Figure 1 Intergranular Corrosion
Intergranular corrosion is the type of corrosion that occurs at the grain boundaries without deeply corrode the metal matrix. The integranular corrosion occurs for two conditions:
The metallographic condition refers to the uneven material or structural that for different causes is established in the grain boundaries. This condition is caused by lattice modifiers (crystalline defects) or from the presence of accumulated heterogeneous phases at the grain boundaries during the crystallization process (impurities) or during a thermal process which has caused their precipitation at the grain boundaries. The environmental conditions determine different attacks on the grain boundaries. These differentiated attacks may be the result of an action of selective dissolution of the metal in the state of higher reactivity, or by the formation of galvanic areas in which the anodic areas are the grain boundaries while the cathodic areas is the metal struturale matrix. The intergranular corrosion may occur even at high temperature through the penetration to the grain boundaries of elements in the gaseous state (eg. Nickel in the presence of sulfur and sulphides). The intergranular corrosion involves many metals such as aluminum, stainless steel, alloys of nicel, etc … The nature of this form of corrosion is very insidious because it attacks in the micro-structural level without having the metal corrosion products visible on the outer surface . This form of corrosion thus leads to a depletion of the bond between a grain and another, with an impoverishment of the mechanical characteristics, and a propagation of the crack between the collapsed grain boundaries, that is, where the application of the effort is of greater intensity. In severe cases, this form of corrosion of the metal leads to the ginning (Figure 1) with disastrous and catastrophic consequences. Focusing on stainless steels, these are susceptible to intergranular corrosion when subjected to heat treatment such as to “sensitize” to this form of corrosion. The sensitization of stainless steels occurs following any heat treatment between 400 and 900 ° C (eg. TIG welding and MIG). At this temperature the carbon, present in the alloy, it tends to form carbides with chromium, which is also present in the alloy. These carbides precipitate at grain boundaries, i.e. the points of greater activity, and subtract elemental chromium necessary to give the passive layer and increase the corrosion resistance of the steel.
Figure 2 The chromium content trend close to the grain boundaries of a stainless steel
Figure 2 shows the trend of elemental chromium relative to the two grains section closest sensitized. Near the ordinate, the chromium content is very high because of the intense Formazine of chromium carbides at the grain boundary. Immediately after, the content of chromium collapses rapidly to levels such as to be less than the limit beyond which the steel is passive with the simple exposure to air it. This decrease of elemental chromium will lead to intergranular corrosion if the steel comes in contact with the corrosive environment suitable. The factors that contribute to raising awareness are:
- Carbon content: the higher the content, the more there is the risk of this form of corrosion;
- Awareness temperature for austenitic range is 650-700 ° C;
- Sensitization of time: the maximum degree of sensitization takes place after about 10000 minutes;
- Grain size: grain coarse grains will be more susceptible to sensitization than fine-grained;
- Cold working: increases the areas of precipitation of chromium carbides;
- Addition of alloy elements: adding titanium or niobium percentages, it promotes the formation of titanium carbide or niobium (because most closely related with the carbon) which stabilizzanoil grain boundary and leaves intact the chromium present in the alloy.
The prevention methods include:
- Avoiding sensitizing heat treatments;
- Avoiding steels sensitized;
- In promoting the use of steel stabilized with titanium or niobium;
- In restoring the steels previously sensitized through a heat treatment again solubilizes the chromium carbides and limits the metal stay for awareness temperatures;
- Using a particular corrosion inhibitors.