Avaliação da resistência à corrosão de aços IF revestidos com zinco e ligas de zinco-ferro destinados à industria automobilística

Abstract

One of the most important changes in the last twenty years regarding steel application as a structural element, was the use of zinc and zinc-iron steel coated products for situations which require a major corrosion resistance. The consumer and competitor pressures responsible for the upsurge in the use of galvanized steel, which wasexperienced in the 1980s, have already been referred to. A further factor, which contributed to this, however, was the coincident uplift in the quality of the coated products. A much wider range of galvanized sheet products now became available from technologically advanced continuous strip production lines (both electrogalvanized and hot dip) offering coating specifications which recognized the special requirements of the automotive producer (surface finish, formability, soldability, thickness control, etc.).The corrosion resistance of interstitial free steel (IF), electrogalvanized and hot-dip galvanized with zinc and zinc-iron alloy, phosphatized and painted in the automotive industry, was evaluated by means of accelerated corrosion tests. Cyclic accelerated corrosion tests (SAE J2334:2003/Manual mode and GM 9540P:1997) and an Accelerated Outdoor Test by Intermittent Spraying of a Salt Solution (ISO 11474:1998)were conducted in order to evaluate the corrosion resistance of the steels, all of which are standard tests used by the automotive sector.For this purpose, ultra low carbon steels zinc (EG and GI) and zinc-iron (GA galvannealed steel) coated with coating mass of 60/60 g/m2 and 40/40 g/m2 were produced in an industrial scale. As a reference for the evaluation of the corrosion resistance, an ultra low carbon steel (IF), phosphated and painted were used.The test specimens used in this study were properly painted by an automotive industry.Before the accelerated corrosion tests the metallic coating (Zn & Zn-Fe), phosphatelayers and the dry paint film were characterized.As result, it was observed that, for the same zinc mass, the steel coated with zinc-iron alloy had presented better corrosion resistance than that of the steels coated only with pure zinc. The better performance of the zinc-iron alloyed material was attributed to its morphological characteristics. The better paint adhesion (adhesive bonding) experienced with the fissure/rougher surface.It has been demonstrated that the zinc-iron layer mass play an important role in the corrosion resistance of phosphated and painted steels galvannealed steels.As a result, it was found that using the test SAE J2334/Manual Operation took three completed tests (4320h or 6 months) to achieve measurable and reliable data, as with the test GM 9540P it took only one complete test (1920h or 2 to 3 months). To get thetest SAE J2334/Manual operation's results quicker it is necessary to act alone or together on the concentration of corrosive agents and/or the time of exposure to salt solution and humidity conditions of the specimens exposed in the test.The test GM 9540P, after 3840h, has showed very aggressive for the steel sheets used in this study, mainly for the galvannealed steel, GA40, of lower coating mass.The results of the accelerated outdoor test by intermittent spraying of a salt solutionhas showed the same tendency verified for the cyclic accelerated corrosion tests SAE J2334 (180 cycles) and GM 9540P (80 or 160 cycles).