Surface coating with corrosion resistance


Which surface coating best provides threaded products with ideal corrosion resistance?

Corrosion resistance of products depends upon their environment.

For example, one type of coating compared to another might provide significantly better resistance to corrosion in a salty environment, whereas the second type might give superior protection where acid vapour is present in the air. For comparing the corrosion resistance of different surface coatings, the salt spray test as defined in DIN 50021 or ASTM B117 is employed. Under the conditions of this test and for equal coating thicknesses, there is a rough hierarchy one can use:

· electrolytically galvanized, white passivated · electrolytically galvanized, yellow passivated (50% better than white passivated);
· electrolytically galvanized, olive-green passivated (not unusual in the trade! - 100% better than white passivated);
· Dacromet (approximately 10 times better than zinc plated, white passivated).

NB: See also the question concerning hydrogen embrittlement! If a sizable coating layer is possible, the best solution under normal atmospheric conditions is generally hot dip galvanizing. However, the corrosion resistance of hot dip galvanizing cannot be determined by a salt spray test. If hot dip galvanized threads are used, allowance must be made for different thread tolerances and modified mechanical properties.