Do Your Own Compatibility Testing

If you work with metals, you’ve probably seen compatibility problems. Materials compatibility problems happen when process chemicals or chemicals in the air cause undesirable changes to the product. Rust is one example of what happens when there’s incompatibility: Iron reacts with water during cleaning or with water in the air. 

It’s helpful to check for compatibility problems before setting up a new cleaning process, before adding a new product and before changing a process. We suggest you do your own compatibility studies and that you do dynamic compatibility studies. It’s not complicated; with a little testing, you can avoid headaches. 

How do you spot incompatibility? Sometimes it’s obvious. If the part dissolves in the chemical, there’s an acute materials compatibility problem. Other clues of incompatibility include undesirable surface modification, deformation, release of materials from the product, weight changes and dimensional changes. 

An undetected compatibility issue can produce immediate or long-term catastrophic product failure. Chemicals can damage cleaning or fabrication equipment, which leads to process disruption, equipment repair or even the need to purchase new capital equipment. 

So why worry? Isn’t it simple to just look at a compatibility table? No. Compatibility tables don’t give you what you need because they are not standardized. Some use letter grades, others use numbers, and some use words such as “excellent” or “fair.” Tables are also incomplete. Most don’t show the synergistic, non-additive impact of many mixtures. Exposure of a metal to Chemical A may not cause damage; exposure of that metal to Chemical B may not cause damage. However, a mixture of those two chemicals can show dramatic reactivity with the product. Also, compatibility tables show results of testing one material of construction at a time. 

If you have a complex product or if you process different materials in the same tank, it’s best to use the “minestrone” method. Put a mixture of materials of construction in a container with the chemicals to be tested, and see if any of the materials degrade. The minestrone method is simple, and it’s a great improvement over testing products one at a time. But like most compatibility tables, the method is static. The test mixture sits in the chemical, usually at room temperature. 

We recommend doing a dynamic compatibility study, which considers compatibility under actual process conditions. Process conditions include not only the chemicals and materials of construction, but also temperature, force and time. In fact, the same factors that make for an effective cleaning process also contribute to compatibility problems. 

Temperature impacts compatibility. For example, one group was successfully using acetone wipe-down to clean many different metals before coating. They tested a specially designed, heated acetone degreaser (don’t try this on your own). Acetone is extremely flammable; it is important to use specially designed low flashpoint equipment. When heated, acetone combined with water from the air and reacted with magnesium produces a surface “bloom.” Testing before buying paid off; they avoided investing in the wrong cleaning machine for their application. 

The forces involved in the cleaning process combined with process time also impact compatibility. Ultrasonics can magnify the cleaning effectiveness of chemicals and can also magnify undesirable impacts on materials of construction. How long should you run the process? Use a reasonable multiple of the expected time of exposure of the product to the chemical or chemical mixture. If the expected time of exposure is 10 minutes maximum, a two-month study is probably overkill. The point is to stress the process enough to exacerbate a potential problem. 

You cannot avoid all problems with materials compatibility testing. Although a two-month compatibility study might not be relevant to reactivity with the product, fixturing could see that magnitude of exposure to process conditions. Eventually, the materials used in fixturing may break down, even with the most resistant materials. There is no substitute for ongoing inspection, maintenance and periodic replacement of worn-out parts or fixtures.  

 

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Published in the June 2017 issue.