Corrosion Performance of EG^� Sheet

Electrogalvanized sheet steels are widely used in the automotive industry for exposed body panels. They offer the improved corrosion resistance of zinc while providing the excellent surface smoothness of cold-rolled steel for exposed, painted applications.

Although the ultimate test of corrosion resistance is how a product performs in its final configuration, accelerated laboratory testing is an important aspect in the initial evaluation of how different sheet products will behave in a corrosive environment. Accelerated tests can also provide an early insight as to how the components of a total coating system (i.e., substrate, pretreatment, primer, topcoat) will interact and may revel the mechanisms of corrosive failure. This Technical Bulletin will present Bethlehem�s results from salt spray and cyclic tests on unpainted EG sheet as well as painted cold-rolled and EG� sheet.

Salt-Spray:
Unpainted: Unpainted EG sheet was tested in salt-spray (ASTM B117) and our results are shown in Figure 1 for zinc coating masses from 20 to 110 g/m2. The test panels were flat with no deformations and were taken from standard production material. The results are typical of zinc coatings and show that the corrosion resistance is nearly a linear function of the coating mass.

Painted: Table 1 summarizes our results from salt-spray tests on flat, unformed panels of painted EG sheet and cold-rolled steel. Both were prepared in the laboratory using typical production materials with a standard automotive pretreatment, primer and topcoat. A scribe was then make through all coatings to expose the base steel. The test side of the coated panels had 45 g/m2 of zinc. The test period was 500 hours with brief interruptions for inspecting the specimens.

	EG	Cold-rolled
Scribe Creep @ 500 Hours, mm	10.4	0.03
Hours to First Rust	172	24
% Red Rust @ 500 Hours	3	100

The results above are typical values when comparing painted zinc-coated sheet with cold-rolled steel in salt-spray. EG sheet has better red rust resistance at the scribe than cold-rolled, but exaggerated scribe creep. The reasons for this phenomenon are discussed in more detain below.

Cyclic testing was developed to more closely simulate the conditions to which an automobile is actually exposed in service. These tests generally involve alternate periods of exposure to salt solutions and high temperature/high humidity. Often there are also occasional periods of high temperature/low humidity, freezing temperatures and exposure to impinging gravel.

Cosmetic Corrosion: Flat specimens of EG sheet with coating masses from 17 to 100 g/m2 and cold-rolled steel were treated with a standard pretreatment, primer and topcoats. A scribe was made through all coatings to the steel substrate. The prepared samples were than exposed to the environments and steps depicted in the flow diagram at the left. During the test, the number of cycles to first rust in the scribe was noted. At the end of the test, the samples were cleaned to remove loose paint and corrosion products, then evaluated for the amount of creepback at the scribe.

The results after 20 cycles of exposure are shown in Figure 2. This shows that lighter EG sheet coating masses are not effective in preventing scribe creep in this test. In general, however, electrogalvanized steel has significantly better creepback performance than cold-rolled steel and, Figure 3, greater resistance to red rust formation in the scribe.

Perforation Corrosion: Using the same exposure cycle described above, samples of EG sheet and cold-rolled steel were tested for perforation resistance. The panels were first deformed then covered with a standard automotive pretreatment and electrophoretic primer, no topcoats were used. A scribe was then made through to the base steel at the formed areas of the specimens. Coating masses for the electrogalvanized samples ranged from 17 to 95 g/m2.

After 250 cycles exposure to the test environments, perforation had occurred. The results are summarized in Figure 4 and show that, although not linear, the perforation resistance of EG sheet is still a function of coating mass. These results also show that even low coating masses can offer a significant increase in perforation resistance compared to cold-rolled steel.

Corrosion Mechanisms: The results reported above for salt-spray and cyclic testing of painted sheet are consistent with current theory. Closer examination of the underlying mechanisms at work during the corrosion processes helps to understand the performance observed.

Salt-Spray Tests: When painted cold-rolled steel is exposed to the salt-spray environment, the painted surface and steel exposed at the scribe are constantly covered with a liquid film. As the bare steel exposed at the scribe becomes covered with rust, it behaves as the anode, and the steel beneath the paint film becomes the cathode, of a galvanic cell. According to this model, creep proceeds by cathodic delamination. Recent investigations have shown an absence of chloride at the leading edge of the creep front which indicates that the under film region is behaving cathodically. The rate of creep would then be limited by the rate of migration of water and oxygen through the paint film and phosphate to the cathodic sites, and Fe²⁺ ions through the rust layers from the anodic regions. If such layers are good barriers, then the rate of scribe creep is relatively slow for cold-rolled steel in the salt-spray test, as reported above.

In a similar fashion, pretreated and painted zinc-coated steel will also have a liquid film covering the exposed steel at the scribe. In this case, the zinc coating beneath the paint film acts as an anode because of the difference in electrochemical potential with respect to steel which acts as a cathode at the scribe. A major component of subsequent delamination is undermining due to anodic dissolution of the zinc coating. This component can be relatively fast because of the easy access of water and oxygen to support the cathodic reaction at the scribe. There are indications of another cathodic region, located at the leading edge of the creep front. Here, delamination of the paint, and absence of chloride delamination is a second component of creep.

This evidence suggests that the relatively high creep rates for zinc-coated steel in the salt-spray test are the results of two, simultaneous mechanisms: anodic dissolution of the zinc coating and cathodic disbonding of the paint in advance of the dissolving zinc.

Cyclic Tests: In the cyclic test described above, the specimens are subjected to alternating cycles of oven bake, total immersion and exposure to high humidity. During the immersion step, the situation is analogous to the conditions in the salt-spray test. However, during exposure to humid conditions, which comprise the majority of the test time, very little moisture condenses on the paint film, thus limiting the through-film transport of water and oxygen needed for underfilm cathodic activity.

When phosphated and painted cold-rolled steel is exposed to the humid step, moisture condenses on the salt-contaminated rusty surface at the scribe. When this occurs, the scribe behaves cathodically and the underfilm region becomes anodic. Chloride at the leading edge of the creep front and attack of the steel substrate suggest anodic activity beneath the paint. Under these conditions, underfilm creep is no longer limited by mass transport through the paint and phosphate layers, and the rate of attack is greater. Since over 90% of the total exposure time of this cyclic test is in the humid step, the rate of scribe creep of painted cold-rolled steel is greater in the cyclic test than in the salt-spray test.

Exposure of phosphated and painted electrogalvanized steel to the humid step of the cyclic test results in a lack of condensed water on the paint film and a reduction in the cathodic disbondment component of creep. Scribe creep is governed mainly by anodic dissolution of zinc. However, it is believed that the driving potential for the galvanic activity of the zinc/steel couple is significantly lower at the temperature of the humid step (600C) compared to that of the salt-spray temperature (380), resulting in a lower zinc consumption rate and less scribe creep.

While the discussion above on the proposed mechanisms of scribe creep reflects current knowledge, it must be recognized that there is still much work in progress. The definitive answers to the mechanisms involved in these complex systems remains to be found.