Meaning
Localized material degradation occurring along the grain boundaries of an alloy reduces the structural integrity and load-bearing capacity of industrial metal components. Intergranular corrosion occurs when sensitization of stainless steel or titanium alloys leads to chromium depletion at the grain boundaries, making these regions highly susceptible to chemical attack. In Chinese metal fabrication, testing for this defect must comply with the GB/T 4334 standard, which governs the evaluation of corrosion resistance in stainless steels.
Degradation Mechanism
High-temperature processes such as welding can trigger a metallurgical change that precipitates chromium carbides along the borders of the metal grains. This depletion of chromium in the surrounding areas leaves the metal unprotected against corrosive environments. When exposed to acids or high humidity, these chromium-depleted channels dissolve rapidly while the bulk of the grains remains intact.
The metal structure then loses its cohesion and can fail under minimal mechanical stress without showing visible external damage beforehand.
Testing Standard
Evaluating susceptibility to this form of decay requires specialized laboratory tests as defined by Chinese national standards. The standard procedures include chemical immersion tests using boiling sulfuric acid, copper sulfate, or nitric acid to expose the vulnerability of the material’s microstructure. Laboratories accredited by the China National Accreditation Service for Conformity Assessment perform these evaluations on specimens cut from the production batch.
If the test specimen exhibits excessive weight loss or cracking upon bending, the entire batch is rejected for safety-critical industrial applications. Importers of specialized chemicals or high-pressure piping must present these accredited test certificates to avoid administrative rejection by market supervision bureaus. The verification of material certificates is a mandatory step before customs authorities clear structural alloys for use in high-risk projects like chemical processing plants.
Fabrication Method
Preventing material vulnerability involves using low-carbon alloy grades or applying post-weld solution annealing to dissolve the precipitated carbides back into the metal grains. Choosing titanium-stabilized grades also helps, as titanium forms carbides more readily than chromium. Fabricators must maintain strict control over welding heat inputs to avoid prolonged exposure to the temperature range where sensitization occurs.
By optimizing these production variables, manufacturers can ensure that high-value equipment has a long operational lifespan.
