Mailing List Archive Mailing List Archive
After reading some of the posts on tank corrosion I thought it was time for a little science on coating and corrosion. This is probably more then you ever wanted to know about paints. First, Powder Coating: Powder coating is the process of applying a powder paint like material to an electrically conductive surface. The powder is then heated so that it can melt and polymerize into a uniform coating that can be very corrosion resistant because of barrier protection of the underlying metal. That means that water cannot get to the metal. However, if a scratch breaks through the coating and there is no sacrificial element under it (like zinc galvanizing) corrosion will start immediately and can travel under the coating. For tanks there are two problems with powder coating, the first is the requirement for electric conductivity. Powder coats are applied by a static electric charge and many primers are not conductive. Therefor, primers are generally not used. This results in the application of powder directly onto the tank surface without any primer. On bare steal this is not a problem but on zinc you will have a hard time getting really good adhesion due to the oxide layer that forms on the galvanized surface almost instantaneously when it is exposed to air. If there is any remaining paint for the old coat you will have spotting of the new coating. The second problem is from heating to melt the powder and cure it. On steel tanks there is no problem on aluminum their is a BIG problem. Aluminum alloys are just that ALLOYS. They contain other metals like iron, copper, zinc, magnesium, and manganese. Whenever these alloys are exposed above 275F you will start to get eutectic melting. This is melting of these alloying metals within the crystal structure of the aluminum. Metals like copper and zinc will move along the crystal structure and pool on a microscopic level. In stressed parts like tanks this pooling forms weak spots were cracks can start easily. Above 250F you will also start to change the crystal structure of the aluminum which in itself can form stresses and cracking. As powder coating typically is done above 300F, powder coating should never be done on an aluminum tank. If an aluminum tank is exposed to temperatures above 250F it should be scrapped. So powder coating will work somewhat on steel tanks but never on aluminum. So what can you do? Try looking around the local airport for a small scale aircraft paint facility that will do small jobs. Have them completely strip the tanks with a chemical stripper or Plastic Media Blast (PMB) system. Never use sand blasting on aluminum or galvanized tanks. Have them apply an aircraft grade chromated primer and then either you or them apply a high quality aircraft urethane topcoat. Never use an epoxy topcoat as it will react with the suns UV rays and start breaking down. The chromated chemicals in the primer will act as a sacrificial element protecting the underlying metal and if a scratch breaks through it will protect the bare metal in the scratch for a long time. These primers will protect aluminum and steel in salt spray chambers for at least 2000 hours. That would be about 5 years on a boat. The primers to look for are compliant with US Federal specification TT-P-1757 (Zinc Chromate) or Mil-P-23377 solvent base epoxy or Mil-P-85582 water base epoxy. Both of the Mil primers contain either Strontium Chromate or Barium Chromate. For top coat I recommend a material meeting Mil-P-85285. This system of chromated primer and high quality urethane top coat will give very good protection and is quite resistant to impact damage (think about a jet fighter at mach 2 in a rain storm). So get a good clean surface, prime it, and topcoat it. Pete Johnson -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]