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Thanks for straightening that out. I have seen several sets of tanks over the years that were powder coated, and they *appeared* to be very durable. I was unaware of the migration issue. I was aware of the heat issue. I have used epoxy paint on AL tanks with good results, however. What is the problem with the UV and epoxy? Its OK to use it as a primer, but not as a top coat? Thanks! Scott ----- Original Message ----- From: <Gilldiver@ao*.co*> To: <techdiver@aquanaut.com> Sent: Thursday, August 10, 2000 2:25 PM Subject: Re: ScubaPro Faber Tanks-Paint Bubbles and Rust > 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'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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