Corrosion Protection for Industrial Coating Contractors
Article 02: Common Forms of Corrosion
Recognizing How and Where Steel Is Being Attacked
Corrosion does not always appear as an even layer of rust. It can spread across an entire surface, concentrate inside a pit, develop beneath a coating, or attack one metal because it is connected to another.
Big Idea: The appearance and location of corrosion provide clues about what caused it, how serious it may be, and what must be corrected before a new coating is applied.
Why the Type of Corrosion Matters
A coating contractor may be hired to abrasive blast and repaint a structure, but the visible rust is only part of the story. Before the surface is covered again, someone must determine why the corrosion developed and whether the cause has been corrected.
Uniform surface rust, deep isolated pits, corrosion beneath a disbonded coating, and attack between dissimilar metals do not represent the same condition. They can require different surface preparation, inspection, repair, and engineering decisions.
The contractor is not normally responsible for diagnosing structural integrity or redesigning the corrosion-control system. However, the contractor should recognize unusual conditions, document what is found, and report concerns before the evidence is removed by blasting or covered by a new coating.
Once the surface has been cleaned and coated, important clues may be lost. Photographs, measurements, inspection notes, and clear communication before surface preparation can protect both the asset owner and the contractor.
1. General or Uniform Corrosion
General corrosion is distributed relatively evenly across a large area of exposed metal. The surface may show widespread rusting, scaling, or general metal loss rather than a few isolated areas of deep attack.
This form of corrosion is often easier to see and evaluate than highly localized corrosion. Because the attack is spread across the surface, the remaining metal thickness may be more predictable than it is with deep pitting.
Where Contractors May Find It
- Uncoated structural steel exposed to weather
- Large areas where an old coating has worn away
- Tank exteriors with widespread coating breakdown
- Steel exposed to continuous condensation
- Poorly maintained industrial structures
Contractor Concern
Widespread rust must be removed to the degree required by the specification. Remaining mill scale, rust scale, embedded contamination, and insufficient surface profile can prevent the new coating from developing proper adhesion.
2. Pitting Corrosion
Pitting is a localized form of corrosion that produces cavities or holes in the metal. A surface may appear acceptable over most of its area while a small number of pits penetrate deeply into the steel.
Pitting can be more dangerous than uniform corrosion because substantial penetration may occur with relatively little total metal loss. Corrosion products can also cover the opening and hide the true depth of the damage.
Where Contractors May Find It
- Tank floors and lower shell areas
- Structures exposed to chlorides or standing water
- Steel beneath deposits or failed coatings
- Marine and wastewater environments
- Areas where moisture repeatedly collects
Contractor Concern
Abrasive blasting may reveal pits that were not visible during the initial inspection. Deep pits can retain spent abrasive, dust, salts, moisture, and corrosion products. These contaminants must be removed before coating.
Do not fill or bridge severe pits with coating simply to improve appearance. Significant pitting, perforation, or suspected section loss should be documented and referred to the responsible inspector, owner, or engineer.
3. Crevice Corrosion
Crevice corrosion develops in narrow, shielded spaces where moisture becomes trapped and oxygen movement is restricted. The chemistry inside the crevice can become different from the chemistry on the freely exposed surface.
This difference can create an electrochemical cell that concentrates corrosion inside or immediately adjacent to the crevice.
Common Crevice Locations
- Lap joints
- Bolted connections
- Gaskets and flanges
- Overlapping plates
- Areas beneath deposits
- Spaces beneath supports and attachments
- Tight joints that cannot be properly cleaned or coated
Contractor Concern
Coating over the outside of a crevice does not guarantee that contamination and corrosion inside the crevice have been controlled. The specification may require sealing, stripe coating, caulking, welding, disassembly, or another treatment approved by the responsible authority.
4. Galvanic Corrosion
Galvanic corrosion can occur when two different metals are electrically connected while exposed to a conductive environment. One metal becomes more anodic and corrodes preferentially, while the more cathodic metal receives protection.
The severity of the attack depends on several factors, including the metals involved, the conductivity of the environment, the electrical connection, and the relative exposed surface areas.
The Area Effect
A small anodic area connected to a large cathodic area can experience severe, concentrated attack. This area relationship can make a small coating defect especially important when the surrounding cathodic surface is large.
Where Contractors May Find It
- Steel connected to copper or copper-bearing materials
- Carbon steel connected to stainless steel
- Dissimilar fasteners and structural components
- Metal transitions in piping systems
- Marine structures containing several metal types
Contractor Concern
Do not assume that coating only one of two connected metals will solve the problem. The design of the isolation and coating system should be reviewed by the responsible corrosion professional. Improperly coating only the anodic metal can create a small exposed anode connected to a much larger cathodic surface.
5. Differential-Aeration Corrosion
Differential-aeration corrosion develops when different areas of the same metal receive different amounts of oxygen. The area with less oxygen can become anodic relative to the area with greater oxygen availability.
Where Contractors May Find It
- Beneath dirt, scale, or biological deposits
- At waterlines inside tanks
- Inside narrow crevices
- Under disbonded coatings
- Where part of a surface remains wet longer than surrounding areas
Contractor Concern
Removing deposits may reveal concentrated attack beneath an area that appeared only dirty before cleaning. The revealed condition should be inspected before additional surface preparation or coating application continues.
6. Underfilm Corrosion
Underfilm corrosion develops when moisture, oxygen, salts, or chemicals reach the steel beneath a protective coating. Entry may occur through a holiday, crack, pinhole, damaged edge, porous film, loss of adhesion, or mechanical injury.
Once corrosion begins, it can spread laterally beneath the coating. Corrosion products may lift the coating, increase disbondment, and expose additional steel.
Common Warning Signs
- Rust staining from coating defects
- Blistering
- Rust creepage from scratches or holidays
- Cracking and flaking
- Loss of adhesion around edges and welds
- Raised areas that sound hollow when appropriately examined
Contractor Concern
Applying another coat over poorly adhered material will not stop corrosion beneath it. Unsound coating and underlying corrosion products must be removed to the extent required by the repair specification.
7. Filiform Corrosion
Filiform corrosion appears as narrow, thread-like tracks beneath a coating. It commonly begins at a coating defect, cut edge, fastener, scribe, or poorly protected opening and then travels beneath the film.
It is frequently associated with coated aluminum and thin metal components, although similar-looking underfilm patterns may be found on other substrates.
Contractor Concern
The affected coating must be removed far enough to expose sound, uncontaminated material. Simply sanding the visible thread and applying touch-up paint may leave active corrosion beneath the surrounding film.
8. Erosion-Corrosion
Erosion-corrosion combines chemical or electrochemical attack with the physical removal of protective films or corrosion products by moving liquid, gas, or suspended solids.
Where Contractors May Find It
- Pipe elbows and directional changes
- Pump components
- Slurry-handling systems
- High-velocity water service
- Areas exposed to turbulence or impingement
Contractor Concern
A coating suitable for quiet immersion may not withstand abrasion, turbulence, or impact from suspended solids. The coating system must be selected for both the chemical exposure and the mechanical service conditions.
9. Microbiologically Influenced Corrosion
Microbiologically influenced corrosion, commonly called MIC, occurs when microorganisms create or contribute to conditions that accelerate corrosion.
The microorganisms do not necessarily consume the metal directly. Their activity can alter local chemistry, create deposits, produce corrosive byproducts, or establish low-oxygen conditions that support localized attack.
Where Contractors May Find It
- Wastewater facilities
- Pipelines and storage tanks
- Cooling-water systems
- Marine and freshwater structures
- Low-flow or stagnant-water areas
Contractor Concern
Unusual deposits, odors, slime, concentrated pitting, or recurring corrosion should be reported. Specialized testing and treatment may be required before the surface is coated.
10. Stress-Corrosion Cracking
Stress-corrosion cracking can occur when a susceptible material is exposed to a particular corrosive environment while under tensile stress.
The surface may show fine cracks even though widespread rust or metal loss is not obvious. The stress may come from service loads, forming, welding, fabrication, or residual stress within the component.
Contractor Concern
A coating contractor should never assume that a crack is merely a coating defect. Cracks in the substrate, welds, or heat-affected areas must be documented and evaluated by qualified personnel before coating work continues.
11. Corrosion Beneath Insulation
Corrosion beneath insulation develops when water enters an insulated system and remains in contact with the metal surface. The insulation and outer jacket can hide the damage from normal view.
Moisture may enter through damaged jacketing, failed seals, penetrations, seams, supports, or areas where water collects. Temperature cycling can also draw moisture into the system.
Contractor Concern
Removing insulation may reveal severe corrosion that was not included in the original coating scope. The contractor should document the condition before cleaning removes the evidence and obtain direction concerning inspection, repair, and coating-system requirements.
12. Flash Rusting
Flash rusting is the rapid formation of rust on freshly prepared steel exposed to moisture. It may occur after waterjetting, wet abrasive blasting, washing, condensation, or unexpected weather exposure.
The amount and acceptability of flash rust depend on the project specification, coating manufacturer's requirements, and approved surface-preparation standard.
Contractor Concern
Do not coat over flash rust simply because the work must remain on schedule. The surface condition must meet the specification and coating manufacturer's requirements before application begins.
Field Rule: If corrosion appears different from what the specification or work scope anticipated, document it and obtain direction before hiding the evidence beneath a new coating.
How Coating Failures Can Reveal Corrosion Patterns
The location and shape of a coating failure often provide clues about the underlying corrosion process.
- Failure at edges and welds: May indicate inadequate film build, poor stripe coating, or difficult surface geometry.
- Rust beneath blisters: May indicate moisture or contamination beneath the coating.
- Rust spreading from a scratch: May indicate underfilm corrosion and poor resistance to creepage.
- Repeated failure at a waterline: May indicate changing oxygen conditions or repeated wetting and drying.
- Attack around dissimilar-metal connections: May indicate galvanic corrosion.
- Deep attack beneath deposits: May indicate crevice, differential-aeration, or microbiologically influenced corrosion.
- Early rust across freshly prepared steel: May indicate flash rusting caused by moisture exposure.
These observations do not replace a formal failure analysis, but they help contractors recognize when additional investigation is necessary.
What the Contractor Should Document
Before removing corrosion products or failed coating, document:
- The exact location of the condition
- The apparent size and shape of the affected area
- Whether the corrosion is general or localized
- The presence of pits, holes, cracks, deposits, or blistering
- The condition of nearby edges, welds, bolts, and connections
- Whether dissimilar metals are present
- Whether moisture, salts, chemicals, or biological deposits are present
- The date, time, and environmental conditions
- Clear overview and close-up photographs
- Who was notified and what direction was received
A scale, ruler, pit gauge, or other appropriate reference in a photograph can help show the size of the condition. Structural measurements and acceptance decisions should be made by authorized and qualified personnel.
When the Contractor Should Stop and Ask
Stop work in the affected area and request direction when you discover:
- Unexpected deep pitting
- Perforation or suspected section loss
- Cracks in the steel, welds, or heat-affected zones
- Severe corrosion beneath insulation or deposits
- Unexpected chemical or biological contamination
- Evidence that the specified coating is unsuitable for the actual service environment
- Damaged cathodic-protection components
- A surface condition that cannot meet the specification using the approved preparation method
Stopping to obtain direction is not a sign of poor productivity. It is part of protecting the asset, the project, and the contractor.
Key Takeaways
- Corrosion can be general, localized, hidden, mechanically accelerated, or influenced by microorganisms.
- Pitting can cause serious penetration even when total metal loss appears small.
- Crevices and deposits create conditions that can concentrate corrosion.
- Dissimilar metals can produce galvanic corrosion when they are electrically connected in a conductive environment.
- Underfilm corrosion can begin at a small coating defect and spread beyond the original damage.
- Corrosion beneath insulation can remain hidden until severe damage has occurred.
- Flash rust must be evaluated against the specification and coating manufacturer's requirements.
- Unexpected corrosion conditions should be documented before surface preparation removes the evidence.
- The contractor should report structural concerns rather than covering them with coating.
Bottom Line: Rust tells you corrosion has occurred. Its pattern, location, and severity may tell you why it occurred and what must be corrected before the structure is coated again.
Knowledge Check
- What is the difference between general corrosion and pitting corrosion?
- Why can corrosion inside a crevice be more severe than corrosion on the surrounding surface?
- What conditions are required for galvanic corrosion?
- How can a small coating holiday lead to widespread underfilm corrosion?
- Why should a contractor document corrosion before abrasive blasting?
- What should a contractor do after discovering unexpected deep pitting or cracks?
View Knowledge Check Answers
1. General corrosion is distributed relatively evenly across a broad surface. Pitting is concentrated in small areas and can penetrate deeply into the metal.
2. Restricted oxygen movement and trapped moisture can create a different local environment inside the crevice, causing the area to become anodic and experience concentrated attack.
3. Two different metals must be electrically connected and exposed to a conductive electrolyte.
4. Moisture and contaminants can enter through the holiday, initiate corrosion, and spread beneath the surrounding coating as corrosion products cause additional disbondment.
5. Abrasive blasting can remove corrosion products and coating patterns that provide evidence about the location, type, and severity of the original condition.
6. Stop work in the affected area, document the condition, notify the responsible party, and obtain direction before continuing.
Coming Next
Article 03: Evaluating the Structure and Service Environment
The next article explains what contractors should examine before surface preparation begins, including substrate condition, existing coatings, contamination, service exposure, accessibility, and conditions that may require clarification.
