Comparison of Corrosion Performance of Zinc and Iron-Zinc Alloy Coatings in Accelerated, Atmospheric, and Highway Automotive Tests

The corrosion performance of electro-plated and hot-dip zinc, galvannealed, and electroplated iron-zinc alloy coated sheet was investigated. Several coating thick-nesses of each material were tested after painting with a standard automotive paint. Four corrosion tests; Ford APGE and Chrysler cyclic-humidity, Volvo atmospheric and highway exposure were employed. The relative cosmetic and perforation corrosion performance of the materials in each test is presented. It is concluded that the realistic highway exposure test most accurately evaluates cosmetic corrosion resistance. The results show that a 5 to 10 micron (µm) coating of a zinc alloy with 10 to 20 percent iron is best. The result of the perforation testing, principally an extended Chrysler test show that the aformentioned iron-zinc alloy coating is as good as a 10 to 15 µm coating of electroplated zinc.

Because of the increasing interest by automobile manufacturers in use of zinc and zinc-alloy coatings for corrosion protection of body panels, U. S. Steel Research has conducted an extensive investigation to evaluate various coatings for this application. The investigation consisted of two parts, (1) initial screening and (2) a comprehensive qualification evaluation. The results of the first part were previously reported.^I)* Briefly, the screening included evaluation of the corrosion performance of three types of alloy coatings, (1) codeposited iron-zinc, (2) duplex iron and zinc layers, and (3) heat-treated iron-zinc duplex coatings and several commercial coatings. The materials were evaluated in an over-the-road corrosion test and in-door cyclic-humidity test as well as conventional salt-spray tests. Before testing, all materials were either painted with an acrylic three-coat paint system, including a cathodic electroprimer, or with a cathodic electro-primer by itself. These studies concluded that a 5 to 10 micron (µm) co-electroplated zinc alloy with 10 to 25 percent iron showed the most promise, and are the bases for the qualification evaluation, the results of which are presented in this paper.