Corrosion of valves is one of the main causes of valve failure. Corrosion can be caused by several forms or causes. It can be roughly divided into six types of corrosion. Corrosion is the natural waste of metals to their ore. Corrosion chemistry emphasizes the basic corrosion reaction of M0M + electrons, where M0 is a metal and metal in which m is a positive ion, so long as the metal (M0) retains electrons, he is still a metal. Otherwise it will be eroded. Physical forces have most of the time physically and chemically working together to invalidate valves. There are many common varieties of corrosion that are mainly overlapping. The corrosion resistance mechanism is due to the formation of a thick protective corrosion film on the metal surface.
When two different metals are in contact and exposed to a corrosive liquid and electrolyte, forming a galvanic cell, the current causes the anodic element to corrode to increase the current. Corrosion is usually near the local contact point. Reducing corrosion can be achieved by electroplating dissimilar metals.
High temperature corrosion
To predict the effect of high temperature oxidation, we need to examine these data: 1) metal composition, 2) atmosphere composition, 3) temperature, and 4) exposure time. However, it is well known that most of the light metals (those that are lighter than their oxides) form a non-protective oxide layer that, over time, gets thicker and falls off. There are also other forms of high temperature corrosion including vulcanization, carburization, and the like.
This happens in the crevices, which impede the diffusion of oxygen, resulting in high and low oxygen regions, resulting in differences in solution concentration. In particular, narrow gaps may occur at the joints or weld joint defects, and the slit width (usually 0.025 to 0.1 mm) is sufficient to allow the electrolyte solution to enter, so that the metal in the seam and the metal outside the seam form a short-circuit primary battery and occur within the seam. Strong corrosion of local corrosion.
When the protective film is destroyed or the corrosion product layer decomposes, local corrosion or pitting occurs. The membrane ruptures to form the anode and the unbroken membrane or corrosion product acts as a 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, it is not uniform on the metal surface or roughness.
Intergranular corrosion occurs for a variety of reasons. The result is almost the same along the metal grain boundaries, the destruction of the mechanical properties. The intergranular corrosion of austenitic stainless steels without appropriate heat treatment or contact sensitization of 800–1500° Fahrenheit is affected by many corrosives (427–816°C). This can be eliminated by pre-annealing and quenching at 2000°F (1093°C) using low carbon stainless steel (c-0.03 max) or stabilized tantalum or titanium.
The physical force that breaks from the wear dissolves the metal by protective corrosion. The effect depends mainly on force and speed. Excessive vibration or metal bending can also have similar results. Cavitation is a common form of corrosion pump. Stress corrosion cracking High tensile stress and corrosive atmosphere can cause metal corrosion. Under static loading, the tensile stress on the metal surface exceeds the yield point of the metal, and the corrosion concentrates on the area where the stress acts. The result shows a localized corrosion. Alternating metals and establishing high stress concentration components can avoid this type of corrosion through early stress relief annealing, or use appropriate alloy materials and design options. Corrosion fatigue We usually associate static stress with corrosion.
Stress can cause corrosion cracking and cyclic loading can lead to fatigue corrosion. Fatigue corrosion results from exceeding the fatigue limit under non-corrosive conditions. Surprisingly, these two kinds of corrosion exist at the same time, the more harmful. This is why we use the best anti-corrosion measures under alternating stress.