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issue: September 2007 APPLIANCE Magazine

Metals
A Comparison of Rolled and Polished Stainless-Steel Appliance Finishes


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by Catherine Houska and James Fritz, PhD, TMR Consulting

This study compares the corrosion performance of rolled and polished stainless steel used in appliance finishes.

Figures 1 (Sample 38, left) and 2 (Sample 12, right): Rolled-on Type 430 stainless-steel finishes from two suppliers

The durability, cleanability, and attractive appearance of stainless steel make it a popular choice for appliances and for architectural applications. The most commonly used stainless-steel finishes have a directional linear polish pattern (i.e., ASTM A480, Nos. 3 and 4 finishes) and are achieved by two primary methods. The traditional and most common method is to apply the finish using abrasive polishing belts, which will be referred to as “polish.” The second method used to impart a directional linear pattern is to press it into the stainless steel, which will be referred to as a “rolled-on” finish. Rolled-on finishes are produced by passing a coil of stainless steel through specially prepared rolls on a temper mill, which presses a pattern into the surface, simulating the appearance of the more traditional abrasive polished finish. Some suppliers of rolled-on finishes have claimed that these finishes provide better corrosion performance than polished finishes and that a rolled-on finish on Type 430 stainless steel provides equivalent corrosion performance to a polished Type 304 finish. This investigation uses laboratory measurements to determine the relative corrosion performance of abrasively polished and rolled-on finishes on Type 340 and 430 stainless steel.

The Corrosion Study

The stainless-steel samples used in this investigation were obtained directly from the surface finish suppliers and from the inventory of various stainless-steel service centers. Only samples that could be identified by surface finish supplier were evaluated. The goal was to obtain similar finishes from multiple sources so that the sampling was representative of the typical products supplied to fabricators.

In total, 41 samples were tested. This survey included Types 304 and 430 stainless-steel samples with rolled-on finishes from the two primary North American producers of this finish. The remaining samples were abrasively polished Types 304 and 430 samples from nine different suppliers. Only one of the abrasive-polished samples was from a supplier outside of North America. Several highly polished ASTM A480 Nos. 7 and 8 samples were obtained as part of this group so that their relative corrosion performance under these conditions could simultaneously be documented.

The type of polishing (i.e., wet versus dry), abrasive media, and other aspects of finish application determine whether microcrevices and other surface imperfections are likely to be created. Corrosion can start prematurely if there are inclusions; microcrevices; particularly rough or torn finishes; deep, narrow grooves or gouges; or other surface textures that trap corrosive substances. A broad range of surface textures are possible within the rather general and imprecise requirements of ASTM A480 Nos. 3 and 4 sheet finishes. Therefore, it was important to obtain a wide range of polished samples and to document the surface condition prior to testing. Table I summarizes the finishes, surface-roughness data, and finish characteristics observed under high magnification prior to corrosion testing.

The most accurate means of comparing the corrosion performance of different metals and finishes is to place samples in the service environment and wait to see if corrosion appears over time.  For these tests to be accurate, all of the samples must be exposed to exactly the same conditions (including atmospheric pollution, salt deposits, weather conditions, maintenance frequency, and cleaning solutions) for extended periods of time. Atmospheric corrosion testing is helpful for determining the relative performance of metals in exterior applications, but these tests are expensive and time-consuming. Most materials decision makers use accelerated laboratory corrosion testing to compare relative performance, but the testing must be representative of the service environment to be relevant.

The accelerated laboratory tests that are commonly used to compare the relative corrosion resistance of stainless steels generally provide a ranking of the material’s relative resistance to a specific mode of corrosive attack but do not necessarily predict performance in particular environmental conditions.  For example, test methods such as ASTM G48 and G150 measure the relative resistance to chloride pitting and/or crevice corrosion in seawater and industrial environments.

Salt spray or fog cabinet testing is more representative of constant exposure to seawater spray or splashing because high salt concentrations are deposited on the surface and continuously dampened, creating a continuously corrosive environment. Unless an application is on an offshore platform, such a severe environment would be unusual in actual practice. Even on applications such as docks and piers, stainless steel is only likely to be subject to saltwater spray or splashing during high tides or storms, so exposure to saltwater spray is not typically continuous.

Unfortunately, these tests bear little resemblance to the cyclical wetting and drying of surfaces that occurs in most applications.  Furthermore, they produce rapid severe corrosion of the less corrosion-resistant stainless steels that have a long record of success in interior and low-corrosion exterior environments, namely Types 430 and 304. This rapid corrosion in laboratory tests makes comparisons of surface finish, which contributes to corrosion performance, more difficult to assess.

The goal of this study was to rank the relative corrosion resistance of stainless steels in more common mildly aggressive environments with cyclic wetting and drying periods that can concentrate surface contaminants. ASTM G85 includes a wide range of corrosion test procedures. The A5 test was designed for testing painted carbon steels and uses a dilute salt spray, specifically a 0.05% sodium chloride (salt) and 0.35% ammonium sulfate and water solution. Samples are exposed for 500 hours to a continuous cycle of 1 hour of salt spray followed by 1 hour of forced-air drying. Although it is still a highly accelerated test, it more closely simulates real-life exposures.

Unfortunately, without modification, this test is not appropriate for testing stainless steel. Ammonium sulfate inhibits or prevents stainless-steel corrosion. Even without the ammonium sulfate addition, the very low sodium chloride addition might not initiate pitting corrosion, or it would produce very low Type 430 and 304 corrosion rates, making sample comparisons difficult within the test period.

European researchers developed a modified ASTM G85 A5 test in an effort to find a relatively short-duration test for stainless steel that would produce results that were more representative of real-life performance. They also wanted the test to be suitable for the less corrosion-resistant stainless steels used in architecture and appliance applications. The length of the ASTM G85 A5 corrosion test and the cyclical wetting and drying cycles were retained, but ammonium sulfate was eliminated from the electrolyte solution, and the sodium chloride (salt) concentration was increased to 3%. The results obtained by the researchers more closely correlated with long-term atmospheric testing and practical industry experience than traditional salt-spray tests. Furthermore, this test method was found to have sufficient sensitivity to rank the quality of abrasively polished surfaces.

Because of the reported success of this experiment, these same ASTM G85 A5 modifications were used to test the rolled-on and abrasively polished stainless-steel samples included in this study. The corrosion testing was done using computerized testing equipment specifically designed for this test at an independent corrosion lab.

Prior to testing, all of the samples were examined and photographed under high magnification to document the surface condition. The presence and frequency of gouges, tearing, overlapping areas that create microcrevices, and other imperfections that increase the probability of corrosion were noted. Since the relationship between surface roughness and corrosion performance is well documented, the surface roughness (Ra, Ry, Rz, and Rq) was measured. The samples were degreased and cleaned, and the edges were taped prior to corrosion testing. During the course of the test, the samples were photographed and examined at 100-hour intervals to determine whether corrosion was visible and to document the percentage of the surface that was affected. The initial surface evaluations and test results are summarized in Table I.

PDP = Proprietary directional polish PNDP = Proprietary nondirectional polish

Type 430 Samples

With the exception of a Type 430 sample with an ASTM A480 No. 7 polished finish, these samples had corrosion staining covering 16 to 50% of their surfaces at the conclusion of the test. The sample with the No. 7 buffed finish only had corrosion staining on 10 to 16% of the surface. It also had the lowest surface roughness.

Four of the 12 Type 430 samples tested were rolled-on finishes from two different producers. Figures 1 and 2 show representative rolled-on Type 430 finishes from each producer after corrosion testing. Both had corrosion staining covering 33 to 50% of the surface. Figure 1 has less corrosion staining and is the smoother of the two finishes (Ra 31). Figure 2 had a surface roughness of Ra 49, and under magnification, there was visible surface gouging that was probably the result of damage caused by worn pattern rolls and galling. The results shown in Figure 1 also appeared to have been produced by worn rolls but, rather than gouging, it had a less well-defined surface pattern. It was found that 75% of the rolled-on Type 430 samples had corrosion staining covering 33 to 50% of their surfaces.

Figures 3 (Sample 43, left) and 4 (Sample 11, right): Mechanically polished, ASTM A480 No. 3 finishes on Type 430 stainless-steel finishes from two suppliers

Figures 3 and 4 depict abrasively polished ASTM A480 No. 3 samples. Under magnification, the surface of Figure 3 was generally cleanly cut, and it had somewhat lower corrosion levels (16 to 33% of the surface) than the rolled-on finish samples. Figure 4 was representative of the poorest quality abrasively polished finishes tested with 33 to 50% of the surface exhibiting corrosion staining. Under magnification, it had visible surface tearing, smearing of the surface (which created microcrevices), and an uneven appearance. It appeared to have been produced by dry polishing with a worn aluminum oxide abrasive belt. It was found that 43% of the polished Type 430 samples had corrosion staining on 16 to 33% of their surfaces while only one (25%) of the rolled-on Type 430 samples was in this surface corrosion range.

Based on the researchers’ assessments of the percentage of surface area that was affected by corrosion staining, the abrasively polished Type 430 finishes statistically provided somewhat better overall corrosion performance than the Type 430 rolled-on finishes. Therefore, the performance difference was minor; the average randomly obtained off-the-shelf sample of Type 430 rolled-on finish did not provide better corrosion performance than the average Type 430 abrasively polished finish.

Type 304 Samples

On average, the Type 304 samples tested had considerably less visible corrosion staining than the Type 430 samples. At least minor corrosion staining was expected on all of the samples tested because of the relatively high salt (sodium chloride) exposure levels. The lowest corrosion rates (less than 0.03% of the surface) were found on the smoothest, abrasively polished Type 304 samples. This included an ASTM A480 No. 8 sample and proprietary finish samples with visible, directional fine polishing lines equivalent to a very fine No. 4 finish. The corrosion on these very smooth abrasively polished samples was so low that close visual examination was necessary to see the scattered very small pits. The lowest corrosion rates were associated with low surface roughness levels and surfaces that were very cleanly cut (no surface tearing or microcrevices) when examined under high magnification.

Figure 5 (Sample 19): A representative rolled-on Type 304 stainless-steel finish

Figure 5 is representative of the Type 304 rolled-on finish samples that were tested. When the rolled-on samples were examined under high magnification, some level of roll wear was evident on all of the samples, and there were deeper, narrow valleys and other surface irregularities that could trap corrosive contaminants. The performance of the rolled-on Type 304 samples was compared with polished Type 304 samples (including proprietary finishes) that had the appearance of a No. 3 or 4 finish. It was found that 43% of the polished Type 304 samples provided equivalent or better corrosion performance than the rolled-on Type 304 samples.

 

Figures 6 (Sample 10, left) and 7 (Sample 33, right): Samples from two suppliers of Type 304 stainless steel with an ASTM A480 No. 4 finish

Figures 6 and 7 illustrate the range of abrasively polished ASTM A480 No. 4 finishes tested. Figure 6 had more surface staining than Figure 5 (16 to 33% of its surface), and Figure 7 had very little corrosion staining (1 to 3% of its surface). When examined under high magnification, it was apparent that the reason for the higher level of corrosion staining on Figure 6 was poor polish quality. It was a visibly nonuniform finish with a rough texture and some surface tearing. In comparison, Figure 7 had an evenly cut finish with no visible tearing or microcrevices. The surface roughnesses of the two samples were similar (Ra 11 and 6, respectively).

Of the No. 3 or 4 finishes (including proprietary) on Type 304, more than half (54%) had less corrosion staining than any of the rolled-on Type 430 samples. Furthermore, 96% of the Type 304 abrasively polished finishes with a No. 3 or 4 surface appearance provided better corrosion performance than 75% of rolled-on Type 430 samples that were tested.

The better-quality abrasively polished Type 304 samples with cleanly cut finishes consistently provided better corrosion performance than the Type 430 rolled-on finish samples. Experienced polishing houses that understand the relevant variables can consistently produce high-quality finishes. Therefore, based on independent, random, off-the-shelf sampling, the claim that Type 430 rolled-on finishes are equivalent to abrasively polished Type 304 finishes is not valid.

Conclusions

These results do not confirm previously published claims that rolled-on Type 430 stainless-steel finishes were equivalent to abrasively polished finishes on Type 304 stainless steel. In fact, even poor-quality abrasively polished Type 304 exhibited less corrosion staining than 75% of the Type 430 rolled-on finish samples tested. These samples were from the two major producers of rolled-on finishes. Furthermore, on Type 430, it was shown that good-quality No. 3 abrasively polished finishes can provide better corrosion performance than most off-the-shelf rolled-on finishes. The good-quality abrasively polished No. 4 finishes on Type 304 also provided equivalent or better corrosion performance than the rolled-on finishes on Type 304. 

All of the off-the-shelf rolled-on finish samples appeared to have been produced by rolls with at least some wear, as was evidenced by examination of these surfaces under magnification. Probable galling damage was also observed. It is possible that rolled-on finishes produced using new rolls would have exhibited better corrosion performance, but the samples used in this test were representative of the material that is actually stocked at multiple service centers for use by customers. Good-quality polished finishes from multiple service centers were consistently equivalent to or outperformed rolled-on finishes when both were applied to the same type of stainless steel.

Appliance companies that are concerned about corrosion staining and want linearly textured, directional polished finishes are encouraged to work with experienced suppliers of high-quality products. Specification of variables such as surface roughness, abrasive belt type, and polishing method (wet versus dry) should be considered. Use of general categories of specification for surface finish (e.g., ASTM A480 No. 4) are not currently precise enough to assure uniform appearance and corrosion performance. Surface roughness provides an important indication of corrosion performance, but other factors such as wet polishing, use of silicon carbide belts, frequency of belt replacement, and quality control can be more important determinants of corrosion performance.

About the Author

Catherine Houska is a metallurgical engineer with more than 26 years of experience. She specializes in stainless-steel finishes, selection, and atmospheric corrosion in aesthetic applications.

James Fritz, PhD, is a metallurgical engineer and NACE-certified materials selection design specialist with more than 30 years of experience with stainless steels and other corrosion-resistant alloys.

If you wish to contact Houska or Fritz, e-mail lisa.bonnema@cancom.com.

 

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