Summary for coating testing regimen Discussion
The thread asks for a fair way to compare a current painted steel coating system with a proposed hot-dip galvanized plus powdercoated system for offshore oil service, where parts suffer heavy mechanical abuse and long exposure to harsh conditions. The focus keyphrase is coating testing regimen, and the replies lean toward practical comparison rather than relying on a single lab standard. William Doherty argues that standard salt spray unfairly disadvantages HDG and recommends real-world worst-case exposure, such as immersing samples at the half-tide mark or placing them in the Atlantic or North Sea. Eric Bogner suggests preparing test panels of both systems, then adding adhesion tests, heat/cold cycling, and salt spray to simulate wear. Another answer says salt spray can still be useful as a direct comparison if the panels are scribed to the base metal first, since that helps represent damage in service. The overall message is that natural weathering is best, but accelerated tests should mimic actual abuse.
Q. We have a customer who is interested in comparing his existing paint coating to a proposed galvanized and powdercoated solution.
Currently, steel components are shot blasted and painted. The paint is a 2 pack epoxy with typically a zinc phosphate primer
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The steel components then suffer serious mechanical abuse, being used in the offshore oil industry. They get shunted by forklift, loaded on to boats by cranes, offloaded onto oil rigs, back to boats, back to shore, off somewhere else, left in the yard for months, etc etc. In short the coating doesn’t last.
The coating serves two purposes. Corrosion protection and identification. (Colour).
In considering using hot dip galv for corrosion protection, it is felt that this will far better withstand the rigorous handling, and the powdercoating over the top, will provide the identification colour. While the powdercoat might end up chipped and scratched just as the paint does, the galv underneath will be more robust and prevent corrosion for longer.
But this is a big step to change, so some form of sample testing comparison is required to validate the assertion that the proposed is a better solution.
Salt spray is an obvious suggestion, but it is widely acknowledged that standard salt spray, or even acetic salt spray is not a representative test on galv. It is widely used on painted steel samples though, and even while this form of testing is also notoriously not reproducible, it is at least comparative in a single testing, from one sample to another.
Can the experts here suggest some form of testing, either salt spray, modified salt spray, or perhaps something entirely different, in order to be able to make a fair comparison between these two coating systems, to predict the relative performance of real life material in such an environment as described?
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
 | A. For this sort of use, you may have to devise a test. Considering it’s for off-shore drilling, salt spray isn’t entirely inappropriate. Mechanically abuse to the point of multiple breaks in the powder coat, followed by salt spray is somewhere near real life. Observe how severe the mechanical abuse is expected to be then bang it with a hammer, or drag behind a truck as seems appropriate. Control the number of hammer blows or miles dragged. ;-) Certainly hot dip-powder coat will prove far superior.
A. Geoff, The Academics usually catch up a decade or two later. It is possible to fudge Salt Spray results but as your process is genuinely superior why bother? Regards, William Doherty A. Good day Geoff. Eric Bogner A. Hi Geoff, Considering your situation, I would suggest that salt spray would be a suitable test, especially as it is a direct comparison of performance and the environmental conditions that the parts are seeing in-service. A simple salt spray may take a long time to produce results, so the panels need to simulate the damage likely to be seen in use. The standard way to do this is simply to scribe a cross on the panel to the base material before placing in the salt spray cabinet. We have used this method of coating evaluation for a number of years and as a direct comparison method it is a good indicator of comparative performance. Obviously the only real test would be natural weathering, but as these tests typically run for years rather than weeks it is rarely practical. Accelerated weathering tests suffer from the same arguments than salt spray often suffers from in that they are not a good indicator of in-service performance and their true use is in comparative testing. Certainly for initial trials I would stick to salt spray, as a friend often says to me “keep it simple stupid”. Brian Terry |
