Erosion corrosion is generic term covering various corrosion damage mechanisms. The erosion corrosion is usually localized to certain areas of equipment and in piping of petrochemical industry. All of the erosion corrosion damage mechanisms usually have very high corrosion rate.
Read Also: What is Erosion Corrosion? Mechanism, Prevention, Factors.
Erosion Corrosion in Piping.
The catastrophic failures in the industry have been attributed to the erosion corrosion. In all of the damage mechanisms in this group the basic principle is the aggressive corrosion wash out due mechanical impingement of abrasive particles in the fluids. The protective oxide layer is either not developed at all or keeps depleting fast exposing the fresh metal surface to the corrosive atmosphere.
Most vulnerable locations are the direction and configuration changes in the piping, such as elbows, tee junctions. The examples of configuration changes which can cause the turbulence in the flow include downstream of the flow control valves, reducer sections, and downstream of pumps etc.
Erosion corrosion usually occurs at locations of high velocities and turbulence flows. The conditions could be developed by the mechanical reason like the configuration changes or due to intermixing of two streams of different velocities or phases. Erosion corrosion can occur both in the presence and in the absence of suspended matter in the flow streams, however un‐dissolved abrasive sediments cause accelerated erosion.
Erosion corrosion is the major problem for the process piping, and less active in the process equipment like vessels and tanks, unless the design of equipment does not provide effective safeguard against the high velocity targets and turbulent flows.
Proper designing, control of flow velocities and the control of flow stream conditions play major role in mitigation of the erosion corrosion. The safeguards for the equipment is usually provided in the configuration and the design of equipment, for example impingement plates, inlet deflectors, and vortex breakers etc.).
Similarly in case of piping the proper design of the piping and control of the flow velocities is the first line of defense against the erosion, and erosion corrosion. SAES‐L-132 (Material Selection for Piping Systems) table 1 gives the maximum recommended velocities for different materials in different process environments.
There are various damage mechanisms which can be grouped as the erosion corrosion phenomena. Most of the acid corrosions can be designated as erosion corrosions if flowing streams have these acids in significant components. Prominent erosion corrosion induced by the acidic corrosions are discussed before. Ammonium bisulfide in the high speed flow, causes sewer erosion corrosion as discussed in this website.
Cavitation is a form of erosion in which various tiny vapor bubbles are formed and collapse immediately on the pump impellers. The shock waves generated by the collapsing bubbles causes the cavitation damage on the impeller. Further details of this phenomenon can be seen from API‐RP‐571 Para 4.2.15.
Similar kind of phenomenon is responsible in the accelerated erosion of the pipe metal downstream of the welds on the high speed pump discharge lines. The vortices are formed in the high speed liquids flow downstream of the high root penetration. Tiny low pressure zones keep generating and collapsing inside vortices. The collapsing impact is usually high enough to break the metallic surface and cause accelerated erosion at these locations.
Further details on this damage mechanism can be seen from API‐RP‐571 Para 4.2.14.
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