Salt Spray Test Results

The 1000 h salt spray test supports the results obtained from EIS studies. At the end of the salt spray test, no corrosion 

This shows the excellent corrosion resistance of siliconized epoxy coating systems towards salt water. The surfaces of the siliconized epoxy-coated specimens looked bright even after removing the coating. This is due to the inherent water repelling nature of silicone, which avoided the transmittance of corrosive species through the metal surfaces by forming a protective insulative layer due to its surface active properties. 

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Fig. 7. ASTM B-117 salt spray test results of (a) the chromate conlrol on AA 2024-T3 (b) the acrylate-epoxy based system on AA 2024-T3 (c) the novolac epoxy-polyurethane based coating on AA 2024-T3 (d) the polyurethane-based coating on AA 6061 (e) the epoxy-based system on HDG and (f) the chromate control on HDG [30],...

Cerium nitrate and cerium chloride with additions of acetylacetone are, according to the salt spray test results, the most efficient corrosion inhibitors, when used as additives to hybrid organic-inorganic coatings. This was also demonstrated by the results of the EIS measurements. Cerium nitrate and, with reservations, cerium chloride in combination with acetylacetone are, according to the salt spray test performance, possible corrosion protection additives to the hybrid coating system used in this study. 

Figure 3.9 Salt spray test result for the coating systems AXl, BXl and Ax2, BX2.

Figure 3.16 Salt-spray test result of coating system 1 after 1000 h exposure of 3.5% NaCl. (b) Salt-spray test result of coating system 2 after 1000 h exposure of 3.5% NaCl. (c) Salt-spray test result of coating system 3 after 1000 h exposure of 3.5% NaCl. (d) Salt-spray test result of coating system 4 after 1000 h exposure of 3.5% NaCl.

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

The reproducibility of test results between labs using the neutral salt spray tests has not been consistent, but the repeatability, within one lab, is better, and the test has value in comparing variations in coating systems. Correlation of hours of exposure in the salt spray test to actual performance of the plated part in service, even in marine atmospheres, is not consistent and usually avoided. A classic example is that cadmium deposits outlast zinc deposits on steel in salt spray tests and clean marine atmospheres, yet zinc outlasts cadmium when exposed to real, industrial atmospheres, because of the presence of sulfur-bearing corrodents in industrial environments. An important variable in salt spray testing is the position of the surface to be tested. Whereas the surface of test panels is specified to be 15—30° from the vertical (40), when salt spray testing chromated zinc-plated specimens, this range has appeared excessive (41). 

Salt spray tests, humidity tests, and other accelerated tests, some usiag sulfur dioxide and carbon dioxide, have shown favorable results for tin—2inc ia comparison with 2iac, cadmium, and fin deposits. Chromating improves the performance. 

As of this writing the 2inc alloys are too new to have actual corrosion resistance data, except for that based on accelerated tests. Zinc—nickel usually shows better results than 2inc-cobalt in salt spray tests. The reverse is tme when the Kesternich test is used. Tin—2inc performs well in both salt spray and Kesternich tests, but appears only to equal 2inc plating and 2inc—nickel in humidity tests. 

Ductile and easily buffed chromium deposits having satisfactory corrosion resistance have been produced thus 0.005 mm-thick chromium deposits applied to steel by chemical deposition or by eiectrodeposition gave simiiar results when subjected to a salt-spray test . 

Resistance to corrosion Most authors who compare resistance to corrosion of electroless nickel with that of electrodeposited nickel conclude that the electroless deposit is the superior material when assessed by salt spray testing, seaside exposure or subjection to nitric acid. Also, resistance to corrosion of electroless nickel is said to increase with increasing phosphorus level. However, unpublished results from International Nickel s Birmingham research laboratory showed that electroless nickel-phosphorus and electrolytic nickel deposits were not significantly different on roof exposure or when compared by polarisation data. 

The results will also be influenced by the concentration of NaCl solution sprayed —some metals are affected more by one concentration than another — for example, zinc is corroded most by a concentrated brine (20%), while iron is corroded most by a dilute brine (3%) synthetic sea-water is less corrosive to these metals than either brine. In view of the many other ways by which the conditions within a salt-spray box differ from those of exposure to a natural sea-coast environment, there seems to be no great advantage in making-up complicated synthetic sea-waters for use in salt-spray testing. However, tablets for this purpose are commercially available. 

The salt spray test has seemed to yield the most consistent results when used to establish the relative merits of different aluminium alloys in resisting attack by marine atmospheres. The best results have been secured when the spray has been interrupted for so many hours each day . 

Thus it appears that by incorporating parameters such as pore resistance and coating capacitance to the existing theoretical impedance model dealing with metal dissolution one would obtain valuable overall information (14,27). Complemented by results from regular immersion and salt spray tests it should be possible to find satisfactory solutions to corrosion problems of coated metals (9 ). 

It Is clear on analyzing the results obtained from specimens 18 and 19 as well as from 20 and 21 that film degradation and coating Integrity can be followed more efficiently by Impedance measurements than by salt spray testing (Table II and Figures 1 and 2). 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

To determine if short-term electrochemical test results were related to longer term salt spray exposure results, a two-part test was conducted, and results were correlated to results from salt spray testing. The first part of the test involved measurement of impedance immediately upon exposure and after 24 hours of... 

Figures 31.23 and 31.24 show typical scanned images of SO2 and Prohesion salt spray-tested [7B] panels, respectively. Visual observation of these images reveals that the plasma-modified panels of [7B] have outperformed both control panels in the SO2 salt spray test. These plasma film combinations were prepared on deoxidized [7B] surfaces without any plasma cleaning pretreatment. Figure 31.23 also shows an image of a panel that had simply been deoxidized prior to the application of E-coat, which performed excellently in the SO2 salt spray test. Figure 31.25 compares the corrosion width obtained by the two methods. The comparisons shown in Figures 31.19, 31.22, and 31.25 indicates that the results obtained by the two methods do not match, partly due to the different duration of tests, and that samples which show good results in one test do not do as well in the other test.

Figure 31.29 summarizes the corrosion widths along the scribed lines that were calculated from (1) SO2 salt spray-tested and (2) Prohesion salt spray-tested A1 alloy panels and their corresponding control panels. As seen from Fig. 31.29, the corrosion test results showed that the plasma coating systems based on the chromate-free spray primers provided excellent corrosion protection for the A1 alloys studied. 

The corrosion widths of Prohesion salt spray-tested IVD Al-coated Al panels were calculated and are summarized in Figure 32.6. As is evident from the data, after 12 weeks of Prohesion salt spray testing, IVD/plasma polymer/spray paint systems showed better corrosion protection overall than IVD/plasma polymer/E-coat systems. All the IVD/plasma polymer/spray paint systems outperformed the cathodic E-coated controls and showed corrosion test results comparable to those of the Deft primer oated controls. 

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