These six forms of corrosion cause your valve to fail
Valve corrosion is one of the main causes of valve failure. There are several forms or causes of corrosion, which can be divided into six forms. Corrosion is the natural waste of metals into their ores. The chemistry of corrosion emphasizes the basic corrosion reaction m0m + electron, where M0 is a metal and M is a positive ion metal. As long as the electron retained by the metal (M0) is still a metal. Otherwise it will be corroded. There are physical forces. Most of the time, physical and chemical effects will make the valve fail together. There are many common varieties of corrosion, which mainly overlap each other. The mechanism of corrosion resistance is due to the formation of a thick protective corrosion film on the metal surface.
When two different metals are in contact with and exposed to corrosive liquids and electrolytes to form a galvanic cell, the current causes the corrosion of anode parts to increase the current. Corrosion is usually localized near the contact point. Corrosion reduction can be achieved by electroplating dissimilar metals.
High temperature corrosion
In order to predict the effect of high temperature oxidation, we need to test these data: 1) metal composition, 2) atmosphere composition, 3) temperature, and 4) exposure time. However, it is well known that most light metals (those lighter than their oxides) form a non protective oxide layer, which will fall off as time goes by. There are also other forms of high temperature corrosion, including vulcanization, carburization and so on.
This happens in the gap, which hinders the diffusion of oxygen, resulting in high and low oxygen areas, forming the difference of solution concentration. In particular, there may be a narrow gap at the defect of the connector or welded joint. The gap width (generally 0.025 ~ 0.1mm) is enough to allow the electrolyte solution to enter, so that the metal in the gap and the metal outside the gap form a short-circuit galvanic cell, and strong local corrosion occurs in the gap.
When the protective film is damaged or the corrosion product layer is decomposed, local corrosion or pitting corrosion occurs. Film rupture forms an anode and an unbroken film or corrosion product as a cathode. In fact, a closed circuit has been established. In the presence of chloride ions, some stainless steels are prone to pitting corrosion. Corrosion occurs on the metal surface or rough parts due to these non-uniformity.
There are many reasons for intergranular corrosion. The result is almost the same destruction of mechanical properties along the metal grain boundary. Intergranular corrosion of austenitic stainless steel at 800 – 1500 ° f is affected by many corrosive agents (427 – 816 ° C) without proper heat treatment or contact sensitization. This 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.
The physical force of wear fracture dissolves the metal through protective corrosion. The effect mainly depends on force and speed. Excessive vibration or metal bending 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. Under static load, the tensile stress on the metal surface exceeds the yield point of the metal, and the corrosion is in the area of concentrated stress. The result shows that it is a local corrosion. In the alternative corrosion of metals and the establishment of components with high stress concentration, this 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 will lead to corrosion cracking, and cyclic load will lead to fatigue corrosion. Fatigue corrosion occurs when the fatigue limit is exceeded under non corrosive conditions. Surprisingly, if these two kinds of corrosion exist at the same time, the harm is greater. This is why we should use good anti-corrosion measures under the action of alternating stress.