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Joseph Schinner of Akzo Nobel Coatings Inc. on
March 8, 2011:
The real answer is the ubiquitous, "it depends." A more useable answer doesn't fit in a single sentence, so I will summarize the best I can. This response applies to coatings over metal, especially ferrous ("galvanized" or not) and aluminum. A scribe, whether vertical or an X-hatch, is a multi-tasker but has the common foundation of being a standardized form of scratch damage through a coating to the substrate. Sometimes a coating is only as good as it handles damaged areas, especially if damage is expected or unavoidable.
Yes, there are coating/substrate situations where the coating looks great until a damage point is introduced, and the undercutting becomes so severe that the coating comes off in flakes or sheets. In a nutshell, the scribe provides a atarting point for undercutting of the film. This provides information on film adhesion under test conditions (especially wet film adhesion in humidity conditions).
The old-fashioned knife point dig at the middle of the X-hatch is still used to show inherent adhesion. It also is a negative control area to show the conditon is sufficient to cause corrosion (in the scribe)and coating protection is needed. The corollary is that it shows the effectiveness of the passivation or sacrificial effects of a corrosion inhibitor to prevent undercutting. Without using some sort of damage areas when doing accelerated testing, the test itself could be grossly misleading to the positive side. Some testers use direct or reverse impact, with or without cross-hatch scribing instead. Some use impact AND scribe- it all depends on damage expected and tested for. A scribe can be used for most accelerated testing, but practicality dictates where it is more beneficial: salt fog, cyclic corrosion testing, real time Emmaqua®, Florida, etc. It sometimes is used with water fog and Cleveland condensing humidity, but these tests are specifically for blister resistance where a scribe is not necessary if the other environments are used. Sometimes, even the QUV, Weather-Ometer® panels are scribed, but common sense here dictates caution because these are primarily gloss/color drift tests that a scribe interferes with on small panels. Bottom line: it is usually best to incorporate a scribe in test procedure somewhere.
Lee Edelman of Independant on
March 8, 2011:
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Yes, scribes shouold be used to test for rust creepage.
The test panel needs to be scribed, and the surface needs to be exposed to the corrosive elements. Undercutting and adhesion need to be checked.
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