Corrosion is one of the main valve is a valve failure, corrosion has caused several forms or reason, can be roughly divided into six forms of corrosion. Corrosion is the natural waste of the way to get the metal into their ore. Corrosion chemistry emphasizes the basic corrosion reaction M0M + electrons, where M0 is a metal and m is a positive ion metal, as long as the metal (M0) retains the electron, he is still a metal. Otherwise it will be corroded. Physical forces have most of the time physical and chemical interactions can make valves fail. There are many common varieties of corrosion, mainly overlapping each other. The corrosion resistance mechanism is due to the formation of a thick protective corrosion film on the metal surface.
    Types include:
    Electrocorrosion
    When two different metals are in contact with and exposed to corrosive liquids and electrolytes, forming a galvanic cell, the current causes the anode member to corrode and increase the current. Corrosion is usually near local contact points. Reducing corrosion can be achieved by electroplating dissimilar metals.
    High temperature corrosion
    In order to predict the effects 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 that are lighter than their oxides) form a non-protective oxide layer that becomes thicker and becomes detached over time. There are other forms of high temperature corrosion including vulcanization, carburization, and the like.
    Crevice corrosion
    This happens in the gap, which blocks the diffusion of oxygen, causing high and low oxygen regions, forming a difference in solution concentration. In particular, there may be narrow gaps in the joints or welded joints. The slit width (usually 0.025~0.1mm) is enough for the electrolyte solution to enter, so that the metal in the joint and the metal outside the joint form a short-circuit galvanic cell and occur in the seam. Localized corrosion of strong corrosion.
    Pitting
    When the protective film is broken 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. Some stainless steels are easily pitting in the presence of chloride ions. When corrosion occurs, on metal surfaces or rough parts, these are not uniform.
    Intergranular corrosion
    There are many reasons for intergranular corrosion. The result is almost the same as the metal grain boundary, the destruction of mechanical properties. Intergranular corrosion of austenitic stainless steels without proper heat treatment or contact sensitization at 800–1500° Fahrenheit is subject to many corrosive agents (427–816 ° C). This can be achieved by pre-annealing and quenching at 2000 °F (1093 °C) using low carbon stainless steel (c-0.03 max) or stabilized tantalum or titanium.
    Frictional corrosion
    From the physical forces of wear and tear, the metal is dissolved by 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 pumping. Stress corrosion cracking High tensile stress and corrosive atmosphere can cause metal corrosion. Under the static load, the tensile stress of the metal surface exceeds the yield point of the metal, and the corrosion concentrates the area where the stress acts, and the result shows a local corrosion. In the case of alternating metal corrosion and the establishment of high stress concentrations, this corrosion can be avoided by early stress relief annealing, or by using appropriate alloy materials and designs. Corrosion fatigue We usually associate static stress with corrosion.
    Stress can cause corrosion cracking, which can cause fatigue corrosion. Fatigue corrosion is produced by exceeding the fatigue limit under non-corrosive conditions. Surprisingly, both of these corrosions are more harmful. This is why we use anti-corrosion measures under alternating stress.
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