What is erosion Where does it occur



Erosion corrosion is a material removal as a result of mechanical surface removal (erosion) and corrosion, whereby the corrosion is triggered by the destruction of protective layers as a result of the erosion.

According to the definition, the erosive attack is directed against a protective layer and not against the material itself. The material is later subject to corrosive attack from the surrounding oxygen atmosphere after the protective layer has been destroyed.

As a result of erosion corrosion, material erosion occurs on the affected component and material transport in the flowing medium, both in single-phase flow (water) and in two-phase flow (water / steam).

After the protective layers have been washed away, erosion corrosion causes erosion, which usually originates from a point of attack. The material erosion generally occurs in the form of smooth, partly metallic, shining washouts. The removed surface shows trough-like, wavy or shoulder-like structures.

The protective layer is preferably removed by flow turbulence, so that imperfections such as edges, elevations, depressions or deflections are particularly at risk.

Influencing variables

The lack of saturation of the liquid layer near the wall with corrosion products that form oxide layers is the cause of erosion corrosion. This happens as a result of a rapid exchange of substances between the boundary layer and the core flow.

Accordingly, erosion corrosion only occurs when passive layers (or more generally: corrosion-inhibiting oxide layers) are removed from the metal surface. An increase in the chromium surcharge in the steel increases the resistance to erosion corrosion well below passivity (> 12 to 13% by weight).

If the removal rate of the medium is higher than the rate at which a new protective layer is formed by an electrochemical reaction, erosion corrosion can occur.

Influences on erosion

- mechanical material properties

- flow conditions (speed, geometry)

Influences on corrosion and protective layer formation

- Medium properties (pH value, oxygen content, temperature)

- electrochemical material properties or the chromium content in steel

parameter

Due to numerous laboratory tests, it is known which parameters have an influence on the development of erosion or erosion corrosion:

1) material

2) pH of the medium

3) Flow velocity of the medium

4) temperature of the medium

5) Oxygen content of the medium

6) Geometric conditions

Occurrence

Based on the assumption that erosion corrosion only occurs in the liquid phase, this can also be related to two-phase flows (water / water vapor), assuming that a coherent liquid film is present on the wall surface and that the water velocity is the decisive flow velocity for the erosion process is used in the film close to the wall. In particular, unalloyed or low-alloy steels (with a low chromium addition) in a temperature range of 80 to 250 degrees Celsius are affected, with maximum material removal in a temperature range of approx. 180 degrees Celsius to be expected.

Categories: Materials Science | corrosion