Corrosion is one of the main factors causing valve damage pipeline, pipe, valve and other attached system software selection

Corrosion is one of the main factors causing valve damage pipeline, pipe, valve and other attached system software selection

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        Corrosion is the damage and quality change of raw materials under the effect of various conditions. The corrosion of metals is mainly caused by organic chemical corrosion and spot chemical corrosion, while the corrosion of non-metallic materials is generally caused by immediate chemical and physical chemical effects.

Corrosion is one of the main factors causing valve damage, therefore, in valve applications, corrosion maintenance is the first consideration.

First, the form of valve corrosion

Metal valve corrosion has two forms, namely symmetrical corrosion and partial corrosion. The rate of symmetrical corrosion can be evaluated by the mean annual corrosion rate. Metal raw materials, high purity graphite, laminated glass, porcelain and concrete, according to the size of corrosion rate can be divided into 4 grades: corrosion rate lower than 0.05mm/a for high quality; The corrosion rate between 0.05 and 0.5mm/a is excellent. Corrosion rate in 0.5~1.5mm/a can also be used; Corrosion rate of more than 1.5 mm/a is not suitable for, tu surface of valve, valve seat, pulse dampers, the small spring all sorts of valve generally use level of raw materials, such as oil plate, single valve for secondary or tertiary raw materials, such as to high pressure, toxic, flammable, flammable, radioactive material valve,, the use of the little corrosive materials.

1, symmetrical corrosion

Uniform corrosion occurs on all surfaces of a metal. Such as stainless steel plate, aluminum, titanium and so on in the air oxidation natural environment formed a layer of protective film, film metal corrosion symmetry. There is also a condition where the metal surface corrodes off, and this type of corrosion is most risky.

2, partial corrosion

Local corrosion occurs in some parts of the metal, its form is porous corrosion, gap corrosion, intergranular corrosion, delamination corrosion, internal stress corrosion, fatigue corrosion, selective corrosion, damage corrosion, cavitation corrosion, vibration corrosion, hydrogen corrosion, etc..

Gap corrosion usually occurs on the metal of passivated film or protective film, because there are shortcomings on the metal surface, aqueous solution can destroy the active positive ions of passivated film, so that the passivated film is partially destroyed, into the internal structure of the metal, into the corrosion hole, it is one of the corrosion forms of metal destruction and safety risks.

Gap corrosion occurs in natural environment such as welding, riveting, gasket or precipitation. It is a unique form of pitting corrosion. The way to avoid this is to clear the gap.

Intergranular corrosion is the dislocation network corrosion caused by the deep metal structure along the surface. The main causes of intergranular corrosion are the unreasonable heat treatment process and cold drawing, besides the residue deposited by dislocation. The welding joints of martensitic stainless steel are very easy to cause chromium poor zone and suffer corrosion. Intergranular corrosion is a common and risky form of corrosion in low alloy steels. To avoid intercrystalline corrosion caused by various low alloy steel valves, there are: to carry out heat treatment and tempering heat treatment “solution, that is, heating to 1100℃ upstream and downstream water quenching, using low alloy steel with titanium and niobium, and the carbon content of 0.03% below, reduce the cause of carburized chromium.

Delamination corrosion occurs in lamellar structure. The corrosion first develops vertically inward, and then corrodes the chemical substances on the parallel surface. Under the expansion force of the corrosion, the surface is lamellar.

Internal stress corrosion occurs when corrosion and tensile stress occur at the same time. Ways to avoid internal stress corrosion; According to the heat treatment process to remove or reduce the internal stress formed in electric welding and cold drawing, improve the unreasonable valve structure, prevent stress, choose organic chemical maintenance, coating corrosion resistant building paint. Add desulfurizer, increase pressure internal stress and other countermeasures.

Corrosion fatigue occurs at alternating positions of internal stress corrosion of the same efficacy, causing the metal to crack. Heat treatment process can be completed to remove or reduce internal stress, surface shot peening treatment and phosphating treatment, chromium, nickel, but pay attention to the coating can not have tensile stress and hydrogen diffusion phenomenon.

Selective corrosion occurs in raw materials with different chemical composition and residue. In a certain natural environment, some elements are corroded and leached, and the remaining uncorroded elements are honeycomb. There are generally copper dezincification, alloy copper desalinization, pig iron graphitization and so on.

Damage corrosion is a corrosion form formed by liquid damage to metal and corrosion replacement effect. It is a common corrosion of valves. This kind of corrosion occurs in the protrusion. Ways to avoid: use corrosion-resistant and wear-resistant raw materials, improve the overall design, choose pipeline cathodic protection, etc.

Cavitation corrosion, also known as cavitation and cavitation, is a unique form of corrosion damage. It is the bubble formed in the liquid, in the destruction of the shock wave, the working pressure can reach 400 atmospheric pressure, so that the metal protective film damage, and even tear metal particles. Then corrosion film, this whole process continues, so that the metal corrosion. To avoid cavitation corrosion, materials resistant to cavitation corrosion can be used, production surface with high smoothness, ductility protection layer and pipeline cathodic protection, etc.

Friction corrosion is the mutual understanding of two components at the same time bearing load, contact area due to vibration and rolling damage. Friction vibration corrosion occurs in the screw connection, valve seat and close parts of the connection, rolling bearing and shaft between the position. You can choose grease, reduce friction, surface phosphating treatment, the use of carbide cutting tools, and its use of spray tile solution or cold pull to enhance the surface strength of the way safety protection. Corrosion is the damage caused by the spread of hydrogen atoms in organic chemistry to the internal structure of metal, and its forms are hydrogen bulges, hydrogen embrittlement and hydrogen corrosion.

Strong steel and steel with nonmetals are very prone to hydrogen bubbling. Crude oil containing sulfates and hydrides is prone to hydrogen bubbling. Choose hollow steel instead of ordinary carbon steel with holes, take plastic and plastic maintenance, add desulfurizer can avoid bulging.

The lattice constant of the strong steel is relatively high in rotation. After hydrogen atom enters, the lattice constant of 4 should change greatly, resulting in financial brittle crack. Carbon steel containing nickel and lead should be used to prevent the use of high strength steel plate with high hydrogen brittleness. Hydrogen embrittlement should be prevented or reduced in electric welding, electroplating process and pickling passivation. Hydrogen enters the metal at constant high temperature and pressure, and is destroyed by an exothermic reaction with a constituent element called hydrogen corrosion. Low alloy steel is thoroughly resistant to hydrogen corrosion.

3, non-metallic corrosion

Non-metallic corrosion has the same size as metal corrosion. Most non-metallic raw materials are non-electric conductors, which are not easy to cause organic chemical corrosion, but extreme chemical or physical corrosion, which is also the key difference from metal corrosion. Non-metallic corrosion is not necessarily gravity-free but usually weight gain. For metallic corrosion, gravity-free is specific. Non-metallic corrosion is mostly caused by physical effects, which are very rare. Non-metallic internal structure corrosion is common, while metal corrosion is dominated by surface corrosion.

After metal raw materials and media contact, aqueous solution or air will slowly spread to the internal structure of the data, so that a series of corrosion transformation of non-metal, according to the categories and types of non-metal raw materials, their corrosion form is different. The morphology of corrosion is melting, swelling, bubble, soft, there will be dissolution, fading, quality change, aging, hard bottom, rupture and so on. However, from a revolutionary point of view, non-metallic materials have better corrosion properties than metal materials, but not the compressive strength of metal materials, but less heat resistance than metal materials.


Post time: May-12-2022

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