Corrosion of valves is one of the main reasons for valve failure. There are several forms or causes of corrosion, which can be roughly divided into six types of corrosion. Corrosion is the natural waste of getting metals into their ores. The chemistry of corrosion 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 an electron, he remains a metal. Otherwise it will be corroded. Physical Forces Most of the time physical and chemical actions work together to cause valve failure. There are many common varieties of corrosion, mostly overlapping each other. The mechanism of corrosion resistance is due to the formation of a thick protective corrosion film on the metal surface.
When two dissimilar metals are in contact and exposed to corrosive liquids and electrolytes, a galvanic cell is formed, and the current increases the current causing the anode to corrode. Corrosion is usually localized near the point of contact. Corrosion reduction 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 known that most light metals (those that are lighter than their oxides) form a non-protective oxide layer that gets thicker over time and then peels off. There are also other forms of high temperature corrosion including vulcanization, carburizing, and more.
This all happens in crevices, which impede the diffusion of oxygen, creating regions of high and low oxygen, resulting in differences in solution concentration. In particular, there may be narrow gaps at the defects of connectors or welded joints, and the gap width (usually 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 the occurrence of a short-circuit galvanic cell occurs in the gap. Localized corrosion of strong corrosion.
When the protective film is destroyed or the corrosion product layer is decomposed, localized corrosion or pitting corrosion occurs. The membrane ruptures to form the anode and the unbroken membrane or corrosion product acts as the cathode, effectively creating a closed circuit. Some stainless steels are prone to pitting corrosion in the presence of chloride ions. Corrosion occurs due to these inhomogeneities on the metal surface or rough parts.
Intergranular corrosion occurs for a variety of reasons. The result is almost the same breakdown of mechanical properties along the metal grain boundaries. Intergranular corrosion of austenitic stainless steels at temperatures of 800–1500°F is susceptible to many etchants (427–816°C) without proper heat treatment or contact sensitization. This condition can be eliminated by pre-annealing and quenching to 2000°F (1093°C) with low carbon stainless steel (c-0.03 max) or stabilized niobium or titanium.
The physical force of fracture from wear, dissolving metals 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 in pumps, and stress corrosion cracking, high tensile stress and corrosive atmospheres can cause metal corrosion. Under the action of static load, the tensile stress of the metal surface exceeds the yield point of the metal, and the corrosion is concentrated in the area where the stress is applied, and the result is shown as a localized corrosion. In parts where metal alternately corrodes and builds up high stress concentrations, this corrosion can be avoided by early stress relief annealing, or by selecting appropriate alloy materials and designs. Corrosion fatigue We usually associate static stress with corrosion.
Stress can lead to corrosion cracking, and cyclic loading can lead to fatigue corrosion. Fatigue corrosion occurs when the fatigue limit is exceeded under non-corrosive conditions. Surprisingly, the coexistence of these two types of corrosion is even more harmful. This is why under the action of alternating stress, we have to use better anti-corrosion measures.