Corrosion Protection for Industrial Coating Contractors
Article 1: Understanding Corrosion
Why Steel Corrodes and Why the Coating Contractor Matters
Before you can protect steel from corrosion, you need to understand what corrosion is, what allows it to continue, and why a coating defect can become an active corrosion site.
Big Idea: Corrosion is an electrochemical process. Protective coatings control corrosion by separating the metal from the environment needed to sustain that process.
Corrosion Is More Than Rust
When most people hear the word corrosion, they picture orange rust on a piece of steel. Rust is one visible product of corrosion, but the corrosion process begins before that orange material appears.
Corrosion is the deterioration of a material through a chemical or electrochemical reaction with its environment. For steel, corrosion occurs when iron atoms lose electrons and begin changing into corrosion products.
The important word for the industrial coating contractor is
electrochemical. Corrosion involves both a chemical reaction and the movement of electrical charge. If the conditions required for that reaction remain present, the metal can continue to deteriorate.
A coating contractor does not need to become a corrosion engineer to perform quality work. However, understanding the basic process makes it easier to recognize contamination, surface-preparation problems, coating defects, environmental risks, and conditions that should be reported before work continues.
Steel Wants to Return to a More Stable Condition
Manufacturing steel requires energy. Iron ore is processed, refined, and converted into metallic iron and steel. That finished steel contains stored energy and is less stable than the naturally occurring compounds from which it was produced.
When unprotected steel is exposed to moisture and oxygen, it begins moving back toward a more stable condition. The resulting compounds include iron oxides and iron hydroxides that we commonly call rust.
That is why corrosion protection is an ongoing battle. Steel does not need encouragement to corrode. It needs a properly designed and maintained system to prevent or control the reactions that cause corrosion.
The Four Parts of a Corrosion Cell
An active electrochemical corrosion cell requires four basic parts:
1. The Anode
The anode is the location where metal loss occurs. Iron atoms at the anodic area give up electrons and enter the surrounding electrolyte as electrically charged ions.
This is the part of the corrosion cell where the steel is actually being consumed.
2. The Cathode
The cathode is the location where another electrochemical reaction uses the electrons released at the anode. The cathodic area is protected from metal loss while the corrosion cell is operating.
Anodic and cathodic areas can exist on the same piece of steel. They may be separated by only a very small distance.
3. The Electrolyte
The electrolyte is a conductive liquid or moist material that allows ions to move between the anodic and cathodic areas.
Water is a common electrolyte. Soil moisture, seawater, condensation, process water, wastewater, and water containing dissolved salts can all support corrosion.
Pure water is not as conductive as water containing dissolved materials. Salts, acids, alkalis, and other contaminants can make moisture more conductive and increase corrosion activity.
4. The Metallic Path
The metallic path allows electrons to travel from the anodic area to the cathodic area. The steel structure itself commonly provides this path.
If all four parts are present and electrically connected, the corrosion cell can operate.
Field Rule: Corrosion can be controlled by eliminating, isolating, or changing one or more parts of the corrosion cell.
What Happens at the Anode?
At the anode, iron atoms lose electrons. After losing electrons, the iron atoms become positively charged iron ions and leave the steel surface.
The released electrons remain in the metal and travel through the metallic path toward a cathodic area. At the cathode, those electrons participate in another reaction, often involving oxygen and water.
The iron ions can react with oxygen, water, and other materials in the environment to form corrosion products. These products may appear as loose rust, tightly adhering oxide, tubercles, staining, scale, or deposits beneath a coating.
The visible rust is evidence of the process. The important damage is the metal that has already been lost.
Why Corrosion Does Not Occur Uniformly
A steel surface may look uniform, but small differences can create separate anodic and cathodic areas. These differences can include:
- Variations in oxygen concentration
- Different moisture levels
- Deposits that trap water or contaminants
- Differences in temperature
- Welds and heat-affected zones
- Different metals in electrical contact
- Scratches, holidays, and damaged coating
- Mill scale or partially removed corrosion products
- Soluble salts remaining on the surface
- Crevices, lap joints, bolts, and restricted areas
These differences help explain why one small area may experience deep pitting while the surrounding steel shows much less damage.
Conditions That Can Increase Corrosion
Moisture
Water provides the electrolyte needed for many corrosion reactions. Rain, condensation, humidity, wash water, immersion, leaking process equipment, and moisture in soil can all contribute.
Oxygen
Oxygen commonly participates in the cathodic reaction. Differences in oxygen availability can also create corrosion cells. Areas beneath deposits or inside crevices may behave differently from freely exposed areas.
Salts
Chlorides and other soluble salts can attract moisture, increase conductivity, and remain active beneath a coating. Steel can look visually clean while still carrying harmful invisible contamination.
Temperature
Higher temperatures often accelerate chemical and electrochemical reactions. Temperature also affects condensation, coating cure, service exposure, and the movement of moisture through a coating film.
Acids and Alkalis
Chemical exposure can change the corrosion rate and attack both the substrate and the protective coating. The coating system must be selected for the actual service environment.
Electrical Contact Between Different Metals
When dissimilar metals are electrically connected in the presence of an electrolyte, one metal may become more anodic and corrode preferentially. This is known as galvanic corrosion.
How Protective Coatings Control Corrosion
A protective coating primarily controls corrosion by separating the steel from water, oxygen, salts, chemicals, and other parts of the service environment.
A coating does not need to stop every individual molecule from moving through the film. It must reduce the movement of moisture, oxygen, ions, and contaminants enough to prevent a damaging corrosion cell from developing at the steel surface.
Different coatings provide protection in different ways. Some primarily act as barriers. Some contain inhibitive pigments. Zinc-rich primers can provide galvanic protection when properly formulated, applied, and electrically connected to the steel.
Regardless of the protective mechanism, the coating system must be properly selected, applied to a properly prepared surface, cured under acceptable conditions, and inspected for defects.
What Happens When the Coating Is Damaged?
A scratch, pinhole, missed area, holiday, thin spot, damaged edge, or area of coating disbondment can expose steel to the environment.
Once water and contaminants reach the exposed steel, a corrosion cell can develop. Corrosion products may build beneath the edge of the coating. Because many corrosion products occupy more volume than the original metal, they can create pressure that lifts or separates the surrounding coating.
The defect can then grow beyond the original point of damage. What began as a small holiday may become blistering, underfilm corrosion, rust creepage, flaking, or widespread disbondment.
This is why small application defects matter. A defect that looks minor during application can become the starting point for a much larger failure in service.
Where Cathodic Protection Fits
Cathodic protection controls corrosion by changing the electrochemical behavior of the protected structure. The system supplies protective current so that exposed areas of the structure behave as cathodic rather than anodic sites.
Cathodic protection is commonly used on buried pipelines, tank bottoms, marine structures, submerged equipment, water systems, and other assets exposed to conductive environments.
The coating and cathodic-protection system are intended to work together. A sound coating reduces the amount of bare steel exposed to the environment. That reduces the protective current required from the cathodic-protection system.
As coating damage increases, current demand can increase. Disbonded coatings and other materials can also shield the steel and prevent protective current from reaching the area where it is needed.
Remember: Cathodic protection does not make poor surface preparation or poor coating application acceptable. The coating remains the primary corrosion barrier.
Why This Matters to the Coating Contractor
The coating contractor controls or influences many of the conditions that determine whether a corrosion-protection system succeeds.
- Was the surface inspected before work began?
- Were oil, grease, salts, dirt, and other contaminants removed?
- Was the required degree of surface cleanliness achieved?
- Was the surface profile within the specified range?
- Were environmental conditions acceptable?
- Were the coating materials properly stored, mixed, and applied?
- Did edges, welds, bolts, pits, and difficult areas receive adequate coverage?
- Was the required dry-film thickness achieved?
- Were holidays and other defects found and repaired?
- Were cathodic-protection components protected from damage?
- Was the completed work properly documented?
These are not separate concerns. They are connected parts of the corrosion-protection system.
A Simple Field Example
Consider a steel tank located outdoors near a coastal environment. Salt deposits settle on the surface. Moisture from humidity and condensation dissolves those salts and creates a conductive electrolyte.
If the tank coating is continuous and well adhered, it separates the steel from much of that environment. If the coating contains a holiday at a weld, moisture and dissolved salts can reach the steel.
The exposed area can become anodic. Metal loss begins. Corrosion products form and spread beneath the coating. The surrounding film may blister or disbond. Additional steel becomes exposed, and the damaged area grows.
A small defect that could have been located and repaired during inspection may eventually require a much larger maintenance repair.
Warning Signs Contractors Should Recognize
Conditions that may indicate active corrosion or an increased corrosion risk include:
- Rust staining emerging from coating defects
- Blistering or loss of coating adhesion
- Rust creepage from scratches, welds, edges, or holidays
- Deep pits hidden beneath corrosion products
- Moisture trapped beneath deposits, insulation, or disbonded coating
- White, green, or other colored corrosion products on nonferrous metals
- Accelerated attack near connections between dissimilar metals
- Recurring failure in the same location
- Unexpected coating failure around cathodic-protection components
- Evidence of chemical exposure not addressed by the existing coating system
Recognizing a warning sign does not mean the contractor should independently redesign the corrosion-control system. It means the condition should be documented and reported to the responsible owner, inspector, engineer, coating manufacturer, or corrosion specialist.
The Contractor's First Corrosion-Control Checklist
Before surface preparation begins, ask:
- What material is being coated?
- What type of environment will the structure face?
- Will the structure be atmospheric, buried, submerged, or immersed?
- Are chemicals, salts, heat, abrasion, or moisture present?
- Is there visible corrosion, pitting, section loss, or coating disbondment?
- Are soluble-salt or other contamination tests required?
- Is a cathodic-protection system present?
- Are there anodes, cables, test stations, isolation joints, or electrical connections that must be protected?
- Does the specification match the actual field condition?
- Who has authority to resolve unexpected conditions?
Asking these questions early can prevent costly disagreements, coating failures, and repair work later.
Key Takeaways
- Corrosion is an electrochemical process that causes material deterioration.
- An active corrosion cell requires an anode, a cathode, an electrolyte, and a metallic path.
- Metal loss occurs at the anodic area.
- Moisture, oxygen, salts, chemicals, temperature, and dissimilar metals can influence corrosion.
- Protective coatings control corrosion primarily by separating the steel from its environment.
- Small coating defects can become active corrosion sites and grow into larger failures.
- Cathodic protection and protective coatings are complementary corrosion-control methods.
- The contractor's surface preparation, application, inspection, and documentation directly affect long-term performance.
Bottom Line: A coating contractor is not simply covering steel. The contractor is interrupting a corrosion process that will restart anywhere the protective system is incomplete.
Knowledge Check
-
What are the four parts required for an active electrochemical corrosion cell?
-
At which part of the corrosion cell does metal loss occur?
-
Why can soluble salts increase corrosion activity?
-
How does a protective coating help control corrosion?
-
Why can a small coating holiday become a larger coating failure?
-
How do protective coatings and cathodic protection work together?
View Knowledge Check Answers
1. An anode, a cathode, an electrolyte, and a metallic path.
2. Metal loss occurs at the anode.
3. Soluble salts can attract moisture and increase the electrical conductivity of the electrolyte.
4. A protective coating separates the steel from water, oxygen, salts, chemicals, and other parts of the corrosive environment.
5. Moisture and contaminants can reach the exposed steel, allowing corrosion products to form beneath the surrounding coating and cause additional disbondment.
6. The coating reduces the amount of steel exposed to the environment, while cathodic protection helps control corrosion at holidays, scratches, and other exposed areas.
Coming Next
Article 2: Common Forms of Corrosion
The next article examines general corrosion, pitting, crevice corrosion, galvanic corrosion, underfilm corrosion, and other forms of attack industrial coating contractors may encounter in the field.
