SIX FORMS OF CORROSION IN VALVES

SIX FORMS OF CORROSION IN VALVES

Corrosion of valves is one of the main reasons for valve failure. Corrosion is the way metal gets into their ore through natural waste. Below we mainly analyze the corrosion mode of the valve.

 

1.Electrical corrosion

When two different valves metals are in contact and exposed to corrosive liquids and electrolytes, forming a galvanic cell, the current causes the anode to corrode and increases the current. Corrosion is usually near the point of contact. Reduction of corrosion can be achieved by plating dissimilar metals.

 

2.High temperature corrosion

To predict the effects of high temperature oxidation, we need to examine these valves data: metal composition, atmosphere composition, temperature and exposure time. However, it is well known that most light metals (those lighter than their oxides) form a non-protective oxide layer that will fall off as it gets thicker and thicker over time. There are also other forms of high temperature corrosion including vulcanization, carburizing, and so on.

 

3. Crevice corrosion

This valves crevice corrosion situation occurs in the gap, which hinders the diffusion of oxygen, resulting in high and low oxygen regions, forming a difference in solution concentration. In particular, there may be a narrow gap at the defect of the connector or welded joint, and the gap width (typically 0.025 ~ 0.1mm) is sufficient to allow the electrolyte solution to enter, so that the metal in the gap and the metal outside the gap constitute a short-circuit primary battery，and strong local corrosion occurred in the joint.

 

4. Pitting

When the valves protective film is damaged or the corrosion product layer is decomposed, local corrosion or pitting occurs. The membrane ruptures to form the anode and the unbroken membrane or corrosion product acts as the cathode, and a closed circuit has actually been established. In the presence of chloride ions, some stainless steels are susceptible to pitting. When corrosion occurs, metal surfaces or rough parts are not uniform due to these.

 

5.Intergranular corrosion

Intergranular corrosion occurs for a variety of reasons. The result is almost the same along the metal grain boundary, the mechanical properties are destroyed. Without proper heat treatment or contact sensitization, the intergranular corrosion of austenitic stainless steels at 800–1500 ° F is subject to many corrosives (427–816 ° C). This condition can be eliminated by pre-annealing and quenching at 2000 ° F (1093 ° C), using low carbon stainless steel (c-0.03 max) or stable niobium or titanium.

 

6.Friction corrosion

Physical force from abrasion and fracture, dissolves metal through protective corrosion. The effect depends mainly on force and speed. Excessive vibration or metal bending can have similar results. Cavitation is a common form of corrosion pumps. Stress corrosion cracking, high tensile stress and corrosive atmospheres can cause metal corrosion.

Stress can cause corrosion cracking, and cyclic loading can cause fatigue corrosion. Fatigue corrosion is caused by exceeding the fatigue limit under non-corrosive conditions. Surprisingly, the presence of these two types of corrosion is more harmful. This is why we use the best anti-corrosion measures under alternating stress.

 

 

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