evaluation method is called the porosity test. It is an old but useful technique [R.W. Zurilla and V. Hospadaruk, SAE Transactions, Paper No. 780187 (1978)] based on the low electrical conductivity of zinc phosphate coatings. If a low voltage D.C. potential is applied to the (unpainted) phosphated metal panel, the current should be low due to the insulating property of the zinc phosphate. If coverage is incomplete or there are pores and channels going down to the substrate, then conductivity will be high. Porosity as measured by conductivity provides an indication of zinc phosphate quality. My lab used to do this measurement on a routine basis by monitoring the oxygen reduction current at -0.55 V vs. a saturated calomel electrode in a pH 12 sodium hydroxide solution. Values of 0-5 [micro]a/[cm.sub.2] were considered acceptable and one large series of specimens produced on different auto paint lines ranged from 0.3 to 5.5 [micro]a/[cm.sub.2]. In another investigation, a series of iron phosphates gave values of 8-21 [micro]a/[cm.sub.2] and salt spray scribe creep increased with increasing porosity. Bare steel gave values of 20-28 [micro]a/[cm.sub.2].

In conclusion, there are a number of ways of evaluaring metal pretreatments and if you are a user of such products or supply paint to go over them, it is a good idea to have the ability to test them.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].

By Clifford K.Schoff

Schoff Associates