Prepare to Powder Coat and Put Off the White Stuff Six preliminary steps can help fight off long And short-term corrosion. By Jon P. McGraw Alloy Casting Co. This article originally printed in the 2002 July-August issue of Fabricator Magazine, an official publication of the National Ornamental & Miscellaneous Metals Association.
owder coating aluminum castings can sometimes be a problem. Immediate problems appear when the powder coat bubbles because of gas evolution from the casting during the heating cycle. Long-term problems happen when white corrosion develops under the coating, and flaking and peeling starts. Even though powder coating is a sophisticated process, these two problems do occur. Fighting off their occurrence is possible by taking correct preliminary steps. Preventative action can mean the use of a better base metal, preheating to degas the casting, extensive chemical cleaning, proper acidic etching rinsing with deionized water, and adequate powder curing. A lack of attention to any one of these steps can mean a rough surface and/or a coating that eventually will flake.
In the above example, the ball cap with the dull white corrosion was made with a standard aluminum casting alloy. The ring shown was made with a marine-grade alloy and shows no corrosion. It has retained its metallic shiny surface. Both samples were placed together in a chlorine corrosive environment.
One: Use a Better Base Metal When most aluminum castings are placed in a seacoast environment they demonstrate white corrosion. This corrosion is caused by salt air penetrating the coating and allowing a galvanic battery action between the aluminum and the dissimilar metals in the aluminum. These dissimilar metals are mainly copper
I N T H I S A RT I C L E . . .
• An industry supplier of aluminum castings and aluminum hardware lists six ways to avoid two problems that occur when powder coating aluminum • Short term problem, like bubbling, result from gas evolution during the heat cycle. • Long term problems, like flaking and peeling, stem from white corrosion setting in between the coating and the base metal. For more info call Jon McGraw at (800) 527-1318
These samples were placed in a severe chlorine atmosphere to replicate a corrosive environment. The sample on the right is a typical casting showing a white-chalky surface. The center powder coated sample shows corrosion that has penetrated from the inside out of a typical casting. The marine-grade ball aluminum cap on the left shows no corrosion. Even with two saw cuts that were put through the coating and into the ball prior to the corrosive environment, the marine-grade metal did not corrode under the powder coat nor did the coating flake off
Left: Dissimilar metals in almost any form can cause severe corrosion. The green area shows severe galvanic corrosion caused by a marine –grade aluminum ball cap attached to a stainless steel bolt. Right: This ball plug was made with a standard casting alloy, degassed-and double coated. The ball exhibits few bubbles but even the double powder coating did not prevent corrosion from penetrating where the coating had been thinly applied. Green speckles show corrosion penetration.
and zinc. The cause of the problem can be eliminated by using a marine-grade aluminum alloy having magnesium as a major component. Even under the most severe conditions, the marine-grade alloy stays shiny and coherent under the powder coat and eliminates white corrosion.
Two: Degas the Metal
The second step for excellent powder coating of aluminum castings is to run a preheat cycle to degas the metal. The preheating expels the contained hydrogen that was entrapped as a natural result of the casting process. Aluminum sand mold castings are made in a mixture of sand, clay and approximately 4 percent water. The impact of the molten aluminum on the damp sand creates steam of which a portion is absorbed into the casting as hydrogen. During the powder coat heating cycle, the hydrogen is expelled and shows up as a bubbly surface. This problem can be eliminated by first using a preheat cycle that is 50% to 100% higher that the normal powder curing temperature and for a time cycle 30 to 100 percent as long as the powder curing time. A good degassing cycle will eliminate unsightly bubbles. The proper selection of a marine-grade alloy and degassing can greatly reduce problems with powder coating on aluminum castings. But even with those actions, there are two more essential steps that need to be taken: good cleaning and good surface etching.
Three and Four: Cleaning and Etching
Good cleaning entails alkaline decontaminating to remove organic oils, waxes and lubricants, and acidic cleaners to remove inorganic rust, scale, weld-
ing smut and to etch the surface. Contaminates and moisture left on the surface will detract from powder adhesion and give a poor coating life.
Five: Deionized Water Rinsing
After acidic etching the surface must be rinsed to remove contaminates and rinsed sufficiently to give a “water breakfree” cascade off the surface. Deionized water is the best media for this procedure. After rinsing, the casting should be dried and free of powder or streaking on the surface. A powdery film indicates poor rinsing or old, heavily contaminated cleaning fluids.
Proper curing of the powder coat is also critical for good longevity. A weak, soft coating will not stand up to the elements. As indicated above it takes several correct steps to enhance the life of powder coating. A misstep in any one can cause flaking. I have seen castings on a gate application where the coating on cast aluminum spear finals was in excellent shape but only 24 inches lower on the same picket the cast aluminum collars were badly corroded with the powder coat flaking off. It was difficult to determine which of the many initial steps had been adequate for the finial but inadequate for the collar. Note the photographs on this page showing actual job applications where the powder coat process proved to be inadequate. Other reports from the field suggest that imported aluminum castings exhibit much more out-gassing and corrosion than domestic castings. This is probably due to higher levels of dissimilar metals and Appendix 14
contaminates and lower quality foundry practices at foreign shops. The important steps for powder coating are the same important steps for other coatings as well, be that fluoropolymers, wet paints, or hybrid combinations. A good powder coating system is a sophisticated process. There are many sequential steps that must happen correctly in order to obtain good results. These detailed steps are highly technical and more than can be covered in this short article. So the next time you send a job with aluminum castings “out to be powder coated” and worry about the results, think of the many steps that are essential to providing a smooth, long lasting coating. Reduce those worries and insure good results by taking preventative action. Make sure the job is not returned.
Below and right: White corrosion has deteriorated the powder coating on the floor flange below and on the metal rings of this cast iron gate.