Scribed test panels

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.

When a test panel is subjected to an environment that is less corrosive, such as the Filiform test, the water sensitivity of the adhesion of a primer shows more clearly. In the Filiform test, scribed coated panels are exposed to HCl vapor for a... 

Figure 28.23 shows the typical pictures of Prohesion tested sheets with different scribes and depths. From the visual examination of the tested panels, a general conclusion can be made that the U-type scribe resulted in much more corrosion through the corrosion test, but the scribe depth had very little effect on the corrosion test results. 

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). 

All corrosion-tested panels were scanned and the corrosion widths along the scribed lines were calculated as described in the experimental procedures. Figure 32.4... 

ASTM D1654 and corrosion delamination and corrosion around a scribe in a coating on a test panel or other test specimens. The ISO standard describes one method that involves the use of pictorial standards. Both standards include numerical rating of failure... 

Racks were made as per standard [12, 13] using mild steel angles and channels with height and width (1.50 x 2.10 m ). The racks were coated with epoxy paint to prevent rusting. Porcelain washers, brass nuts and bolts were used to fix the test panels at 45° with respect to base. Three types of test panels uncoated, coated and scribed coated were fixed in the racks and test racks at three exposure sites are shown in Figs. 2.6a, b and c. 

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. 

Corrosion performance was evaluated by the scab corrosion test. The coated panels were scribed and subjected to 25 cycles as follows 15 min immersion in 5% NaCl solution, 75 min air-dry at room temperature, followed by 22.5 h exposure to 85% relative humidity (RH) and 60°C environment. The tested samples were examined visually for failure such as corrosion, him lifting, peeling, adhesion loss, or blistering. The distance between the scribe line and the unaffected coating was measured as the corrosion creepage. 

Evaluation of salt fog (ASTM B-117) and humidity resistance (ASTM D-2247) after 300 h of exposure was conducted on scribed panels of cold rolled steel (CRS), phosphatized steel, oily CRS, and aluminum. In accordance with test protocols, performance was rated from 2 to 10,2 representing the most extensive rusting and largest blisters and 10 representing the absence of any rust or blisters. Additionally, the frequency of the blisters was assessed by indicating dense (D), medium (M), or few (F). 

Figure 10.8 Scanned image of the surface of two alloy panels showing adhesion failure caused by the omission of O2 plasma treatment of the substrate prior to plasma film deposition and application of the primer (Deft 44-GN-72 MIL-P-85582 Type I Waterbased Chromated Control Primer), a) Panel after Skydrol LD4 fluid resistance test, which had the O2 plasma treatment prior to film deposition and primer application, b) Panel after scribed wet (24-h immersion in tap water) tape test, which had not been treated with the O2 plasma treatment prior to film deposition and primer application.

After completing corrosion testing exposure, the panels were rinsed with distilled water and visual observations were made. The panels were then subjected to Turco 5469 paint stripper solution to strip off the E-coat or spray primers (including the controls) from the scribed surface, so that the effect of corrosion beneath the coatings and away from the scribes could be viewed. These panels were then used to estimate the average corrosion creep widths, in order to compare the corrosion performance of the different sample systems [5]. 

Pitting corrosion away from the scribe was observed on almost all panels of both controls, which have chromate conversion coatings, after the corrosion tests. Pitting... 

Scribed surface of E-coated panel with different surface pretreatments for corrosion testing... 

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. 

The results of the concurrent EIS measurement are consistent with what were found with Prohesion test results shown in Figure 28.23. The earlier failure of the coating system due to partial delamination at the interface resulted in severe corrosion on the U-shaped scribed panels. The concurrent EIS measurement reveals the importance of the lateral diffusion of salts initiating from the damaged interface. This situation could be explained by Figure 28.26, which schematically depicts the pathways of electrolyte to the sampling site of EIS measurement. 

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.

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. 

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