Exposure to road salts, UV radiation, heat, moisture and chipping from kicked-up road debris can quickly degrade an automotive coating system.
Icing salt suffer from salt-containing atmospheres. PV modules have been tested since 1995 according to IEC 61701 Ed. 1 ‘Salt mist corrosion testing of photovoltaic (PV) modules’. The test procedure corresponds to IEC 60068-2-11 with constant test conditions. Since 2012, PV modules are tested according to the 2nd edition which. The ASTM B117 standard for operating a salt fog or salt spray cabinet, aka the “Salt Spray” or “Salt Fog” test, is a widely used corrosion test required by many companies and government agencies, including the US DoD and DoE. The purpose of salt fog testing is to collect corrosion resistance data.
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Exposure to road salts, UV radiation, heat, moisture and chipping from kicked-up road debris can quickly degrade an automotive coating system. To continually improve coating performance, companies seek to mimic and accelerate real-life exposure conditions in the laboratory.
Recent years have seen a marked improvement in automotive coatings technology, and laboratory testing has played a key role.
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A common test involves the purposeful damage of the coating layer to determine a property referred to as rust creep, a quantitative measure of how far the corrosion travels along the substrate/coating interface to either side of a mechanically-induced scratch during exposure to a corrosive environment—typically a neutral salt spray or cycle corrosion test. Rust creep results are used to determine whether a coating process meets the automotive manufacturer’s stringent requirements.
The scribing process can be performed on standardized flat test panels, or on the actual automotive component, if size and geometry permit. Some test specifications call for a single linear scribe, whereas others specify an intersecting set of two scribes, such as seen in Figure 1.
Test Panel Prep
Testing was performed on off-the-shelf 3- by 5-inch cold rolled steel test panels. Fifty panels underwent a zinc phosphate pretreatment followed by electrocoating using a high edge build (HE) cathodic epoxy formulation. Of these, half were set aside and labeled as group “E.” The remaining panels had a powder coating applied on top of the electrocoated surface and were labeled as group “P.” Using ferromagnetic techniques, average coating thickness was determined to be 25 and 100 microns, respectively.
Four scribing tools commonly used by the automotive industry were used to scribe the test panels. The tools are described in Table 1 and shown in Figure 2.
Table 1. Description of Scribing Tools Used in This Study
Scribing Tool | Description |
Sikkens Model 463 | Square blade |
Van Laar Model 426 | Rounded Tip |
Clemen Model 428 | Rounded Notched Tip |
Box Cutter | Razor Blade |
Tech A
Tech B
Square: Sikkens (463)
2
2
Rounded: Van Laar (426)
2
2
Notch: (428)
2
2
Razor Blade
2
2
Total Panels Each Coating System:
16
Image a.
Image c.
Exposure and Measurement
The 16 sectioned panels were taped along the cut edge to minimize rust run-off, then all 32 panels were placed in a programmable cyclic corrosion chamber using a slotted panel rack. The panels were exposed to six cycles of ISO 11997-1 (2005), Cycle B, a standard cycle corrosion test consisting of a combination of neutral salt spray exposure according to ASTM B117/ISO 9227, 100 percent condensing humidity according to ISO 6270-2 CH, drying and dwelling. One full corrosion cycle is a week in duration. Figure 5 shows a typical panel after the six-week exposure. Rusting was observed at the panel hanging hole as well as along the scribe. No other surface rusting or edge rusting was observed on any of the panels.
a.
Conclusions
The variation of the rust creep was surprisingly low as a function of scribe type and operator. The standard formula for calculating rust creep is as follows: Creep = (wc-w)/2, where wc is the average raw creep measurement and w is the original width of the scribe. Once this formula was applied to the raw data shown in Figure 6, the variation in calculated rust creep was only 0.6 mm across all four scribing tools and operators for the electrocoated panels and only 0.4 mm for the powder coated panels.
Other conclusions resulting from this study include:
- The Sikkens scribe provided the most uniform rust creep readings among operators.
- The rust creep measured along the scribe made by the relatively narrow razor blade resulted in the greatest standard deviation between individual rust creep measurements for both the Group E and Group P panels. This was likely attributed to the relatively deep penetration into the substrate and significant deformation and non-uniform displacement of the coating.
- The rounded carbide tip of the Van Laar 426 scribe resulted in the lowest average observed rust creep. This characteristic is likely the result of less aggressive damage imposed on the surface of the test panel and minimal tearing of the coating.
- Addition of the powder coating layer lowered the overall degree of rust creep for all scribe types used in the study.
- There was a correlation when comparing Operator A to Operator B to rust creep measurement, in that Operator A’s scribes resulted in lower creep measurements in six of the eight test conditions. This indicates that scribing characteristics, such as applied pressure and general technique, may influence final rust creep measurements.
Tom Ackerson, PE is the laboratory Director at IMR Test Labs, Louisville; he can be reached at [email protected]. Jennifer Breetz is also with IMR Test Labs and can be reached at [email protected]. The authors wish to acknowledge the assistance of the following: Chuck Gault, Max Coatings, Birmingham, Alabama, for providing the coating services; Max Calenberg and Justin Barnes of IMR Test Labs, Louisville, Kentucky, for assistance in sample preparation.
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