Improving Corrosion Protection

How can you improve a powder coating system to prevent corrosion on parts? Powder Expert Rodger Talbert suggests adding blasting and priming.

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Q. We make metal enclosures for a number of industrial applications, including control cabinets. We have an older powder coating system with a five-stage iron phosphate washer; dry-off oven; two powder booths with automatic and manual application equipment; and a convection cure oven. We apply a single coat of a high-quality, durable UV powder. Still, we have problems with corrosion on the cabinets, and we are not sure of the best way to tackle improvement on our system.

A. An organic coating over ferrous metal can provide a lot of protection against moisture penetration and oxidation, but some rules must be followed to get the best possible outcome. The surface has to be treated in a way that will ensure a good bond. All foreign matter (dirt, dust, oils, wax, markers, and so on) must be 100-percent removed to prevent setting up a corrosion cell. The surface can be enhanced with zinc phosphate or another superior, corrosion-inhibiting conversion coating. The coating material must be of high quality, and it must be applied completely and at the right thickness. 

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Start with the fabrication of your enclosures. Make sure all metal is free of excess burrs and sharp edges. If you must bend the sheet metal over into a tight fold, make sure the inside of that fold can be treated and covered with powder so rust cannot bleed out when it is exposed to the outdoors. Next, focus on cleaning. You can clean with a good chemical cleaner or by abrasive blast so long as all organic soils are removed and there is not scale, weld stains or other inorganic material left on the surface.

There are different ways to provide protection once the soils are removed. One way is to use a conversion coating that adds corrosion protection. Zinc phosphate, chrome treatment, zirconium treatment or similar materials can provide barrier protection under a powder film. You also can include a powder primer before you apply the topcoat. The primer will add significant protection, with or without a conversion coating.

In your case, the five-stage washer may do a decent job of cleaning, but the iron phosphate does not add significant corrosion resistance. It may be hard to convert to something better, however, because the equipment needs to be made of stainless steel, you need very high-quality water (deionized or reverse-osmosis filtered) and you have to have the right number and size of stages.

What I would add in your situation is blast and prime. If you do a good job of blasting, you will have a good surface for bonding. If you add a layer of zinc-rich powder primer, you will add major additional barrier to moisture penetration and a reactive layer of zinc for corrosion protection. This should be the most cost-effective and reliable way to improve your corrosion resistance.

Q. We need a method to determine the best possible line density of every part we run. Are there any guidelines or methods available to calculate this best possible density?

A. Everyone in the industry stresses that rack density helps achieve better efficiency, and this is true. But how do you determine how much surface to hang per foot? You could do it by common sense, trial and error, and learning as you go. Not all bad, but lacking in scientific support. You can add some time and motion study, recording of film-build control, and careful evaluation of rejects and causes for the light coating and the heavy coating. This will give you a step up in data and actual verification, but it still is somewhat lacking in methodology. 

One way to support your racking setup and part spacing is to evaluate the number of guns and amount of time you have to spray (line speed). You will want to understand how much powder you can deliver per gun, how much surface has to be covered per part and how long the parts will be in front of the guns. 

A critical part of this evaluation is to know the relative efficiency of your spray guns at a given output. Transfer efficiency is best at the lowest effective output. Ten pounds per hour per gun will provide better transfer efficiency (TE) than 20 pounds per hour per gun. If TE was the only goal, however, every coater would deliver a very low amount of powder. The problem is that it will take longer to coat the part. Time is an important factor due to production requirements and labor. So the gun has to be able to deliver enough powder to get the part coated in a reasonable length of time, and therefore we turn the gun up to get the job done. 

The next step is to understand that there is a limit to how high we can go in output before the TE is so poor that it is no longer effective. Experience and lab testing have shown that at an output of around 40 pounds per hour, the TE begins to drop below 50 percent, and other problems begin to surface such as orange peel, inconsistent film build and poor coverage in Faraday areas. An effective limit to output is typically between 30 and 40 pounds per hour. Remember, lower output provides better TE, but we must accept some time limitations. So 20 is better than 30, and 30 is better than 40. Go as low as you practically can.

Once you have established a gun output rate and have a decent idea of TE, you can calculate how much surface you can cover per hour. You will also need to have a target film build and specific gravity for the powder:

• Calculate coverage per pound:
(192.3 × TE) ÷ (thickness in mils × specific gravity) = coverage per pound of powder
• Determine total gun output: 
Number of guns × pounds per hour per gun = total gun output per hour (Gun output per hour ÷ 60 minutes = gun output per minute)
• Determine how much surface can be covered: 
Pounds per minute of output ÷ coverage per pound of powder = potential square footage of coverage per minute

The final step is to calculate the surface area of your parts, factor in your line speed and make sure you do not exceed the potential that can be covered per minute with your planned racking arrangement. A difficulty factor may come into play. It may take longer to coat a complex part that has many Faraday areas, and that will reduce the potential coverage. There are probably three levels of difficulty: very simple (flat panel), somewhat difficult (bends or other geometry) and very difficult (deep recesses and complex shapes like castings). If you follow these guidelines, however, you will have a good idea of how many parts can be effectively hung and coated per minute on your line.

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