Issue link: http://www.e-digitaleditions.com/i/901126
Rust-proofing a water cooling tower exposed to biocides, algaecides, and metal corrosion inhibitors Q I'm replacing galvanized sheet metal on a cooling tower. The metal will be sub- jected to water with biocides, algae- cides, and metal corrosion inhibitors in it. In addition, at any given time, it may be submersed, drying, or wet from splashing. Would powder coating be a way to rust- and corrosion-proof the metal to lengthen its service life? B.O., Richmond, Va. A Depending on the quality of the galvanizing, powder coat- ing may not provide a signifi- cant improvement to protect the steel from corrosion. Normally, hot dipped galvanized steel is powder coated to im- prove the aesthetics of the part more than to significantly improve the corro- sion resistance of the design. Galvanizing applies molten zinc to the steel surface. This zinc is designed to be sacrificed to save the steel from corro- sion. Over the life of the product, the amount of zinc lessens as it degrades (like a dissolving bar of soap). Applying powder coating to the surface can in- hibit this method of protection. Even- tually, the powder coating will delami- nate from the steel as the galvanized layer dissolves. When this happens, there is often enough galvanizing left on the steel surface to provide continued protection for some time to come. It's true that the powder coating will protect the galvanizing from immediate corrosion effects. However, the prod- uct's aesthetics will already be compro- mised as the coating delaminates and exposes the underlying galvanized sur- face, often in spotty areas. I hope this in- formation helps. —N.L. Investigating clear powder coating for postforming Q We are a specialty awards manufacturing company that is presently investigating the use of clear powder on our metal (brass, steel, and aluminum) products. To date, we've had a mixed success rate. I should mention that our products need to be fabricated after powder coating. We can't coat the parts after they are cut or punched. Our customers are also used to getting a very smooth product. We have narrowed our search to a cou- ple of powder coating brands but still have the problem of orange peel that varies from heavy to light from test to test. We're also seeing chips on the cut or punched edges. I've spoken to our vendors, and they're uncertain where our problems originate, although they have suggested our oven may be partly to blame. We might want to look into using infrared (IR). We use a gas con- vection oven that reliably reaches 320°F to 350°F. It won't hold a 400°F temper- ature very well. At present, we bake the clear powder at 320°F for 20 minutes. Is there any way to get completely away from the orange peel effect, and is it pos- sible that our oven could play a part in the lack of a good quality end product? Is it possible to end the edge chipping? I have a feeling that the powder is simply failing to crosslink properly, and a good bond with the substrate is not taking place. I may not have given you enough information to formulate a proper an- swer, but any advice is appreciated. J.S., Mesquite, Tex. A As we've noted many times in this column, and as is com- mon knowledge in the indus- try, powder coating and orange peel go hand in hand. There are ways to minimize orange peel, but it seems to be always lurking in the coating wait- ing for us to make a misstep and bring it to the surface. Any heavy film will bring out the or- ange peel, which will telegraph through the clear coat. I assume the awards you are coating are of certain configurations and not flat sheets of metal. Usually, the configuration of the part will help cam- ouflage the orange peel. Your supplier can tell you what the crosslink tempera- ture of the powder is and the time re- quired at temperature. There is equip- ment that can prove this. Your supplier should have an oven recorder that will give you this information. On the sur- face, 20 minutes seem adequate, but you need to know how many minutes the part is at the required temperature, not just total time in the oven. You need to be absolutely certain where you are on the cure curve before you begin to work on the problem from any other angle. In a coating shop, you only do one thing at a time. That rule goes back a hundred years and still holds true today. Once you have determined cure, and fixed any over- or undercure, you can attack the chipping issue. —G.T. 24 POWDER COATING, November 2017 George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers