Corrosion is the gradual deterioration of a metal as a result of its physicochemical interaction with the surrounding environment. In industrial equipment and structures, it can lead not only to cosmetic damage, but also to loss of wall thickness, leaks, coating failure, reduced reliability and a shorter service life.
To choose an effective protection method, it is useful to distinguish between electrochemical corrosion, which typically occurs in the presence of an electrolyte, and chemical or dry corrosion, where the metal reacts directly with a gaseous environment without a liquid electrolyte.
Key takeaways
- ISO 8044:2024 is the current international vocabulary standard for corrosion of metals and alloys.
- Electrochemical corrosion involves anodic and cathodic reactions, an electronic path through the metal and an ionically conductive environment, typically an aqueous electrolyte.
- Galvanic corrosion can occur when dissimilar conductive materials are electrically connected and exposed to a common electrolyte.
- Ordinary rusting of carbon steel in humid air is primarily an electrochemical process, not a typical example of dry chemical corrosion.
- Chemical or dry corrosion is associated with direct reaction between a metal and a gaseous environment without a liquid electrolyte, often at elevated temperature.
- Effective corrosion control usually combines material selection, good design, coatings, environmental control, inhibitors, inspection and, where appropriate, cathodic protection.
What is corrosion?
ISO 8044:2024 defines terminology used for corrosion of metals and alloys. In engineering practice, corrosion is understood as a harmful interaction between a metallic material and its environment that changes the properties of the metal and can affect the function of the material, its environment or the technical system as a whole.
Corrosion does not always look like red-brown rust. Depending on the material and service conditions, it may appear as:
- uniform metal loss,
- pitting,
- crevice corrosion,
- galvanic corrosion,
- stress-corrosion cracking,
- under-film corrosion,
- high-temperature oxidation or other reactions with gases.
This is why an effective protection strategy should begin by identifying the corrosion mechanism and the actual service environment rather than simply choosing a generic anti-rust product.
Electrochemical vs chemical corrosion: what is the difference?
| Criterion | Electrochemical corrosion | Chemical / dry corrosion |
|---|---|---|
| Typical environment | Water, condensate, soil or another ionically conductive electrolyte. | Dry or nearly dry gas without a liquid electrolyte. |
| Mechanism | Anodic oxidation and cathodic reduction occur simultaneously. | The metal surface reacts directly with a component of the gaseous environment. |
| Typical example | Rusting of steel in a humid environment, galvanic corrosion, corrosion in soil. | High-temperature oxidation, sulfidation or reaction with other dry gases. |
| Typical protection | Coatings, isolation of dissimilar metals, inhibitors, cathodic protection and control of the electrolyte. | High-temperature resistant alloys, protective scales or coatings, and control of temperature and gas composition. |
What is electrochemical corrosion?
Electrochemical corrosion occurs when anodic and cathodic reactions can take place on the metal surface and there is a path for both electron and ion transfer. On anodic areas, metal atoms are oxidised and enter the environment as ions. At cathodic areas, a reduction reaction takes place.
For carbon steel exposed to a thin film of moisture containing dissolved oxygen, local anodic and cathodic regions can form on the same surface. The resulting electrochemical reactions eventually produce iron oxides and hydroxides commonly referred to as rust.
Galvanic corrosion: more than simply putting two metals together
Galvanic corrosion may occur when two dissimilar conductive materials are electrically connected and exposed to a common conductive electrolyte. One material becomes more anodic under the actual service conditions and may corrode faster, while the more cathodic material is protected.
It is not technically correct to rely on a universal rule such as “the metal with the lower potential always corrodes”. The actual behaviour depends on the electrochemical potentials of the materials in the specific environment, passivation, temperature, flow, surface condition and other factors.
Why does the area ratio matter?
Galvanic corrosion is strongly influenced by the relative exposed areas of the two materials. A particularly unfavourable configuration can occur when a small anodic area is coupled to a much larger cathodic area, because the anodic current is concentrated over a smaller surface.
How can the risk of galvanic corrosion be reduced?
- select material combinations that are suitable for the actual electrolyte and service conditions,
- electrically isolate dissimilar metals where practical,
- avoid designs that trap water or conductive contamination around the joint,
- consider the exposed area ratio of the coupled materials,
- select and maintain an appropriate coating system,
- assess the complete assembly, not only the two components at the joint.
What is chemical or dry corrosion?
Chemical corrosion is generally used to describe direct reaction between a metal surface and a gaseous environment without the involvement of a liquid electrolyte. It is particularly important in equipment operating at elevated temperature.
Typical mechanisms include:
- oxidation – reaction of the metal with oxygen,
- sulfidation – reaction with sulphur-containing gases or compounds,
- halogenation – reaction with halogens or halogen-containing species,
- other high-temperature reactions depending on the gas composition and alloy.
The resulting scale may be protective, or it may be porous, cracked or poorly adherent. Whether it slows further corrosion depends on the metal, the composition and structure of the scale, temperature and the rate at which the layer grows.
Passivation: why do some metals resist corrosion better?
Some metals and alloys form thin, adherent and relatively stable passive films. Aluminium, for example, develops a thin oxide layer, while the corrosion resistance of stainless steel depends on a chromium-rich passive film.
Passivity does not mean immunity. Chlorides, unsuitable pH, elevated temperature, crevices, contamination or mechanical damage can destabilise the passive condition and promote localised corrosion.
What most often accelerates corrosion in practice?
- persistent moisture and condensation,
- chlorides and other salts, including marine exposure and road salt,
- acidic or alkaline aqueous environments that are unsuitable for the material,
- elevated temperature,
- deposits and crevices that retain electrolyte,
- galvanically unfavourable material combinations,
- damage to protective coatings,
- stray electrical currents,
- poor inspection and maintenance practices.
How can metal be protected against corrosion?
1. Good design
Corrosion prevention starts at the design stage. Components should, where possible, be designed to drain freely, avoid stagnant zones and minimise crevices and inaccessible areas that trap moisture or contamination. HSE specifically highlights drainage, minimising crevices, avoiding dead spots and ensuring access for cleaning and inspection as important design measures.
2. Appropriate material selection
The material should be selected for the real process fluid, temperature, contaminants, pressure, mechanical load and foreseeable upset conditions. A material that performs well at one temperature may not remain suitable at a higher temperature in the same medium.
3. Protective coatings
Paints, epoxy systems, metallic coatings and other barrier layers reduce contact between the metal and the corrosive environment. Their effectiveness depends on surface preparation, coating continuity, thickness, adhesion and timely repair of damaged areas.
4. Temporary protection during storage and transport
Protective oils, waxes, sprays and other temporary corrosion preventives can be useful during manufacturing, storage and transport. The required protection period, environment, packaging and need for later removal should all be considered before selecting the product.
5. Corrosion inhibitors
Corrosion inhibitors reduce the rate of corrosion in a specific environment. Depending on the chemistry, they may influence anodic or cathodic reactions, support passivation or form a protective film. Their suitability must be assessed for the actual material and process medium.
6. Cathodic protection
Cathodic protection is an electrochemical corrosion-control technique in which the protected metal is made the cathode of an electrochemical cell. AMPP lists applications such as underground tanks and pipelines, water tank interiors, ship hulls, docks and other buried or submerged metallic structures.
Cathodic protection can use sacrificial anodes or impressed current systems. It requires proper design, monitoring and maintenance and is not a universal solution for every metal component.
7. Environmental control
Where the process allows it, reducing humidity, preventing standing water, controlling chlorides, maintaining suitable pH or lowering temperature can significantly reduce corrosion risk. In many cases it is more effective to remove the cause of exposure than to repeatedly repair the consequences.
8. Inspection and maintenance
Coatings, joints between dissimilar materials, welds, drainage points, insulation systems and areas where deposits accumulate should be inspected at intervals appropriate to the risk. Localised corrosion can cause failure much earlier than would be predicted from uniform metal loss alone.
How should corrosion protection be selected for different situations?
| Situation | What should be considered |
|---|---|
| Metal component stored indoors | Temporary protective film, humidity, packaging and required protection period. |
| Outdoor steel structure | Surface preparation, coating system, UV exposure, rain, condensation and maintenance interval. |
| Two dissimilar metals exposed to moisture | Galvanic compatibility, electrical isolation, exposed area ratio and drainage. |
| Buried or submerged pipeline | Coating condition, cathodic protection, electrical continuity and monitoring. |
| High-temperature equipment | Alloy resistance to oxidation or sulfidation and suitability of any high-temperature protective layer. |
| Machine with recurring local attack | Type of localised corrosion, crevices, deposits, chlorides, pH, temperature and the underlying design cause. |
Anti-corrosion products for industrial applications
Melkib offers a dedicated English-language category of anti-corrosion agents for industry. The range includes products intended for temporary protection, maintenance, protective films and other industrial corrosion-control tasks.
The specific product should be selected according to the metal, exposure conditions, required protection period, application method, operating temperature and whether the protective film must later be removed.
FAQ: electrochemical and chemical corrosion
Is rusting of iron chemical or electrochemical corrosion?
In a normal humid environment, rusting is predominantly electrochemical. Moisture or condensation provides the electrolyte, while anodic and cathodic reactions occur on the metal surface.
When do we refer to chemical or dry corrosion?
Typically when a metal reacts directly with a gaseous environment without a liquid electrolyte, for example during high-temperature oxidation or sulfidation.
Is contact between two different metals enough to cause galvanic corrosion?
No. They must be electrically connected and exposed to a common electrolyte. The result also depends on the material pair, environment, passivation and exposed area ratio.
Is stainless steel always corrosion-proof?
No. Stainless steel depends on a stable passive film and can still suffer pitting, crevice corrosion, galvanic corrosion or stress-corrosion cracking under unfavourable conditions. The correct grade must be selected for the actual environment.
Can corrosion be eliminated completely?
Corrosion risk can often be reduced to a very low level or controlled for the required design life, but there is no single universal method that eliminates every corrosion mechanism in every environment.
Need to select corrosion protection for a specific application?
Start with the metal, operating temperature, exposure to water or chemicals, required protection period and whether the protective layer must later be removed. These details make it much easier to choose a suitable anti-corrosion solution.
Technical sources
- Melkib – current Polish counterpart of this article.
- Melkib – current English article page.
- ISO – ISO 8044:2024, Corrosion of metals and alloys — Vocabulary.
- AMPP – Cathodic Protection for Corrosion Control.
- National Physical Laboratory – Guide to Good Practice in Corrosion Control.
- HSE – Corrosion / selection of materials.
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