Intergranular and Galvanic Corrosion of Materials
Intergranular corrosion is a localized form of attack occurring at the grain boundaries of a metallic materials with nominal or no effect on the grains. It affects metal’s strength and ductility. This type of corrosion occurs quickly affecting the metal deeply and damaging it.
While using austenitic stainless steels, the corrosion is caused by carbide precipitation during welding. Carbide precipitation can be avoided by using alloys with carbon content below 0.03% or alloys that are stabilized with columbium or titanium or heat processing after a quick cooling, a process that retains carbides in solution. The best choice is to use a low carbon alloy or stabilized austenitic stainless steel.
Nickel base alloys are exposed to carbide precipitation and precipitation of intermetallic phases when subjected to temperatures below their annealing points. With austenitic stainless steels low carbon concentration alloys are preferred to delay carbide precipitation. For example in Inconel alloy 625, niobium, tantalum or titanium is added to withstand it against chromium or molybdenum carbide precipitation. These elements interact with carbon rather chromium or molybdenum.
Galvanic corrosion is specified as dissimilar metal corrosion is observed in the most uncommon areas, causing major engineer problems. The Galvanic series of Metals shows details how the galvanic current will flow between two metals and which metal will receive corrosion when they are placed in contact of each other. The corrosion rate of metal is also influenced by the relative areas between the anode and cathode. As the current flow is from anode to cathode, the combination of the large cathode region and small anode region is unwanted. Corrosion of anode can be several times higher than if the two zones were identical. Similarly anode zone should be larger than cathode zone.
The passivity of stainless steel is resulted by availability of a corrosion resistant oxide layer on the surface. In many natural conditions, they remain in the passive form and act as cathode to iron or steel. In high chloride concentrations like in marine water or in reducing solution, a variation in active state often occurs. Lack of oxygen also results into changing active state. It happens where oxygen is not freely available for example in crevices and below contamination on slightly fouled surfaces.
Recommended corrosion resistant Metals and Alloys
- Nickel 200 shows supreme resistance to all concentrations of caustic soda at temperatures up to the melting point. It prevents stress corrosion cracking in chlorine salts and also withstands nonoxidizing halides.
- Monel alloy 400 offers excellent corrosion resistance in sulfuric acid, hydrochloric acid and phosphoric acid. It also prevents corrosion in oxidizing conditions such as nitric acid, ferric chloride, chromic acid, wet chorine, sulfur dioxide or ammonia.
- Inconel 600 wire and bar resist dry halogens at high temperatures and are widely used in chlorination systems at temperatures about 538oC or 1000oF. It prevents stress corrosion cracking in all types of chloride salts and also resists corrosion to all types of nonoxidizing halides. So, it is used in applications of dealing with water media.