Crevice corrosion media and materials?

Electrolyte solutions, especially those containing chloride ions and hydrofluoric acid, can easily cause crevice corrosion, posing a threat to inert metals such as stainless steel.

Corrosion generally occurs in water-containing metal gaps

For slurry media containing solids, even in the smallest pipes, socket welding is not used; instead, butt welding is employed? I guess this is also to prevent crevice corrosion, right?

It generally refers to stainless steel products. I understand it to be the same as pitting corrosion, as corrosion occurs due to accumulation

The last edit to this post was made by xlxuiin on 2012-8-22 at 20:33. The ratio of the maximum pit depth to the average corrosion depth is called the pitting coefficient. The higher this value, the more severe the pitting will be. Generally, corrosion pits are covered by corrosion products and are difficult to detect; as a result, sudden accidents often occur due to corrosion perforation (see metal corrosion). 　　Crevice corrosion is a type of corrosion that occurs in the gaps between two connected components. This localized corrosion can arise in gaps between metals joined together by methods such as riveting or bolting, in gaps between metals and non-metals, as well as in gaps formed between deposits on a metal surface and the metal surface itself. 　　Many metal materials are susceptible to pitting and crevice corrosion. Metal materials such as stainless steel and aluminum alloys, which rely on passivation to enhance their corrosion resistance, are also prone to pitting and crevice corrosion. Many environmental media can cause pitting and crevice corrosion in metal materials, especially solutions containing chloride ions. 　　Pitting corrosion: Electrochemical heterogeneity on the metal surface is a major cause of pitting corrosion. Certain defects or weak points often appear in the surface of metal materials or in protective layers such as passivation films (such as at inclusions, grain boundaries, dislocations, etc.), and these areas are prone to forming pitting nuclei. When a metal is immersed in a solution containing certain activating anions (especially chloride ions), pitting can occur as long as the corrosion potential reaches or exceeds the pitting potential (also known as the breakdown potential). This is due to a dynamic equilibrium between the dissolution of the passivation film in the solution and its reformation; the active anions in the solution (chloride ions) disrupt this equilibrium, causing tiny corrosion spots to form on the local surface of the metal, which then develop into sources of pitting. For example, the dissolution of sulfide inclusions on the stainless steel surface exposes the fresh surface of the steel, thereby creating sources of pitting. 　　The development of pitting is a autocatalytic process within the occluded region. In erosional pores with a certain degree of obstruction, the concentration of dissolved metal ions **increases**; in order to maintain charge balance, chloride ions continuously migrate into these pores, resulting in their accumulation. The hydrolysis of high-concentration metal chlorides produces hydrogen ions, thereby creating a highly acidic environment within the etched pores; this further accelerates the dissolution of the metal in those pores, as well as increases the concentration of chloride ions in the solution and leads to increased acidity. The inner wall of the erosion pit is in an activated state (acting as the anode of the corrosion galvanic cell), while the metal surface outside the erosion pit remains in a passive state (acting as the cathode). This creates an activated-passive cell system with a small anode and a large cathode, thereby facilitating rapid development of pitting corrosion. 　　Crevice corrosion is caused by the difficulty of material movement between the media inside and outside the crevice. To this end, the width of the gap should be narrow enough. Its development is also a autocatalytic process within a closed region. For example, in steel components located in media such as seawater, at the initial stage of crevice corrosion, a corrosion process occurs on both the metal surfaces inside and outside the crack, with oxygen reduction serving as the cathodic reaction. As the dissolved oxygen in the gap is quickly consumed, and it is very difficult to replenish it through diffusion, the cathodic reaction of oxygen reduction in the gap gradually stops, resulting in the formation of an oxygen concentration cell between the inside and outside of the gap. The oxygen reduction cathodic reaction occurring over a large area outside the gap promotes the dissolution of the metal anode inside the gap. The dissolution of metal within the gap generates excess metal cations (Me+), which in turn causes chloride ions outside the gap to move into the gap in order to maintain electrical balance. The subsequent hydrolysis of metal ions increases the acidity within the gaps, which in turn accelerates the anodic dissolution of the metal (see figure). Pitting corrosion and crevice corrosion Comparison of pitting corrosion and crevice corrosion. The mechanisms underlying the development of both pitting corrosion and crevice corrosion are similar, but their triggering mechanisms and occurrence processes differ. The former is caused by the passive state of the material or local damage to the protective layer, and develops through the formation of pitting sites; the latter is caused by the electrochemical heterogeneity of the medium, with corrosion starting under confined-cell conditions in gaps. From the perspective of electrode potential, the electrode potential at which crevice corrosion occurs and develops is lower than that for pitting corrosion. In terms of media, crevice corrosion can occur in solutions free of chloride ions, whereas pitting tends to occur only in the presence of specific active anions. 　　The chloride ion concentration in the solution has a significant impact on both types of corrosion; generally, the higher the chloride ion concentration, the greater the likelihood of pitting and crevice corrosion, and the faster they progress. Other halide ions also have a similar effect. Generally, the higher the temperature of a solution, the greater the risk of pitting and crevice corrosion. 　　Preventive measures: An important way to improve the pitting resistance of materials is by adding appropriate alloying elements (such as molybdenum in stainless steel), applying passivation treatments and proper heat treatment, as well as reducing the amount of impurities in the metal material. The main measures to prevent crevice corrosion are to avoid gaps and geometric shapes that can lead to surface deposits in the structure, to prefer welding over riveting, and to use non-hygroscopic materials for gaskets. Electrochemical protection is effective in preventing both pitting and crevice corrosion. Using appropriate metal materials with resistance to pitting and crevice corrosion is also an effective measure to prevent pitting and crevice corrosion.

For acidity, chloride ions, etc., impervious graphite can be used for such equipment