Process related causes
The service life of a heat exchanger may be shortened due to corrosion caused by process related events. Some examples are listed below:
Excessively high brazing temperature or too long time at temperature will lead to excessive Si diffusion in the core. Si diffuses along grain boundaries and this can increase the susceptibility to intergranular corrosion. By maintaining proper time-temperature cycles and thereby minimizing Si diffusion, intergranular attack can also be minimized.
Copper in contact with aluminum will cause a corrosion related failure very quickly. Copper is noble (cathodic) to aluminum and when these two metals are in contact in the presence of an electrolyte, the aluminum will be consumed rapidly. This may occur in a heat exchanger manufacturing facility where both Al and Cu heat exchangers are produced and there is cross-contamination of process routes. It only takes one small Cu chip to land on the surface of Al during some part of the manufacturing process to cause a short-term failure in the Al heat exchanger. If both Al and Cu heat exchangers are to manufactured under the same roof, it is recommended (and practiced) to physically separate the two production routes with a wall and take extensive steps to avoid cross-contamination.
Carbonaceous residues can be generated on the heat exchanger surfaces during the heat cycle from residual lubricants, excessive use of surfactants, binders in flux or braze pastes etc. Carbon plays very much the same role as Cu in that it is noble to Al. In a corrosive environment, carbon residues act as a cathode and Al as an anode, leading to the galvanic corrosion of Al. The best preventative measure is to ensure that the heat exchangers are thoroughly and properly cleaned and degreased prior to brazing. This includes monitoring the flux slurry bath for any signs of organic contamination (for instance oil slicks).
Painting a heat exchanger offers some level of corrosion protection, but is primarily used for cosmetic purposes. Painting will enhance corrosion protection if it covers the entire heat exchanger uniformly and is free from defects. In fact, paint defects or stone chips will accelerate corrosion locally. Many Al producers believe it is better to leave the heat exchanger unpainted to prolong its service life.
Conversion coatings such as chromate or phosphate conversion coatings work differently than painted surfaces. Conversion coatings enhance the natural oxide film on Al, essentially making it thicker and more resistant to hydrolysis. These types of coatings are most often used with automotive evaporators.