What is point corrosion in acid-resistant steel?
Spot corrosion is a localized form of corrosion that occurs on the surface of acid-resistant steel when the protective passive oxide film breaks down. The corrosion often appears as small but deep attacks in the material and can be difficult to detect before the damage has become extensive.
Acid-resistant materials such as AISI 316 and 316L are used to reduce the risk of pitting corrosion in industrial processes where fluids contain chlorides, acids or other aggressive substances.
Process Surplus offers stainless steel and acid-proof tanks in AISI 304, 316 and 316L for the process industry, food production, pharmaceuticals and water treatment where corrosion resistance is crucial.
Function and structure
Acid-resistant process tanks are designed to withstand corrosive environments and hygienic requirements.
Spot corrosion often occurs in areas where the protective oxide film is affected or damaged, for example in the case of:
Welded joints and heat affected zones
Joints and connections
Scratches and mechanical damage
Areas with low fluid circulation
Stagnation or deposits
Tanks may be equipped with agitators, CIP systems, level measurement and other process equipment that affects design and flow patterns.
Materials and corrosion mechanisms
Acid-resistant steels such as AISI 316 and 316L contain molybdenum which improves resistance to pitting and crevice corrosion compared to AISI 304.
316L also has a low carbon content, which reduces the risk of corrosion in welded areas.
The passive oxide film on stainless steel acts as a protective layer against corrosion. When this film is broken down by chloride ions or aggressive chemicals, for example, pitting corrosion can start to develop.
How does spot corrosion occur?
Spot corrosion usually occurs when aggressive substances break the passive protective film on the steel surface.
Common causes are:
Chloride ions from salt or detergents
Acids and chemicals
High temperatures
Lack of cleaning
Stagnant liquid
Mechanical damage to the surface
As the protective layer breaks down, small localized infestations can develop rapidly and create deep holes in the material.
Pressurized and atmospheric tanks
Both pressurized and atmospheric tanks can be affected by pitting corrosion depending on the process environment and the properties of the fluid.
In pressurized tanks, welded joints and stressed areas are particularly important to check as corrosion attack can affect safety and strength.
Atmospheric tanks are mainly affected by chemical exposure, condensation and chlorides in liquid or ambient environment.
Applications where spot corrosion is critical
Point corrosion is particularly relevant in:
Food production with CIP cleaning and chloride-containing environments
Chemical industry with acids and aggressive solutions
Pharmaceutical production with high hygiene requirements
Water treatment using salt and disinfectants
Process industry with temperature variations and chemical handling
Preventive measures
The risk of pitting corrosion can be reduced by:
Choice of acid-proof steel such as 316 or 316L
Polishing and passivation of surfaces
Controlled cleaning and CIP processes
Avoidance of stagnant liquid
Monitoring of chloride levels and pH
Regular inspection of welds and critical zones
Proper process design and material selection are key elements for long life and safe operation.
Volumes and capacity
Point corrosion can occur in both small and large industrial containers depending on process conditions.
Larger tanks may be more sensitive in areas with low circulation or where liquid remains for longer periods.
Process Surplus offers tanks in several sizes and designs depending on the application and corrosion requirements.
Benefits of choosing the right materials
The right materials and finishes bring several benefits:
Reduced risk of pitting corrosion
Longer lifespan
Less maintenance and downtime
Improved hygiene and cleanability
Safer operation in aggressive environments
New and used acid-proof tanks
Process Surplus offers both new and used acid-proof tanks in AISI 316 and 316L with a focus on corrosion resistance and hygienic operation.
Tanks can be equipped with different types of coating, insulation, agitation and CIP systems depending on the application.
Choosing the right solution
Choice of materials and construction should be based on:
Chloride levels and chemical environment
Temperature and pressure
Cleaning processes
Hygiene requirements
Operating conditions and fluid circulation
Processöverskott offers advice on material selection and technical solutions to reduce the risk of spot corrosion and ensure a long service life.
Executive summary
Spot corrosion is a localized and potentially severe form of corrosion that can occur in acid-resistant steels when exposed to chlorides, acids or other aggressive environments.
The right choice of materials, surface treatment, cleaning and process design are crucial to reduce risk and ensure safe and long-term operation in industrial processes.
Frequently asked questions about point corrosion in acid-resistant steel
What is spot corrosion?
Spot corrosion is a localized corrosion damage where small holes form in the surface of stainless or acid-resistant steel as the protective oxide film breaks down.
Where does spot corrosion most often occur?
Corrosion often occurs at welds, joints, scratched surfaces and areas with low fluid circulation or stagnant fluid.
Which materials are most resilient?
Acid-resistant steels such as AISI 316 and 316L have better resistance to pitting corrosion than AISI 304 due to their molybdenum content.
How can point corrosion be prevented?
The risk is reduced by proper material selection, polishing, passivation, good cleaning and control of chlorides and chemical process conditions.
Why is point corrosion important in process tanks?
Point corrosion can weaken the tank’s structure and lead to leaks, breakdowns and increased maintenance costs if not detected in time.