Failures Caused by Corrosion of Valves

Failures caused by corrosion of valves are that valve parts lose their original functions due to the action of corrosive media, and they can be divided into failures caused by crevice corrosion and galvanic corrosion. Failures caused by crevice corrosion refer to the failure caused by corrosion in the crevice of the valve structure. Pei Wang and others observed the macroscopic characteristics of crevice corrosion on the sealing surface of the globe valve disc, and found that pits of different sizes appeared at the bottom of the corrosion area, while the corrosion outside the corrosion area was fewer, as shown in Figure 1.

 
Figure 1 Macroscopic characteristics of the fracture of crevice corrosion
 
Failures caused by galvanic corrosion are that a potential difference is generated by the direct contact of two metal parts with different electrochemical properties in the valve, which makes the metal with high potential corrode slower and the metal with low potential corrodes faster. Jun Yuan observed the characteristics of failures caused by galvanic corrosion of the hot air valve stem’s electroplating layer, and found that there were precipitated substances on the surface of the fracture, and small honeycombs were in the partial areas. The coating layer falls off like a flake and dot. The corrosion products were analyzed by SEM and it was found that there were Cl and S elements.
 
Failures caused by galvanic corrosion
Galvanic corrosion is more damaging than failures caused by crevice corrosion and often occurs on the valve stem where the valve is coated with a corrosion resistant metal coating. At present, there is not enough characteristic analysis for different corrosion precipitation products, and the specific corrosion medium that affects the corrosion characteristics cannot be distinguished. In the future, research in this direction should be considered.
 
Causes of failures caused by corrosion of valves
The valve will inevitably leave a gap in the structure, and crevice corrosion will occur due to the accumulated corrosive medium in the gap. Essentially, crevice corrosion is a special kind of galvanic corrosion that occurs in sufficiently small crevices. Many scholars have done related research about the generation mechanism of crevice corrosion. Zhenbin Ding and others analyzed the specific process of crevice corrosion of valve internals, and found that in the electrolyte environment, when the oxygen in the crevice was consumed, a large number of external chloride ions would migrate to the interior of the crevice. The fluidity in the crevice is poor, so the acidity increases. The oxygen concentration difference battery is formed inside and outside the gap. The anode is outside the gap. The polarizability decreases. The cathode is inside the gap. The metal dissolves to generate hydrogen depolarization, which makes the metal in an activated state and accelerates crevice corrosion.
 
 
Figure 2 Crevice corrosion processes 

For galvanic corrosion, in order to improve the corrosion resistance and wear resistance of the valve, a thin metal layer is usually plated on the surface of the valve parts. Therefore, most failures caused by galvanic corrosion in valves are failures caused by coatings, which in turn lead to galvanic corrosion caused by direct contact between the metal parts and the corrosive medium. Yingbo Xu analyzed the reasons for the failure of the galvanized layer of the zinc coating and found that the coating was not dense enough, and micro-cracks were prone to occur during collision and extrusion. The corrosive medium penetrates the coating through the micro-cracks, causing corrosion of metal. As shown in Figure 3, Yushan Ma and others analyzed the process of failures caused by galvanic corrosion of the 45 steel valve stem. Corrosion pits were initially generated on the surface of the coating. As time goes on, the corrosion spread to the inner layer until it reached the surface of the base metal. A potential difference is formed between the coating and the metal to form galvanic corrosion, which accelerates the failure.
 
 
Figure 3 The mechanism of failures caused by galvanic corrosion
 
Process factors can affect both crevice corrosion and galvanic corrosion. Yang Zhou  and others analyzed the reasons for the failure of the float valve, and found that improper selection of the processing deformation temperature will transform the austenite in the float valve into martensite, reduce the corrosion resistance of the material, and then lead to crevice corrosion and galvanic corrosion.
 
As mentioned above, crevice corrosion will occur when there is a crevice. However, the specific crevice size range needs further study. The speed of failures caused by corrosion is slow at first and then accelerated. It is the focus of future research to analyze the metal species that will form the potential difference for different parts and materials of the valve.