Many customers seek ways to test anodize coating performance in objective ways. We put this post together to describe some well-known testing methods that can be used as a basis for creating requirements.
What testing methods are ultimately adopted will be determined primarily by the desired properties of the anodic coating. For example, architecture firms will value color and environmental resistance highly whereas semiconductor equipment companies will be more interested in corrosion resistance. Below are a couple tests that can be used to compare coatings – most of which we conduct daily.
Perhaps the most basic test of all, thickness, measured in mils or microns, is the starting point for most companies. The thicker the coating, the better overall properties you will get (up until around 3.2 mils on 6061 AL based on our research). However, keep in mind that the thicker the coating, the more expensive it will be since the time in the anodizing tank increases.
Because of this, customers who just need a superficial coating for appearance should only target around 1 mil. On the other hand, customers who need performance coatings should target 2-3 mils.
We typically recommend 2.5-3 mils since it will greatly increase the lifespan of the part and the costs associated with an extra mil of anodizing are far less than base cost to machine the aluminum.
Voltage Breakdown: For customers interested in DC electrical resistance, voltage breakdown is a great test that is easily performed. This test measures the overall resistance to current in terms of voltage per mil with the higher the number, the better the resistance. This test is often used by semiconductor equipment manufacturers that need to ensure that plasma will not cause a short in their etching chambers.
Bubble Test: Bubble tests are designed to test the chemical resistance of the coating – typically chloride resistance specifically. This more advanced test involves putting hydrochloric acid on a part and waiting for the acid to dissolve the coating until bubbles start appearing in large numbers. Naturally, the longer the coating can withstand the acid, the better it can withstand corrosion.
Seal Test/Admittance: This test measures conductivity of multi-frequency A/C current. High admittance can mean that the part was sealed poorly and the anodize coating will then not perform to typical performance standards.
Salt Spray: The salt spray test in its various forms is an accelerated corrosion test that is useful for many industries where environmental exposure is a primary concern. Usually the test is stipulated in hours of salt spray and that number is determined by the specific industry application.
Color: While color has little to do with the performance attributes of a coating, aesthetics are often very important. Here at Semano, we use a X-Rite spectrophotomet for color testing that gives us an objective way to measure color.