【Daily Question】12.21 Methods to prevent crevice corrosion (Reply +3 points; correct answer adds another +3–8 points)

Crevice corrosion is corrosion that occurs within or near a slit or gap due to the presence of such a slit. The gap where crevice corrosion occurs must be wide enough for the corrosive solution to enter, but narrow enough to maintain a stagnant solution. Crevice corrosion usually occurs at the points where the metal surface comes into contact with gaskets, washers, linings, surface deposits, etc., as well as in joints and areas where metals overlap. It is generally believed that crevice corrosion is caused by the oxygen concentration difference between the inside and outside of the crack. Due to the low oxygen level inside the crack, the reduction reaction of oxygen occurs primarily on the metal surface outside the crack. The dissolution of the metal inside the crack generates an excess of positive ions (M+); in order to maintain electrical balance, harmful ions such as C- migrate from the outside into the crack, resulting in an increase in the concentration of metal chlorides inside the crack. The hydrolysis of these metal chlorides produces free acids (H+Cl-), which accelerates the rate of metal dissolution, thus creating a self-catalytic process. Crevice corrosion is often accompanied by a long incubation period, but once it begins, it progresses at an accelerating rate. Dull-state metals are prone to crevice corrosion in environments containing halide ions. The main measures to prevent crevice corrosion are: selecting materials resistant to crevice corrosion ； Implement a reasonable corrosion prevention design ； Electrochemical protection and corrosion inhibitor protection are employed.

What are the methods to prevent crevice corrosion? Answer: Many metal components are connected using screws, rivets, welding, etc., and narrow gaps may appear at defects in these connections or weld joints. Its seam width (generally between 0.025 and 0.1 mm) is sufficient for the electrolyte solution to enter, allowing the metal inside the seam and the metal outside it to form a short-circuited galvanic cell, resulting in severe corrosion within the seam. In a medium, metal undergoes severe localized corrosion in areas with gaps or on surfaces covered by other substances; this type of corrosion is known as crevice corrosion. In addition to being caused by the differences in metal ion concentrations or dissolved oxygen levels within the gap compared to those around it, the main reason for crevice corrosion is the autocatalytic process associated with this type of corrosion; the hydrolysis of metal chlorides in the gap leads to a rapid decrease in pH, which accelerates the dissolution of the metal. Factors affecting it: (1) Width of the gap: The gap must be wide enough for the liquid to enter, but narrow enough to prevent it from flowing away. (2) Media: This type of corrosion can occur in many media, but it is most severe in media containing CL—. (3) Metal material: This type of corrosion occurs easily in metals or alloys that have a passivation layer or oxide film.

Except for gaps. The most fundamental approach is to avoid gaps from the perspective of structural design; appropriate measures must be taken to prevent gaps at the joints between tubes and tube sheets in heat exchange equipment, as well as in the gaps around flanges, gaskets, bolts, and rivets. Clean regularly and maintain a flow rate of ≥1.5 m/s in environments such as seawater to prevent the accumulation of contaminants (including marine organisms) on the steel surface. Stainless steels with high chromium and molybdenum, as well as those with high chromium and molybdenum plus nitrogen, which are resistant to crevice corrosion, are selected. It can also be seen that, in order to address crevice corrosion, starting with material selection is more difficult than addressing pitting, and the economic cost is higher as well.

Where possible, the same material should be used for the accessories, with paint applied to prevent corrosion.

Crevice corrosion: In media such as chloride ions, within the structural gaps of stainless steel or at the gaps formed between deposits and stainless steel, the acidification of the solution (an increase in chloride ion concentration and a decrease in pH value) along with oxygen deficiency lead to the destruction of the passivation film, resulting in punctate or ulcerative corrosion. Control: 1. Structurally, adopt a design that prevents the formation of gaps. Use welding in place of riveting or bolting. Non-absorbent gaskets are used for flange, washer, and bolt connections. 2. In terms of material selection, stainless steels containing high amounts of chromium and molybdenum, or those containing high levels of chromium, molybdenum, and nitrogen, should be used; especially at high temperatures, it is important to choose appropriate materials. 3. Corrosion inhibitors: Add corrosion inhibitors to the medium when conditions permit. 4. Others: Anti-corrosion treatments such as cathode coating, cathodic protection, and increasing the flow rate of corrosive media can be effective in suppressing corrosion under certain operating conditions.

What are the methods to prevent crevice corrosion? Crevice corrosion is primarily caused by the electrochemical heterogeneity of the medium resulting from the presence of crevices. To prevent crevice corrosion, materials resistant to crevice corrosion are primarily chosen ； Implement a reasonable corrosion prevention design ； Electrochemical protection and corrosion inhibitor protection are employed.

1. The most fundamental way to eliminate gaps is to avoid their existence from the perspective of structural design. Appropriate measures must be taken to prevent gaps at the joints between tubes and tube sheets in heat exchange equipment, as well as in the gaps around flanges, gaskets, bolts, and rivets； 2. Electrochemical protection method, cathodic protection ； 3. Paint coated with a corrosion inhibitor ； 4. Use appropriate materials ； 5. Clean sediment in a timely manner ； 6. Use solid materials for sealing gaps ； 7. Gaskets should not be made of moisture-absorbing materials such as asbestos or paper; polytetrafluoroethylene materials should be used whenever possible ；

Many metal components are connected by welding, and narrow gaps may appear at defects in these connections or weld joints. Its gap width (generally between 0.025 and 0.1 mm) is sufficient for the electrolyte solution to enter, allowing the metal inside the gap and the metal outside it to form a short-circuited galvanic cell; intense corrosion occurs within the gap, and this type of localized corrosion is known as crevice corrosion.

1. Eliminate gaps. The most fundamental approach is to avoid gaps from the perspective of structural design; appropriate measures must be taken to prevent gaps at the joints between tubes and tube sheets in heat exchange equipment, as well as in the gaps around flanges, gaskets, bolts, and rivets. 2. Clean regularly and maintain a flow rate of ≥1.5 m/s in environments such as seawater to prevent the accumulation of contaminants (including marine organisms) on the steel surface. 3. Use stainless steels with high chromium and molybdenum, or high chromium and molybdenum plus nitrogen, to resist crevice corrosion.

What are the methods to prevent crevice corrosion? An effective way to prevent such corrosion is to increase the content of chromium and molybdenum in the alloy, especially molybdenum.