Salt spray testing scribe

Salt Spray Test With Different Scribes... 

Figure 31.17 Scanned images of SO2 salt spray-tested (4 weeks) [2A] panels total scanned area 27 cm and total scribe length within the scanned area 16 cm.

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 31.29 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. 

Figure 32.3 shows the scanned images of SO2 salt spray-tested IVD Al-coated 7075-T6 panels one control, and two E-coated panels. The direct application of E-coat to IVD-coated panels (with no plasma treatment) did not provide corrosion protection as good as that of the chromate conversion oated control panel more corrosion creep from the scribed lines was observed on [7I]/E panels than on the [7pI]CC/E... 

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. 

In salt spray test rust appeared on the scribed lines of coated panels with progressive exposure of 1,000, 500 h. After 2,000 h, rust appeared on the scribed lines (<0.5 mm) on coated WS and increased (>1.5 mm) with 4,500 h exposure. 

For barrier coatings, the osmotic forces are much less than in the field in fact, they may be reversed completely from that which is seen in reality. In the salt spray test, corrosion at a scribe or defect is exaggeratedly aggressive compared with a scribe under intact paint. 

Magnified images of selected scribes on primer coated Al 2024-T3 test panels following salt spray testing, (a) darkened area of scribe and (b) shiny area of scribe. 

Figure 7. Paint adhesion loss in salt spray exposure (ASTM B117) as a function of ester content for chain-extended epoxy-amine and epoxy-ester resin based coatings. All coatings applied at 20-25 urn film thickness to SAE 1010 steel test panels, baked, scribed and exposed for 24 hours to salt spray conditions.

It is a well-established practice to test corrosion resistance of a coated panel by exposing a scribed coating layer to a corrosive environment such as salt spray for a prolonged period. Corrosion resistance of the coating is qualitatively evaluated by examining the corrosion that took place near the scribed line. Such a method certainly provides an estimate of the level of corrosion resistance of the coating however, this method does not yield information concerning the mechanisms of corrosion protection. 

After completing test cycles in Prohesion salt spray, the panels were rinsed with distilled water and visual observations were made. Then the panels are subjected to a commercial paint stripper solution (Turco 5469) to strip off the E-coat on the scribed surface to see the corrosion underneath the E-coat film and away from the scribe. 

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