Visual Identification Guide for Different Corrosion Types in

1. Uniform Corrosion

• Macro Appearance: The entire exposed surface thins at a relatively uniform rate, covered by a layer of corrosion products (such as rust on steel). There is an overall reduction in dimensions.
• Micro Characteristics: The surface dissolves in a relatively flat and even manner.
• Identification Key: The corrosion is evenly distributed. It is the most common and predictable form of corrosion, typically managed by designing with a corrosion allowance.

2. Pitting Corrosion

• Macro Appearance: Deep, small holes appear on the metal surface. The diameter of the pits is small, but the depth can be significant, while the surrounding area shows minimal corrosion. The pit openings are often covered with corrosion products.
• Micro Characteristics: Corrosion pits appear hemispherical, bowl-shaped, or narrow and deep, with corrosion products visible inside the pits.
• Identification Key: Extremely hazardous, as it leads to failure through localized perforation despite potentially minimal weight loss. Commonly occurs due to local breakdown of the passive film on stainless steel in chloride-containing environments.

3. Crevice Corrosion

• Macro Appearance: Occurs within and at the edges of crevices where the fastener contacts gaskets, connected parts, or its own threads. The external surface may remain intact, while severe localized corrosion or decay occurs inside the crevice.
• Micro Characteristics: Metal dissolution occurs within the crevice, accompanied by an external oxygen concentration cell effect.
• Identification Key: Similar to pitting, but the corrosion location is explicitly linked to the crevice structure. It is a common issue in flange connections and lap joints.

4. Galvanic Corrosion (Dissimilar Metal Contact Corrosion)

• Macro Appearance: When two different metals contact in an electrolyte, the metal with the more negative potential (more active) corrodes at an accelerated rate, with corrosion concentrated near the contact area; the more positive metal is protected. For example, with a steel bolt on an aluminum part, severe corrosion occurs on the aluminum around the bolt.
• Micro Characteristics: Dissolution of the anodic metal.
• Identification Key: The corrosion pattern clearly correlates with the dissimilar metal pairing. Observe which metal is corroding and whether it aligns with galvanic series predictions.

5. Intergranular Corrosion

• Macro Appearance: Corrosion proceeds along the grain boundaries of the metal. The surface may appear intact, but the metal loses strength and toughness, potentially crumbling into powder upon light impact. Commonly seen in stainless steels (e.g., 304) after welding or sensitization heat treatment.
• Micro Characteristics (Metallographic): Under a metallurgical microscope, grain boundaries appear widened and net-like due to corrosion. This is a classic case for micro-morphology identification.
• Identification Key: The combination of macroscopic strength loss and microscopic grain boundary corrosion is the key to judgment. It frequently occurs in specific heat-treated or welded stainless steels and aluminum alloys.

6. Stress Corrosion Cracking (SCC)

• Macro Appearance: Brittle cracks produced under the combined action of tensile stress (residual or working stress) and a specific corrosive medium. Cracks typically exhibit a dendritic or radial pattern, and the fracture surface may contain corrosion products. Surface cracks may be subtle while internal propagation is severe.
• Micro Characteristics: Cracks mostly propagate intergranularly (e.g., carbon steel in alkaline solutions) or transgranularly (e.g., austenitic stainless steel in chloride environments), with branching at the crack tips.
• Identification Key: The three essential elements are "Stress" + "Specific Medium" + "Brittle Cracks." It is one of the most hazardous and hidden forms of corrosion.

7. Hydrogen Embrittlement (HE)

• Macro Appearance: Manifests as sudden, unexpected brittle fracture with a flat fracture surface and no obvious plastic deformation. Often occurs in high-strength steels (hardness > HRC 34) after pickling, electroplating, or cathodic protection, manifesting as delayed fracture under stress.
• Micro Characteristics: The fracture surface typically exhibits a "rock candy" intergranular appearance or "chicken claw" markings.
• Identification Key: High-strength material, hydrogen source (process-induced), and delayed fracture are the three core clues. Must be distinguished from SCC, as HE does not require the continuous presence of a specific corrosive medium.

8. Fretting Corrosion

• Macro Appearance: A mixed phenomenon of localized wear, oxidation, and corrosion occurring on contact surfaces subjected to vibration and repeated slight relative sliding (e.g., thread pairs, bearing surfaces). It produces reddish-brown (steel) or black (aluminum) oxide debris, accompanied by surface pitting or grooves.
• Micro Characteristics: The surface shows scratch marks, debris, and tiny etching pits.
• Identification Key: Occurs at friction interfaces with slight relative motion, producing oxide debris as a byproduct.