Corrosion margin

The corrosion margin, or corrosion addition, is the thickness added to the structurally required net thickness so the member still meets strength requirements at the end of its design life after wastage. Under the Common Structural Rules the gross offered thickness equals the net thickness from strength and fatigue checks plus a tabulated corrosion addition that depends on the member’s location and the corrosiveness of the adjacent compartments. Thickness-gauging at survey measures the remaining thickness against renewal limits set as a fraction of the original margin.