What Causes Corrosion?
Corrosion takes on many forms. The most common forms of corrosion, as applied to HVAC equipment, are known as galvanic and general corrosion. Each of these corrosion types can lead to equipment failure. As a result, special attention will be given to each of these types of corrosion.
When dissimilar metals are electrically connected in the presence of an electrolyte, a reaction occurs. This reaction is known as galvanic corrosion. There must be a bi-metallic couple between two dissimilar metals in the presence of an electrolyte solution. Without these ingredients, galvanic corrosion will not occur.
Galvanic corrosion most often causes fin degradation, which may ultimately lead to the destruction of the coil. Galvanic corrosion of the unprotected coil begins at the bi-metallic couple between the copper tube and aluminum fin.
As corrosion begins within the copper-to-aluminum bond in the standard coil construction, the aluminum bond in the standard coil construction, the aluminum fin deteriorates. Consumption of the fin continues until coil performance is affected and severe visual deterioration results.
The standard condenser coil is manufactured from copper tubes mechanically bonded to aluminum fins. This bond crates a classic bi-metallic couple necessary for galvanic corrosion. An electrolyte in the presence of the copper-tube and aluminum-fin couple is sufficient to initiate a corrosion reaction. However, elimination of the bi-metallic couple can eliminate galvanic corrosion. This can be accomplished with an all copper coil or through isolation of the two metals with a protective coating. The protective coating will create a barrier between the metallic couple and the electrolyte to prevent galvanic corrosion.
Electrolytes are electrically conductive solutions. Common electrolytes include chloride and contaminants containing chloride from sources such as seawater, road salts, pool cleaners, and household cleaning agents. These electrolytes are typically sodium or calcium chloride compounds. Other electrolyte sources include sulfur and nitrogen compounds generated by combustion of coal and fuel oils.
The most common sources of chloride contamination are marine and coastal environments. Sea spray, mist, and fog contain tiny droplets of salt water which can be transported several miles by ocean breezes and tidal currents. It is not uncommon to experience salt water contamination up to five miles from the coast. As a result, protection from ocean-borne electrolytes in inland areas is recommended.
General corrosion is the degradation of metal caused by a reaction with the environment, such as oxidation and chemical attack of the metallic surface. Copper is susceptible to attack from sulfur-containing gases. The result is the formation of a nonproductive layer on the material surface. Unprotected metal will continue to react with the contaminant and corrode. Under severe, prolonged conditions, the metal continues to corrode until the integrity of the equipment is jeopardized. Unprotected copper in polluted industrial environments can lead to failure of the refrigeration system. Sulfur- and nitrogen-based electrolytes are often the cause of accelerated corrosion in industrial environments.
Since general corrosion consumes metal and forms metal oxides, unsightly surface conditions result. Surface tarnish on copper, such as black, green, brown, or yellow deposits, lead to the perception of poor quality.
Copper-fin coils experience similar attack of the copper metal. Failure of a contaminated copper-fin coil can result from fin degradation and ultimately lead to loss of tube integrity.
A clean copper tube in an uncontaminated atmosphere maintains system integrity. However, in a contaminated atmosphere, metal oxides begin to form on the copper tube. Prolonged exposure to a contaminated atmosphere usually results in tube failure.