Is it normal for seamless stainless steel elbows to have patterns?
Is it normal for patterns to appear on the surface of stainless steel seamless elbows? This needs to be judged based on specific characteristics.
Let’s distinguish between normal patterns and abnormal patterns. Normal patterns are acceptable, while abnormal patterns require caution.
Abnormal patterns that require vigilance:
1. Corrosive patterns
Pitting or pitting corrosion: gray black mottled or small pores (diameter ≤ 1mm) appear on the surface, often caused by chloride ion corrosion. For example, salt spray infiltration in coastal environments can cause damage to oxide films, resulting in localized corrosion penetration.
Defects in the heat affected zone of welds: If there are micro cracks at the joint between the elbow and the pipeline that have not been detected by radiation, they may expand into linear corrosion patterns after use.
This type of pattern carries a risk of leakage and requires immediate cessation of testing.
2. Processing defect patterns
Rolling oxidation line/folding: Linear protrusions or peeling parallel to the tube axis appear on the surface (such as “mountain shaped scale folding”), caused by residual oxides or interlayer slip in the hot rolling process.
Scratch/indentation: A strip-shaped indentation formed by mechanical damage during transportation or installation, which may become the starting point of corrosion.
Acceptable normal patterns:
1. Interference color of oxide film
Rainbow/light blue patterns are formed on the surface after high temperature or acid washing, which belongs to the optical interference phenomenon of passivation film and does not affect the sealing performance. Commonly found in welding heat treatment areas.
2. Uniform brushed pattern
To enhance corrosion resistance, some elbows are mechanically brushed to form straight lines with consistent directions (Ra ≤ 0.8 μ m), which is part of the process design.
If the pattern is accompanied by pressure drop or leakage sound, an air tightness test must be performed according to ISO 5208 A-level standard.
Is it normal for seamless stainless steel elbows to have patterns?
Is it normal for patterns to appear on the surface of stainless steel seamless elbows? This needs to be judged based on specific characteristics.
Let’s distinguish between normal patterns and abnormal patterns. Normal patterns are acceptable, while abnormal patterns require caution.
Abnormal patterns that require vigilance:
1. Corrosive patterns
Pitting or pitting corrosion: gray black mottled or small pores (diameter ≤ 1mm) appear on the surface, often caused by chloride ion corrosion. For example, salt spray infiltration in coastal environments can cause damage to oxide films, resulting in localized corrosion penetration.
Defects in the heat affected zone of welds: If there are micro cracks at the joint between the elbow and the pipeline that have not been detected by radiation, they may expand into linear corrosion patterns after use.
This type of pattern carries a risk of leakage and requires immediate cessation of testing.
2. Processing defect patterns
Rolling oxidation line/folding: Linear protrusions or peeling parallel to the tube axis appear on the surface (such as “mountain shaped scale folding”), caused by residual oxides or interlayer slip in the hot rolling process.
Scratch/indentation: A strip-shaped indentation formed by mechanical damage during transportation or installation, which may become the starting point of corrosion.
Acceptable normal patterns:
1. Interference color of oxide film
Rainbow/light blue patterns are formed on the surface after high temperature or acid washing, which belongs to the optical interference phenomenon of passivation film and does not affect the sealing performance. Commonly found in welding heat treatment areas.
2. Uniform brushed pattern
To enhance corrosion resistance, some elbows are mechanically brushed to form straight lines with consistent directions (Ra ≤ 0.8 μ m), which is part of the process design.
If the pattern is accompanied by pressure drop or leakage sound, an air tightness test must be performed according to ISO 5208 A-level standard.