Metals are generally composed of many crystals. The contacting surfaces of the crystals are called Grain Boundary. Within each crystal, the atoms are in an orderly alignment, but the alignments of the atoms in adjacent crystals are different. The atoms within the grain boundary areas must integrate with atoms of both crystals in dissimilar alignments, therefore, they are in mixed orientations. This means that the energy levels are in high state.

This high energy state of grain boundaries is evident from microscope observations of corrosive solution etched specimen where the grain boundaries are dissolved and the crystals becoming clearly visible.

This type of chemical corrosion remains on the surface and does not advance further, but if heated under certain conditions the crystal grain boundaries will forego chemical composition changes and some selective corrosions will occur. This phenomenon is evident on austenitic stainless steel such as SUS304, as explained below.

The reason for stainless steel's corrosion resistance is the passive surface layer, and existence of chromium is imperative for this. Stainless steel generally contain 12~13% chromium, but the element in question here is the amount of carbon content in steel.

Carbon tends to bind with chromium and easily form chromium carbide. The chromium carbide is formed when stainless steel is heated at 500~800 degrees C for a certain period of time, though the time duration varies depending on the carbon content. At 750 degrees C and 0.06% carbon content, the duration is shortest at less than one minute, and as long as several hundred hours at 500 degrees C.

The chromium carbides form at grain boundary areas. In a carbide form, chromium does not contribute in creation of passive layer. Therefore, exposed stainless steel surface will be lacking the passive layer along the grain boundaries. When stainless steel in this state is exposed to corrosive environments, corrosion along the grain boundaries will progress. This is called Intergranuler Corrosion. A sectional photo of SUS304 stainless steel with Intergranuler Corrosion is shown in [Fig.1]. Principle of this corrosion is based on formation of a battery where the corrosion progresses as the passive layer portion being the positive pole, and the portion without passive layer is the negative pole.

As countermeasures to prevent Intergranuler Corrosion, the following process is applied. A stainless steel material in the final form factor is heated to over 1000 degrees C to dissolve the chromium carbide content, and to promote the divergence of chromium toward the grain boundaries from the other areas, then rapid cooled with water. This is called Solution Heat Treating. Use of stainless steel processed to prevent Intergranuler Corrosion in high temperature applications such as welding is not recommended since Intergranuler Corrosion will occur. For such applications, lowered carbon content SUS304L and SUS316L (0.03%C) are used.
Intergranuler Corrosion does not occur in carbon steel, but occurs with high tensile aluminum alloy (with a few % copper alloyed).