How is the valve corroded?

Valve corrosion is one of the main causes of valve failure. How many forms or causes of corrosion? There are roughly six types of corrosion.

Corrosion is the natural and wasteful way of getting metals into their ores. The chemistry of corrosion emphasizes the basic corrosion reaction of M0M + electrons, where M0 is a metal and M is a positively ionic metal, as long as the metal (M0) retains electrons, it remains a metal, otherwise it will be corroded. Physical forces Most of the time physical and chemical forces will work together to make the valve fail. There are many common varieties of corrosion, mainly overlapping. The corrosion resistance mechanism is due to the formation of a thick protective corrosion film on the metal surface. Types include:

Electric corrosion

When two different metals are in contact and exposed to corrosive liquids and electrolytes, forming galvanic cells, the current causes the anodic piece to corrode and increase the current. Corrosion is usually localized near the point of contact. The reduction of corrosion can be achieved by plating dissimilar metals.

High temperature corrosion

To predict the effects of high temperature oxidation, we need to examine the following data: 

1) metal composition 

2) atmosphere composition

3) temperature 

4) exposure time. 

But it is well known that most light metals (those that are lighter than their oxides) form a non-protective oxide layer that gets thicker over time and falls off. Other forms of high temperature corrosion include vulcanization, carburization, and so on.

Crevice corrosion

This happens in gaps that block the diffusion of oxygen, creating areas of high and low oxygen and creating a difference in solution concentration. In particular, the joints or welded joint defects may appear narrow gap, the gap width (generally in 0.025~0.1mm) enough to make electrolyte solution into the gap, the metal and the metal outside the gap to form a short circuit galvanic cell, and strong local corrosion in the gap.

pitting

Local corrosion or pitting occurs when the protective film is destroyed or the corrosion product layer is decomposed. The membrane ruptures to form an anode and the unruptured membrane or corrosion product acts as the cathode, effectively creating a closed circuit. Some stainless steels are easy to pitting in the presence of chloride ions. Corrosion occurs on metal surfaces or rough parts because these are not homogeneous.

Intergranular corrosion

Intergranular corrosion occurs for a variety of reasons. The result is almost identical mechanical property destruction along metallic grain boundaries. Intergranular corrosion of austenitic stainless steel at 800 -- 1500° F is subject to many corrosive agents (427 -- 816°C) without proper heat treatment or contact sensitization. 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 stabilized niobium or titanium.

Friction corrosion

From the physical forces of wear and tear, the metal is dissolved through protective corrosion. The effect depends mainly on force and speed. Too much vibration or bending of metal can have similar results. Cavitation is a common form of corrosion pump, stress corrosion cracking, high tensile stress and corrosive atmosphere will cause metal corrosion. When the tensile stress on the metal surface exceeds the yield point of the metal under static load, the corrosion concentrates on the area of stress action, and the result shows a local corrosion. In alternating metal corrosion and the establishment of high stress concentration of parts, such corrosion can be avoided by early stress relief annealing, or the selection of appropriate alloy materials and design schemes. Corrosion fatigue We usually associate static stress with corrosion.

Stress can cause corrosion cracking and cyclic loading can cause fatigue corrosion. Fatigue corrosion occurs when the fatigue limit is exceeded under non-corrosive conditions. Surprisingly, when these two kinds of corrosion are present together, they are more harmful. That's why, under alternating stresses, we use the best anti-corrosion measures, right