Metal Cladding

The corrosion resistance of a substrate can be improved by metallurgically bonding to the susceptible core alloy a surface layer of a metal or an alloy with good corrosion resistance. The cladding is selected not only to have good corrosion resistance but also to be anodic to the core alloy by about 80 to 100 mV. Thus if the cladding becomes damaged by scratches, or if the core alloy is exposed at drilled fastener holes, the cladding will provide cathodic protection by corroding sacrificially.

Coating Defect Survey - DC Voltage Gradient

Today, DC voltage gradient surveys have evolved as the most accurate and economic means of locating coating defects.

When a DC current is applied to a pipeline in a similar manner to cathodic protection, ground voltage gradients are created due to passage of current through resistive soil. Well coated pipelines have a high resistance to earth. However, at locations where there are coating defects the resistance to earth is of such way that current can flow through the soil to be picked-up by the pipe. In the vicinity of these defects measurable voltage gradients can be detected at ground level. The larger the defect, the greater the current flow. Increasing the current flow also results in an increased voltage gradient for a given soil resistivity.