Sizing up anodizing: Testing & Quality Assurance

An example of the surface of an anodized cylinder. Image courtesy of Brush Research Manufacturing

To permit the use of aluminum instead of other, heavier metals in industrial applications, many cylinders are anodized to create an extremely hard surface that resists wear and corrosion, is nonconductive and is lubricious. Because anodized surfaces are porous, they also improve adhesion of coatings, as well as accept a variety of dyes for coloring. Given the myriad benefits, anodizing is popular for a variety of cylindrical items, including lift mechanisms for chairs; lift cylinders for hatchbacks; shock absorbers and forks for bicycles; fuel pumps; water pumps; pneumatic and hydraulic cylinders; spool valves; valve stems; and valve bodies.

By definition, however, the anodizing process means that part dimensions grow dimensionally and the surface finish becomes rougher. For a cylinder, that includes both an increase to the OD and a decrease to the ID. There are several types or classes of anodizing methods, and each reflects a range of coating thicknesses. As a rule, thicker coatings provide greater corrosion protection. Therefore, surfaces last longer in harsh environments, such as salt air.

As for surface finish, a hard coat anodized to 0.002″ thickness generally results in an R_a two to three times the original bare metal finish. For example, a machined R_a of 16 easily can become 30 R_a or more after anodizing.

For many parts, this is not an issue. However, when a part is cylindrical and mates with another part, often using a seal, increased dimensions and a rougher surface finish can be problematic.

Anodic coatings are very hard—only slightly less hard than diamond and harder than hard-chrome plating—and increased surface roughness may abrade sealing materials. Seal wear and coating irregularities might provide a path for leaks.

For this reason, parts require a fine surface finish for reliable sealing and long component life. To accomplish this, many companies utilize honing tools as an economical approach for treating the surface before or after the electrochemical process to control the dimensions and create a smoother surface. The results are a cost-effective method to resolving finishing challenges in the anodizing process and the consistent yield of high-quality products at a competitive price.

Honing Tools

Manufacturers traditionally have used grinding, lapping and rigid honing to improve the surface finish of anodized and hard-coat anodized parts. For several reasons, however, machine setups are difficult and must be extremely precise. First, the anodized coating is very hard. Second, the total coating thickness is very thin. Third, the high and low points of the anodized coating are not absolutely symmetrical around the centerline of the cylinder ID. When rigid honing is used with anodized parts, the honing stones contact only the coating’s high points, so areas of the cylinder ID remain untouched.