Use of the scribe and spotface techniques to assess the painted galvanised steel underfilm corrosion A.N.C. Costal J.E.R. Carvalho,^ 1C da Silva,^ P.R.F. Ribas,^ J.C. dos Santos^ ^Department ofmaterials Sciences, Escola de Engenharia Industrial Metalurgica De Volta Redonda, UFF, Av. Dos Trabalhadores, 420 Pb/fa ^6foW^ &/27260-740 BmzzV EMail: ribas@metaleeimvr.ujf.br ^Research Center ofcompanhia Siderurgica Nacional (CSN) ^Generals Motors do Brasil Avenida dos Estados, 2880, Sao Caetano do Sul, SP 09501-970 Brazil Abstract The increasing interest of the steel industry on materials with higher corrosion resistance has led to a continuous development of the zinc-coated steels as well as the corrosion process evaluation techniques. One major goal of the new analysis methods is to reduce the assessing time. A common technique consists in scribing the painted galvanised steel surface in order to expose the steel substrate to the corrosion environment. In this work, an alternate analysis technique for assessing the underfilm corrosion propagation is used with the major benefit of reducing the time of analysis, without altering the basic corrosion mechanisms. In this method, industrially-painted galvanised steel sheets have the painting and coating removed from a 13-mm diameter spot in order to increase the cathodic/anodic area ratio. This increase leads to a lower sacrificial layer protection power thus reducing significantly the total time of the corrosion test as compared to the time consumed during the "scribe" method test. In addition, a new method is proposed for measuring the corrosion propagation which consists in measuring the corroded area under the organic painting thus determining the average corrosion propagation under the organic coating.
218 Surface Treatment 1 Introduction The improvement of the painted zinc-coated steel sheets corrosion resistance depends on the assessment of the corrosion propagation under the protective coating. Such progress may only be quantified if monitored through reliable evaluation techniques. In general it is desired to know if a new material presents a good corrosion resistance when compared to well known materials. This fact makes the corrosion resistance evaluation usually a comparative technique. The steels currently used in the automotive industry present a very good corrosion resistance if are coated with zinc or its alloys followed by a phosphate layer and organic painting. Nevertheless, some small damages may occur to the painting layer causing a dramatic acceleration of the corrosion process, a fact that probably happens during the usual service of vehicles. The accelerated corrosion tests are based on the fact that if a small damage to the painting and metallic coating layer is present, the steel substrate will be exposed directly to the environment. Atfirst,the steel is cathodically protected by the zinc coating layer which in turn is consumed during the corrosion process. For this reason, the corrosion resistance of the coated steels may be associated to the zinc layer corrosion rate or, in other words, to the corrosion propagation rate under the organic layer. The technique used largely in the automotive industry for preparing samples for the accelerated corrosion tests consists in scribing the sample surface such that part of the metallic coating and steel substrate are exposed to the aggressive ambient. Since the damaged area is small, the corrosion propagation under the organic layer is delayed for a certain amount of time. Jordanfl] employed a new type of damage removing a 13-mm diameter circular area of the painting and zinc coatings thus obtaining a larger cathodic/anodic ratio when compared to the "scribe" technique and consequently reducing the "incubation" time. In Jordan study, different coated steels with the same organic coating and the two types of paint damages were subjected to atmospheric corrosion tests (field tests). A similar work was performed by Carneiro[2] who employed different coated materials in accelerated corrosion tests. Both works showed that the "spotface" damage may lead to a significant time reduction for the overall tests when compared to the "scribe" damage. It is worthwhile to emphasise that the spotface technique only brings some benefit if there is a sacrificial metallic layer present or else the corrosion propagation will begin immediately after the ambient exposure. In order to assess the corrosion propagation under the organic layer, Jordan[l], Carneiro[2] and Costa[3] measured linearly the distance from the corrosion front to the original damage limit, by choosing randomly the base points - this is the usual procedure adopted by industry. This measurement method does not allow a good accuracy since corrosion does not propagate uniformly and the millimetre scale used for determining the corrosion advance is not adequate for small lengths. In this work, a new method is proposed for measuring the corrosion propagation which consists in determining the corroded area under the organic painting. We also intended to evaluate the time reduction obtained when using
Surface Treatment 219 the spoftace technique as compared to the scribe technique for damaged painted zinc-coated steel sheets subjected to accelerated corrosion tests as well as to analyse the spotface technique as an alternate method for comparing the corrosion propagation of different material systems. 2 Experimental procedures Two different material systems were studied: (i) zinc-electroplated steel sheets and (ii) steel sheets coated with zinc by immersion and annealing ("galvannealing"). Both materials were painted in an industrial plant, receiving a phosphate coating followed by organic painting. The samples for the accelerated corrosion tests were selected after characterisation of the various layers in order to avoid any significant coating thickness variation from sample to sample as well as paint adherence problems. Thickness of the zinc layer and paint layer were 9.6 and 75 jim, respectively for all samples. The samples were cut in 300 x 100 mm rectangles and had their edges insulated with a special paint in order to avoid localised corrosion on these regions. Two types of damage to the painting and zinc layers were prepared in each panel to permit a comparison between results. The "scribed" region was prepared with a stylus in a straight 40-mm line and deep enough to reach the steel substrate. The "spotface" region was prepared with a 13 mm end mill which went 0,4 mm down to the steel substrate. Figure 1 presents a schematic drawing of the damaged panels. The panels were subjected to a corrosion accelerated test usually employed by the automotive industry namely "scab corrosion test", consisting of submitting the panels to a cyclic exposure to different aggressive environments. Each cycle lasts for one week and consists of five steps. In thefirststep, 24-hour long, the samples were submitted to hot humid circulating air (70 C) for 1 hour; cold air (-10 C) for 30 minutes; immersed in a 5% NaCl solution for 15 minutes; exposed to ambient air for 01:15 hours; and finally maintained in a camera with controlled temperature (60 C) and relative humidity (85%) for 21 hours. In the second, third, and fourth steps, each lasting 24 hours, the samples were immersed in a 5% NaCl solution for 15 minutes; exposed to ambient air for 01:15 hours; and finally maintained in a camera with controlled temperature (60 C) and relative humidity (85%) for 22:30 hours. In the 72-hour long fifth step, the samples were immersed in a 5% NaCl solution for 15 minutes; exposed to ambient air for 01:15 hours; and finally maintained in a camera with controlled temperature (60 C) and relative humidity (85%) for 70:30 hours. After the corrosion test, a dry air jet was applied transversally to the panel damages in order to remove the paint above the corroded region. An adhesive tape was applied to the damaged area to remove any remaining paint. The advance of underfilm corrosion was determined with a millimetre scale by measuring the perpendicular distance from the damage edge (starting point) to the corrosion front. At the scribe, four equally-spaced points on each side were measured while eight equally-spaced points were used around the spotface damage.
220 Surface Treatment.paint STEEL MAGNIFIED CROSS-SECTION OF SPOTFACE O PLAN VIEW OF TEST PANEL MAGNIFIED CROSS-SECTION OF SCRIBE FIGURE 1: Schematic drawing of the damaged panels for the corrosion tests. The alternate method proposed in this work to improve the measurement accuracy consisted of measuring the corroded area thus calculating the average linear advance of corrosion instead of measuring directly the linear advance with a scale. In this alternate method, after the paint removal, each damaged region was photographed with a 4-time enlargement and processed with an image analyser to determine more precisely the corroded area. A simulation was used in order to compare the results with both methods with three different experimenters performing the same measurements after the first corrosion test steps. 3 Results and Discussion One set of samples was used to compare the results obtained with the two measurement methods - linear corrosion advance and corroded area. Infigures2 and 3, the mean corrosion advance obtained from three different experimenters is plotted for all three samples of each test step - samples #1 to #3 correspond to the same step, and #4 to #6 to another step. The corroded area measurement results showed much lower 95% confidence intervals as compared to the linear corrosion advance method. In spite of the fact that this measurement method is more accurate, the confidence intervals, observed in the corroded area measurements for each set of three samples used in the corrosion test steps (figures 4 to 6), are not necessarily smaller since the corrosion process does not propagate uniformly from sample to sample. The linear corrosion advance measurement method was not accurate enough to determine the beginning of the corrosion propagation step.
Surface Treatment 221 1,2; 1,1- _1,0; 0,9; 1 OJ 0,8-0,7- to.fr o 0,5" C/) g 0,4-00,3; 0,2; 0,1- i i - i i - i i 1. : Area method A Linear advance method ^ T T A i i _ : ' - ': i.. ] A A *» J- * 1 2 3 4 5 Electroplated samples FIGURE 2: Corrosion propagation around the scribed region for each individual sample according to the two methods: linear advance and corroded area measurements. Corrosion propagation (mm)5,5-5,0-4,5-4,0-3,5-3,0- _P Area method 1 I A Linear advance method ^ Hh - I'- = I ; 1 2 3 4 5 Electroplated samples FIGURE 3: Corrosion propagation around the spotface region for each individual sample according to the two methods: linear advance and corroded area measurements.
222 Surface Treatment Comparing the corrosion propagation in the two regions (scribe and spotface) for the galvannealed material (figure 4) it is clear that the corrosion propagation around the spotface region starts earlier than at the scribed region. This was also observed for the zinc-electroplated material. Once the underfilm corrosion started it propagated at about the same rate in the two regions. The panel regions with the scribe damage did not present any underfilm corrosion propagation during the first three steps (about 72 hours of exposure) thus characterising the incubation step. This can be seen in all figures presented in this work, independently of the material system. The spotface region of both materials presented corrosion propagation after the first corrosion test step thus showing no or a minimal incubation step. Figures 5 and 6 present the curves obtained with the corroded area measurement method in each damaged region for the two materials showing a significant difference in corrosion behaviour. Although both material systems presented the same metallic coating weight and organic coating thickness, the different origin and method of metallic coating application seemed to interfere in the corrosion properties. The same behaviour was observed in both types of damage (scribe and spotface) and indicated that the use of the spotface technique can save test time since it may give the same information obtained with the scribe technique. When the linear advance measurement was used the confidence intervals for each material system intercepted each other in some steps, making it difficult to obtain a good discrimination of the corrosion resistance between materials. mm spotface scribe ion propaga r\j.&. o O 0 20 40 60 Time (days) 80 100 FIGURE 4: Underfilm corrosion propagation in the painted galvannealed steel measured by the corroded area method.
Surface Treatment 223 O) 8-7- 6-5- 4-3- 2-1 - galvannealed electroplated 5 "-I 20 40 60 Time (days) 80 100 Figure 5: Comparison between corrosion propagation around the scribed region, measured by the corroded area method in the two material systems studied in this work. 18-16- 14-12-.1 c5 10 mm galvannealed - electroplated Figure 6: 8-2 CL 6- m g 'CD 4-2o 2- O 0- -2 20 40 ( Time (days) 80 100 Comparison between corrosion propagation around the spotface region, measured by the corroded area method in the two material systems studied in this work.
224 Surface Treatment 4 Conclusions The results presented in this work showed that the method for assessing the corrosion propagation under the organic film through the corroded area measurement, proposed in this work, presented a better accuracy than the usual method of measuring the linear advance of the corrosion front. It showed to be suitable for determining the onset of propagation of corrosion at the scribed region. The material systems subjected to accelerated corrosion tests presented underfilm corrosion propagation around the spotface region immediately after exposure to the aggressive environment. At the scribed region, this propagation started after about a 4 day exposure. The corrosion propagation rate was similar for the two types of damage. The spotface method gave similar results as the scribe method regarding to the discrimination between the corrosion behaviour of different material systems with the advantage of reducing significantly the assessing time. Acknowledgements The authors wish to acknowledge the steel company Companhia Siderurgica National for the materials and laboratory facilities used in this work, the General Motors of Brazil for the industrial painting of the coated steel panels, the Brazilian National Research Committee (CNPq) and Fundagao de Amparo a Pesquisa no Rio de Janeiro (FAPERJ), for the partial project funding. References 1. Jordan, D. L., Measurement of Underfilm Corrosion Propagation by Use of Spotface Paint Damage", Corrosion/95: Paper Number 384, Houston, Texas: Nace International, pp. 1-18, 1995. 2. J. C. Cameiro, "Avalia$ao da Tecnica "Spotface" Empregada para Analise da Corrosao de A$os Revestidos'% Master thesis, Universidade Federal Fluminense, 1998. 3. Costa, A. N. C.; Ribas, P. R. F., "Analise Comparativa das Tecnicas "Spotface" e "Scribe" na Avalia^ao da Resistencia a Corrosao de A$os Revestidos e Pintados", Congresso Brasileiro Engenharia e Ciencias dos Materials - Anais do CBECIMAT, pp. 530-537, 1998.