Erosion- and turbulence-corrosion (Topic 11021)

Pure erosion is caused by a shear force from a flowing liquid which is higher than the adhesion of the oxide layer on the aluminium surface. The shear force from the liquid may be increased by turbulence or dissolved or suspended solids causing abrasion. The shear force from the liquid is not necessarily caused by liquid flow, but can also be induced by a moving surface for example a propeller.

If the removal of the oxide layer occurs in a corrosive liquid, the corrosion will be enhanced because a bare aluminium surface will be exposed to the liquid for a while until the oxide layer is healed. This is called erosion-corrosion. The rapid self healing of the oxide layer on aluminium is a great advantage with regards to erosion-corrosion, but because aluminium is a fairly soft material, erosion-corrosion can be a serious problem.

The shear force from the liquid on the metal surface is the main design criteria with regards to erosion corrosion. In practice, this is often transferred to flow rate limits. For the same flow rate, the shear force in a small diameter tube will be higher than in a large diameter tube, but the differences will be small for normal flow rates. If the liquid contains no solids and in smooth pipes, with undisturbed flow, the tolerable flow rate will be fairly high, but normally some turbulence must be expected and the flow rate limitations must be decided, based on these critical areas.

The data on flow rate limitations on aluminium are rather limited. An upper limit on 2ñ3 m/s in flowing sea water is often suggested. In addition to limiting the flow rates, it is very important to avoid introducing turbulence sources like for example sharp bends and irregularities around joints, see below.

References [37]