Anodize Corrosion Resistance

Aluminium, despite having a natural passive oxide layer, is susceptible corrosion, especially in corrosive environments. For this reason, many engineers will consider anodizing as a solution to corrosion resistance. Other solutions such as paint or powder coating may also be considered. While there is no one size fits all solution, if performance is the primary consideration, anodize will dramatically outperform the alternatives dramatically. In this article, we will discuss the different choices, quantify corrosion resistance, and different types of anodize to consider. 

Paint – Often considered as the starting point. The primary advantage is that is cheap. As anyone who has ever owned a car will know, paint degrades from just UV exposure over time, let alone perform well against more aggressive chemicals. 

Powder Coat – Very similar to paint, but the performance is certainly a step up – and so is the cost. Powder coat is a usually a good in between solution, but again not recommended if corrosion resistance is the primary consideration. 

Anodize – Unlike the paint and powder coating, anodize is not a coating at all. But rather an oxide that is bonded to the surface of the aluminum chemically. The performance of the corrosion resistance is dependent upon a number of factors which we will discuss further, but first, let’s standardize our yardstick for corrosion resistance. 

Testing Corrosion Resistance

Two primary methods exist for corrosion testing. The first is from the Milspec and is fairly well known. The salt spray test is designed to quantify corrosion, by counting the number of pits after running a set number hours of salt spray exposure. The limitation of this test is that it is simply performed as a pass/fail test. This limits the usefulness of it to determine how much better one anodize type is to another, but is still a good starting point when discussing anodize performance. 

The second method for quantifying corrosion resistance is the bubble test. This is a great way to standardize acid resistance, in an accelerated fashion. The test works by placing a 5% solution of HCL in a tube on the anodize part. When the acid penetrates down to the best aluminum, bubbles will be created and the test will conclude. The time recorded from initiation to penetration is the therefore the yardstick used to quantify corrosion resistance. The limitations of this test is that if your primary corrosion is coming from pH basic attacks like caustic soda, the test results will not be as well correlated. 

Corrosion Performance by Anodize Type

Type 2 Clear Coat: This will provide the weakest form of anodize corrosion protection. This is because of the parameters (electrical) that is used to create it as well as the the thickness, or in this case, lack there of. Type 2 anodize should generally be avoided if designing for corrosion resistance as it is generally a cosmetic coating. Bubble test results ~ 30 minutes. 

Type 3 Hard Coat: This is the coating most people will want when needing corrosion resistance. It is a much thicker and more durable coating that type 2 and benefits from the more aggressive parameters that are used to create it. Typically, hard coat will cost more than clear (though not at Semano) which may be a consideration of cost is a concern. Bubble test results ~4 hours at 3 mils anodize thickness.  

Semano Proprietary Coatings: For the most extreme corrosion resistance, often used in plasma chambers, consider our A200HCD anodize coating which outperforms even the robust hard coat anodize bubble test by about 10x. The cost is going to be typically more than type 3 hard coat as well. Bubble test results ~40 hours at 3 mils anodize thickness. 

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