(1) Electrochemical corrosion
Electrochemical corrosion refers to an electrochemical process in which a contact surface of a heterogeneous metal forms a battery due to a difference in potential between the electrodes, thereby causing corrosion of the anode metal.
Measures to prevent electrochemical corrosion. First, the flow path of the pump is preferably made of the same metal material. Second, the sacrificial anode is used to protect the cathode metal. For example, a small piece of consumables is made of a low-potential metal, and as an anode, an important part of a large part is made of a high-potential metal. As a cathode, the anode metal is first corroded to protect the cathode metal.
(2) Uniform corrosion
Uniform corrosion refers to the occurrence of uniform chemical corrosion of the entire metal surface when the corrosive liquid contacts the metal surface. This is the most common type of corrosion type and the least hazardous type of corrosion.
Measures to prevent uniform corrosion are: take appropriate materials (including non-metal) and consider sufficient corrosion margins in pump design.
(3) Intercrystalline corrosion
Intergranular corrosion is a type of local corrosion, mainly referring to the phenomenon of precipitation of chromium carbide between stainless steel grains. Intergranular corrosion is extremely corrosive to stainless steel materials such as stainless steel submersible pumps or stainless steel self-priming pumps. The material in which intergranular corrosion occurs is almost completely lost in strength and plasticity.
Measures to prevent intergranular corrosion are: annealing stainless steel or ultra-low carbon stainless steel (C < 0.03%).
(4) Pitting corrosion
Pitting corrosion is a type of localized corrosion. This phenomenon is called pitting corrosion due to the local failure of the metal passive film causing a certain partial area of the metal surface to rapidly form a hemispherical pit. Pitting corrosion is mainly caused by Cl.
To prevent pitting corrosion, M0-containing steel (usually 2.5% Mo) can be used, and as the Cl content and temperature increase, the M0 content should also increase accordingly.
(5) Crevice corrosion
Crevice corrosion is a type of local corrosion. It refers to the corrosion caused by the partial destruction of the passive film of the metal due to the decrease of oxygen content in the gap and/or the decrease of pH value when the gap is filled with corrosive liquid. Stainless steel often undergoes crevice corrosion in CL-solutions. Crevice corrosion and pitting corrosion are similar in formation mechanism. Both are caused by the action of Cl- and the local destruction of the passive film. As the Cl content increases and the temperature rises, the possibility of crevice corrosion increases.
The use of metals with high Cr and M0 content prevents or reduces crevice corrosion.
The order of resistance to crevice corrosion is as follows: 12% Cr steel <17% Cr steel < austenitic stainless steel < austenitic 316 stainless steel.
(6) Stress corrosion
Stress corrosion refers to a type of local corrosion caused by the combination of stress and corrosive environment.
Austenitic Cr-Ni steel is more susceptible to stress corrosion in C1-medium. With the increase of Cl content, temperature and stress, stress corrosion is more likely to occur. Usually 70, -80. Stress corrosion does not occur below C. The measure to prevent stress corrosion is to use austenitic Cr-Ni steel with high Ni content (Ni; >25% to 30%).
(7) Erosion corrosion
Refers to a scouring corrosion of a metal surface by a high velocity fluid. Fluid Scour Wear Corrosion is different from the abrasion caused by solid particles in the medium. Different materials have different wear and corrosion resistance properties. The anti-wear corrosion performance is from poor to good: ferritic Cr steel <austenitic-ferritic steel <austenitic steel.
(8) Cavitation corrosion
Corrosion caused by cavitation of the pump is called cavitation corrosion. The most practical and convenient way to prevent cavitation corrosion is to prevent cavitation. For pumps that often cavigate during operation, to avoid cavitation corrosion, use cavitation resistant materials such as cemented carbide, phosphor bronze, austenitic stainless steel, 12% chrome steel, or pneumatic diaphragm pumps or other diaphragms. Pump products can avoid the occurrence of cavitation.
How to prevent corrosion problems in the pump?
Corrosion in the centrifugal pump can affect the wet surface or a small portion of its internal components, which are general or highly localized. Typically, pump failures are caused by highly localized and affected components. Therefore, it is very important to find a solution that reduces the effects of corrosion.
Let's take a look at the different ways to reduce corrosion! Not only will this save you millions of dollars, but it will also extend the life of your pump.
1, material selection
When solving corrosion problems, material selection should be considered first. This not only handles corrosion problems, but also selects the best material to reduce the rate of degradation based on the operating conditions of the centrifugal pump.
Cast iron has a relatively low corrosion resistance and can quickly reduce pump performance. Therefore, pump manufacturers use stainless steel to avoid corrosion.
Please note that the correct material selection is ultimately the responsibility of the user. Only the user can control the liquid being pumped relative to the concentration, temperature, and the like. It is the responsibility of the pump manufacturer to provide the materials of choice under physically good conditions and to provide the correct chemical composition while meeting specified hydraulic requirements. In practice, a trusted pump supplier will provide guidance based on his experience with previous applications involving similar services.
We recommend that you carefully consider the choice of materials, which is one of the most effective ways to improve the corrosion resistance of your pump.
Sources of information that can be referenced for material selection include:
(1) First-hand in-plant experience;
(2) Similar applications in other locations;
(3) Corrosion diagram of hydraulic research institute;
(4) Recommended drawings of mechanical seals;
(5) The experience of the pump manufacturer;
(6) Appropriate literature, texts and similar references.
Normally, no data available will specifically satisfy the conditions, especially if a solution containing several concentrations of several chemicals is pumped. When the temperature is increased by 10 ° C, the corrosion rate can be doubled.
The presence of dissolved air or other oxidant in the liquid may cause the corrosion rate to accelerate separately, otherwise it is considered acceptable. In some