Tian Jin Rong Da Jian Ce You Xian Gong Si

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Product Details

Pipeline crevice corrosion inspection

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Place of Production 天津
Product Category 工程监理和第三方检验
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Product Description

Pipeline crevice corrosion testing, corrosion testing of the joints of the gasket, weld corrosion detection, valve seat crevice corrosion detection, metal sheet corrosion detection
Tianjin Rong Da Testing Co., Ltd. contact: Wang Jingfu
Cell phone: 13102238283 QQ:3144667033
Telephone: 022-23892103
Crevice corrosion, refers to the macro pitting spots or ulceration occurred in metal components cracks, this is a kind of localized corrosion often occurs in the joints connecting washers, screws, rivet, or lap welding of sheet metal joint, valve seat, accumulation between etc.. The reason is: the crevice corrosion corrosion product covering the gap is formed, and medium diffusion limited, resulting in the gap of the medium composition and concentration compared with the surrounding vary greatly, and has gradually formed a "occluded cell corrosion".
Its characteristics are:
1, crevice corrosion can occur in all metals and alloys;
2, can occur in any etching solution;
3. The corrosion potential of crevice corrosion is lower than that of point corrosion potential;
4, slot width must make etching solution into the gap, and to narrow solution stagnation in the gap, to crevice corrosion, sensitive to 0.025~0.1mm width.
The process of crevice corrosion can be divided into the initial stage and the development stage. In the initial stage, the metal anode is dissolved and the oxygen is dissolved in the metal surface. Due to oxygen in the gap can not be exchanged with the outside world, the rapid consumption and difficult to be added, so the oxidation reduction reaction is quickly terminated, and the formation of oxygen concentration difference between the inside and outside the battery. The anodic dissolution process of metal in the gap continues, and the cathodic reaction of oxygen reduction have been transferred to the gap metal surface, the cathode (seam) and small (slot) of the anode area, and the metal dissolution reaction in the joint acceleration, two corrosion products gradually accumulate in the seam. The development, has become a typical occluded cell, then entered the development stage of crevice corrosion. The stage of development, a large number of positively charged metal ions dissolved in solution in the accumulation in the gap, and the chloride ions in the bulk solution to keep the neutral point gap, while the formation of metal salts, followed by chloride hydrolysis, acid was increased, the pH value decreased, and further promote the gap within the anodic dissolution of metal. This process is repeated cycle, known as the self catalytic process of crevice corrosion.
So for the protection of crevice corrosion, can refer to the following points:
The first point, reasonable design, to avoid the gap. For example: superior welding riveting; butt welding lap welding welding is superior; we must ensure the quality, avoid welding holes; screw joint structure, can use low sulfur rubber gasket, dense packing, joint surface to protective coating. In addition, avoid seeper area is design should consider things; maintenance, should be diligent in cleaning, do not leave dirt.
Second, the gap can not be avoided, you can use the cathodic protection. Like in the sea, the use of zinc or magnesium is very active and this kind of activity is relatively large. But there is one point to note, take this approach, attention should be paid to hydrogen embrittlement.
Third, taking into account the inhibitor is difficult to enter the gap, so it can be in the joint surface coated with paint, corrosion inhibitor for steel, for example, the use of paint PbCrO4; the metal sheet can be used separate packing paper impregnated with gas phase corrosion inhibitor for aluminum, and use; paint ZnCrO4. Because of "wood". That's just the way it is.
Fourth point, the use of suitable materials, for some of the important components, the ability to resist crevice corrosion of materials can be a priority, such as high chromium and molybdenum stainless steel, etc.