Erosion corrosion
Erosion corrosion is material removal due to mechanical surface removal ( erosion ) and corrosion . By definition, this type of corrosion is triggered by an erosive attack on the protective layer. Pipes and system parts are preferably subject to this particular form of corrosion through which liquids or a two-component system such as water / steam flow at a higher speed and / or abrasive particles. In addition, for the erosion of the protective layers, which mainly consist of metal oxides, there must be a corrosive attack by the flowing medium on the metal surface with an unfavorable pH value and oxygen and electrolyte contents.
As a result of erosion corrosion, material is removed from the component concerned and material is transported 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, sometimes 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.
The cavitation erosion leads to comparable damage, but is physically related causes - for example, a high flow rate - causing. Corrosion-chemical influences are not the trigger for this type of erosion, but can exacerbate it.
Influencing variables
The lack of saturation of the liquid layer near the wall with corrosion products that form oxide layers is one of the reasons for the development 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 rate of erosion by the medium is higher than the rate at which a new protective layer is formed by an electrochemical reaction, erosion corrosion can occur. The formation of iron oxides - and thus the formation of a protective layer in the boundary layer near the wall (metallic iron | iron oxides) - is therefore less than the erosive erosion in this area. The prerequisites are that no more iron and hydroxide ions are formed in this boundary layer than leave this area due to convection and diffusion processes. In addition to the pH value, the oxygen content and temperature of the water are important parameters for avoiding this type of corrosion.
parameter
Numerous laboratory tests make it known which parameters have an influence on the development of erosion or erosion corrosion. In addition to the parameters pH value, oxygen content and temperature already mentioned , these are the type of material and alloy components and geometric relationships in the corrosion area with the local flow velocity .
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 small amount of chrome) in a temperature range of 80 to 250 degrees Celsius are affected, with maximum material removal in a temperature range of approx. 140-160 degrees Celsius to be expected.
Individual evidence
- ↑ DIN EN ISO 8044, edition: 1999-11; Corrosion of metals and alloys .
- ↑ HE Hömig In: Physicochemical basics of feedwater chemistry. 2nd Edition. Vulkan-Verlag, Essen 1963, p. 232.
- ↑ HE Hömig In: Physicochemical basics of feedwater chemistry. 2nd Edition. Vulkan-Verlag, Essen 1963, p. 233.
- ↑ HE Hömig In: Physicochemical basics of feedwater chemistry. 2nd Edition. Vulkan-Verlag, Essen 1963, p. 234.
- ↑ A. Bursik et al.: Chemical aspects in intermittent power plant operation. In: VGB kraftwerkstechnik 60, issue 6, June 1980, p. 490.