Hard anodizing is particularly useful for ring groove protection. Hardened ring grooves afford optimum sealing characteristics and resist micro-welding between the ring and the ring land. Hard anodizing can be done on the entire piston as well as just the ring lands.
In supercharged and turbocharged applications, more heat transfer from the piston is necessary. Some builders favor the tighter clearances and reduced piston rock of non-anodized pistons. One highly successful builder has stepped back from anodizing for his high-horsepower, long-duration Bonneville engines precisely for that reason, although he is quick to acknowledge its advantages in other applications. So, more often than not, hard anodizing remains a performance-enhancing feature, except in some occasions where very high cylinder pressure and associated heat is created over a long duration, such as a 5-mile run at Bonneville with 3,000 horsepower and 3 bar or greater boost.
In addition to hard anodizing, the pistons are also treated to an anti-friction coating to make them less abrasive on the cylinder walls.
The anodizing process submerges a given metal part in an electrically-conductive acid solution called an electrolyte. In the language of electrical circuits, we find a positive electrode (cathode) where electrons enter the circuit. They exit the circuit via the negative side (anode). In the anodizing process, the piston becomes the negative electrode. An electrical current passing through the solution and exiting via the piston deposits a hard, corrosion resistant layer of oxidation on the part. Hard anodizing typically uses a higher current and a weaker electrolyte solution at a lower temperature.Anodizing makes pistons more resistant to wear because the coating is exceptionally hard. The oxidation layer formed by anodizing becomes part of the parent material and does not flake off. Hard anodized coatings are typically thicker and harder than those used in decorative anodizing applications. In the case of pistons, a decorative corporate dye is often added to marry the added value to the manufacturer’s brand. Diamond’s gray-green coating is such an indicator. A typical hard coating exceeds 25 μm (0.025 mm or 0.0001 inches); this extra material is accommodated in the ring and pin machining operations and the piston to wall recommendations
Some engine builders prefer naked pistons with no anodizing or other surface treatments.
One popular feature offered on performance pistons is top ring groove coating via hard anodizing. This method supports power increases by permitting tighter tight ring clearances. It creates a harder mating surface that is very resistant to micro-welding, thus decreasing ring groove wear and the likelihood of ring failure due to seizure. The typical buildup is .00025-inch, and additional clearance is accommodated during manufacturing. Because anodizing is exceptionally hard, it can add a measure of roughness to the surface. So additional anti-friction/anti-scuff skirt coatings are often applied over it to help control cylinder bore wear.
Anodizing can also be combined with other piston surface treatments such as hard anodizing. Anodizing forms a hard, corrosion resistant layer on the surface of aluminum. There are various types of anodizing treatments, and some that form the hardest surface layer can also leave the surface rougher. Diamond Pistons have raised the technology standard by combining beneficial coating processes. Diamond combines hard-coat anodizing with a baked-on molybdenum dry-film lubricant skirt coating that provides an anti-friction, anti-scuffing surface.