Selected Engineering Myths that Perpetuate Corrosion Failures

Corrosion engineering is a field that is burdened with numerous layman/decision maker misconceptions. Typically, these misconceptions are unchallenged engineering beliefs that often lead to corrosion failures. Some examples, among the many, are:

• Seawater is more corrosive than other natural bodies of water because of its’ high chloride content. If chloride were the corrodent, wouldn’t one expect the corrosion product to be a metal chloride? We know that the seawater corrosion product consists of hydrated mixed metal oxides, i.e.,  rust. Therefore, oxygen must be the corrodent, not chlorides. If the responsible engineer does not even know what the corrodent is, it is unlikely that corrosion prevention or control will be achieved.
• If one has a galvanic couple, corrosion control can be established by painting the anode (corroding member of the couple). By following this method, one is guaranteed a result of promoting premature corrosive failure. The entire galvanic corrosion current will focus upon the component at holidays and at other coating defect sites. The galvanic corrosion rate is governed by the amount of corrosion current that is impressed over a given surface area. The surface area at coating holidays and other defect sites is invariably very small, so the coated component's galvanic corrosion rate would be very high under this circumstance.
• Inlet end erosion–corrosion can easily be controlled by installing stainless steel inlet end inserts. This method introduces a galvanic cell that attacks, e.g. copper alloy tubesheet ligaments and copper alloy tubes at the tube – insert interface.
• Outlet end erosion–corrosion is caused by cooling water flow turbulence and can be controlled by the use of stainless steel inserts. The cooling water flow is not turbulent at the outlet end. Outlet end failures are typically caused by temperature induced air outgassing from the coolant and consequent cavitation. One must identify the location along the length of the tubes where outgassing occurs and then take proper precautions to avoid creating a galvanic cell.
• A galvanic cell requires the presence of two or more dissimilar metals. Galvanic corrosion can be induced on a single metal surface when there is a significant temperature differential on that metal surface. This condition exists, for example, in the upper row tube sections of condensers that have a vacuum priming problem; and the resulting failure mechanism is known as thermogalvanic corrosion.
• Other than its’ specific conductance, the environment has nothing to do with galvanic corrosion. Under a wide variety of conditions, copper can deposit from the coolant on many reactive metal surfaces and create myriads of galvanic cells.
• Microbiologically influenced corrosion (MIC) is not a concern unless the corrosive bacteria’s population is one million cells per ml. of coolant or per gram of corrosion product. Bacterial population is not a reliable factor to use for making MIC susceptibility predictions.  Given a sufficient nutrient supply and other favorable conditions, bacterial populations can increase by many orders of magnitude in a matter of hours.
• Linear polarization devices can be used to measure corrosion rate on line and can be used to obtain feedback on corrosion control effectiveness. Linear polarization devices can only measure the uniform corrosion rate;  and it is the localized corrosion rate that causes corrosive failure.