Salt spray testing pitting

Salt Spray Test Panels coated with the standard chromate conversion coating and CMT were compared with each other in their corrosion resistant properties in several ways. The conventional 5% NaCl/S02 fog chamber tests showed excessive corrosion and pitting within one week on chromate conversion coated (COC) 7075-T6 A1 alloy panels. The CMT coated panels were almost uncorroded and without any pits. The plates in Figure 1 show the conditions of the panels after 7 and 14 days exposure in this environment. Even after 14 days exposure the CMT panels were still far better than COC panels. 

Pitting corrosion, on the other hand, occurs without gross delamination of the coating during the normal duration of salt spray tests (e.g., 1-3 months), and only removal of the coating after the salt spray tests can reveal the presence of pits. Pitting... 

Figure 31.18 Scanned images of Prohesion salt spray-tested (12 weeks) [2A] panels total scanned area 27 cm, total scribe length within the scanned area 16 cm the white spots on [2A] (Ace/0)/TN/E are paint which could not be stripped with Turco paint stripper (not pitting-corrosion sites).

Prohesion salt spray-tested panels in Figure 31.24 show that [7B] (Alk/AH)/T/ E and [7B] (Alk/0)/TH/E systems performed comparably to the controls. Deft primer-coated control panels ([7B] CC/A) displayed extensive pitting corrosion away from the scribe in both tests, indicating that Deft primer may have poor barrier properties. This pitting corrosion away from the scribe was observed on both controls when examined by scanning electron microscopy (SEM). 

Figure 31.28 Scanned images of Prohesion salt spray-tested A1 panels with chromated plasma coating systems prepared by anode magnetron plasmas Primer G could not be removed from the whole Al panel due to the strong adhesion, and only the portion with pitting corrosion occurred underneath could be stripped off with paint stripper solution.

It was noted (from Figure 32.14) that the [2I](0)/T/(Ar)/Dl specimen, which performed well in the SO2 salt spray test, exhibited much larger corrosion widths along the scribed lines, and much pit corrosion away from the scribed lines was visually observed after Prohesion test. The worse performance of Spraylat primer-coated samples likely resulted from its weaker adhesion to plasma-treated IVD surface than Dexter primer, which has been shown earlier from the adhesion test results. Another possible reason might be due to its inferior barrier properties to Dexter primer because there was no pit corrosion observed for Dexter primer-coated samples but many pits on Spraylat primer-coated panels after the Prohesion test. 

Rusting, pitting, and SCC tests frequently provide results with poor reproducibility. The percent surface rusted after a salt spray test can also be quite variable. The amount of rust covering a specimen after the salt spray test varies if the rust initiates at the top of the coupon and runs down over the test surface or initiates at the bottom and covers only a small portion of the specimen (Fig. 9). Rusting is also dependent upon the surface finish smoother finishes are usually more resistant. Because of these factors, it is difficult to use the salt spray test for quality control. 

In either the solution treated or aged condition, it is corrosion resistant to seawater, salt emd other natural environments, oxidizing media, inhibited reducing acids, alkalies, and metallic chlorides at room temperature. In salt-spray tests, aged Ti-13V-llCr-3Al exhibits no pitting and experiences no general corrosion or degradation in mechanical... 

Polarization curves of anodized AM50 after 21 days salt spray testing. side A, corrosion damaged -A- side B, no corrosion damage OCP, open circuit potential and PP, pitting potential (Song 0t at., 2006). 

Cui, 1., 1. Cai, and C. Jia, Pitting corrosion of high strength aluminum alloys in salt spray test, Zhongguo Fushi Yu Fanghu Xuebao, 30, 2010, 197. 

Salt spray testing (SST) was performed using unscratched bare and scratched sol-gel only and 1 8 PANI/sol-gel coated samples. Corrosion of the bare Al alloy surface takes place within 48 h, as shown in Fig. 7.17a. A performance improvement was noted for the sol-gel only coating where the number of pits observed on the surface... 

As shown in Fig. 10.5, the corrosion protection properties of the coating material containing cerium sulphate were very limited. After 336 h of salt spray testing, samples of both concentrations exhibited numerous relatively large pits. The lower... 

Chromate conversion coatings perform poorly in environments containing acidified chloride. In salt/S02 spray tests the substrate metal is heavily pitted after three to four days of exposure. In this work, a new coating was developed which itiproved the corrosion resistance of the conventional chromate coating remarkably. 

Results in Table I show that MIST test road salt spray alone did not produce blush and bloom although pitting was observed at the higher salt concentrations. Acid precipitation had a very pronounced affect at or below pH=4. This pH range is easily obtained on automobiles due to a drop in pH of the solution as a result of evaporation as well as the fact that the pH of precipitation has been less than 4 on many occasions. The appearance of the test panels was similar to that observed on vehicles after one or two years in Montreal or after several years in milder environments such as Detroit or Boston. 

The combination of acid precipitation with road salt spray produced the worst effects on anodized aluminum. As the MIST test pH became more acidic and the amount of salt increased the time to achieve an equivalent milky white appearance was reduced significantly (Table I). At more neutral high chloride concentrations severe pitting occurred while at more acid high chloride concentrations blush and bloom predominated. The latter environment is similar to that existing in the Northeast USA and Canada and, therefore, the results can explain the problems of blush and bloom in these areas. Corrosion surveys by automotive companies and trim producers in these areas have shown that blush and bloom and pitting have become increasingly more severe over the last ten years.5 These problems have led to a shift away from anodized aluminum as an automotive trim material in recent years. 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

Probably the most widely used test for pitting resistance is salt spray. B 117 describes the expeiimentfd arrangements to be used and mandates the use of 5 wt% NaCl at slightly elevated temperature (35X). The test chamber allows the solution to be sprayed within its confines, creating a salt fog environment around the coupons. Such a constantly moist, salty environment is quite aggressive. 

ASTM B 117 is the standard test used for evaluating the corrosion resistance of unpainted aluminum alloy surfaces when treated by anodizing, conversion coating, and for plating processes. Aluminum surfaces, which are anodized and then sealed for corrosion protection, must resist the salt spray environment for at least 336 h without pitting. Conversion... 

Automobiles and mUitaiy equipment frequently are exposed to marine environments and road salt. Beach test sites are available, but simulations in the laboratory are convenient. The ASTM B 117, Method of Salt Spray (Fog) Testing, has been widely used for this purpose to evaluate rusting, pitting, and SCC. Martensitic stainless steels and maraging steels have been exposed to salt spray (and other NaCl environments) to evaluate resistance to SCC prior to use in military equipment and fasteners. Ferritic and austenitic stainless steels have been tested to evaluate resistance to rusting prior to use in automotive applications. 

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