by Richard A. Hays, Deputy Director, Corrosion Policy & Oversight (D, CPO) [OUSD (AT&L)]
Corrosion is most often associated with "rust" on steel and the oxidation of other metals. However, 10 U.S.C. § 2228 defines corrosion is, "the deterioration of a material or its properties due to a reaction of that material with its chemical environment." It is inclusive of the deterioration of all materials, which can be caused through sun exposure, mold and mildew, wind, and other environmental elements.
Understanding the scope of this definition requires an explanation of the "Science of Corrosion." Other sections of the WBDG CPC Source explain the impacts and possible solutions for the control of corrosion.
Environmental Severity Factors Influencing Corrosion
There are several environmental severity factors that influence corrosion. These include but are not limited to:
- Presence of chloride
- Industrial pollutants
- uV radiation
- Microbiological activity
These factors influence the degradation of materials in multiple ways. None are the same, but the impact, unless arrested can be devastating. Corrosion affects metals, wood, pavements, fabrics, roofs, and building systems. The International Standards Organization (ISO) has a straight-forward approach to categorizing a region's corrosivity. As shown on the ISO Corrosivity Map in Figure 2, coastal areas tend to represent a higher corrosion threat due to the presence of chlorides and moisture in the presence of heat.
While this is a good start to understanding the risks associated with environmental severity factors, local area considerations of climate, industrial impacts and intense solar exposures may result in higher corrosion rates than one would expect from only the presence of chlorides. It is prudent to investigate and understand environmental severity impacts for specific building and construction sites to establish the local corrosivity risk. See the Environmental Severity Page for a more extensive explanation related to this risk area.
The Electrochemical Cell and Corrosion
There are several forms of metallic corrosion and all occur because of the electrochemical cell. There are four fundamental components in an electrochemical corrosion cell: An anode, a cathode, an electron path (electrical connection) between the anode and cathode, and, a conducting environment (electrolyte (see Figure 3). All four components must be present for corrosion to occur. Metal at the anode is oxidized causing it to lose mass or "corrode". The electrons that are produced as a result of this reaction travel to the cathode through the metallic path. The over-abundance of electrons at the cathode prevent it from corroding. Most common electrolyte mediums are water, soil, and chloride contaminated concrete. The electron path is an electrical connection, usually with some amount of resistance. It can be internal to the metal itself or external such as a wire, direct connection, or other interconnecting metallic components.
The forms of corrosion in a given system depend on the materials, configuration, operation, and environment. The types and extent of cathodic protection, coating, etc. depend upon the corrosion risk factors discussed in this webpage. See the CPC Source Knowledge Area - Cathodic Protection for more information.
Identifying and Recognizing Common Forms of Corrosion
The common forms of corrosion include (See Table 1, Figures 1 and 4 for the associated image examples):
- Uniform (General) Corrosion
- Galvanic Corrosion
- Erosion Corrosion
- Selective leaching/de-alloying
- Stress Corrosion Cracking
- Solar Ultraviolet degradation
- Others (e.g. Exfoliation, Environmentally Assisted Cracking, Corrosion Fatigue)
Here is a summary description of the major forms of corrosion that you might expect to see in almost any locale regardless of the environmental severity of the location:
Uniform (general) is a form of corrosion that occurs uniformly over the entire exposed surface of a metal. Applying a barrier coating is one of the most common methods to mitigate this degradation
Pitting is a form of localized corrosion that occurs when a corrosive medium attacks a metal at specific points and results in deep cavities in the metal
Crevice is a form of corrosion that occurs when an electrolyte becomes trapped and stagnant in particular locations such as in joints, corners, and under debris. It can be hard to find and extremely aggressive causing high pitting rates and subsequent unexpected failures
Galvanic corrosion results from the formation of a galvanic cell by the galvanic coupling of dissimilar metals (metals having different electrical potentials), which are exposed to an electrolyte. This form can be very aggressive but it may also be one of the easiest to detect and prevent
Erosion corrosion is due to the increased rate of deterioration and loss of a material from the combined effects of corrosion and the repeated motion of the surrounding environment. Most often found in high velocity areas or in fluids containing abrasive materials
Intergranular is a form of corrosion that attacks grain boundaries in materials. It may occur as a result of a galvanic couple between differing phases within a material and may be prevented by avoiding susceptible alloy or heat treatments
Selective Leaching or De-alloying is a localized form of corrosion where a particular element within a material is preferentially attacked and extracted from the material. Many copper alloys are susceptible alloy to this attack
Stress Corrosion Cracking (SCC) is a cracking process involving the combined factors of a susceptible material, corrosive environment and a sustained tensile stress
Solar Ultraviolet Degradation is the process by which organic-based polymers, used in the building and construction industry, undergo photolytic and photo-oxidative reactions during exposure to solar UV radiation
Other types of Corrosion:
Environmentally Assisted Cracking is a combination of a tensile stress, a corrosive environment, and a susceptible material; stress disrupts protective films at crack tip; the corrosion process generates atomic hydrogen that reduces cracking resistance of material; and, usually occurs on very high strength materials such as fasteners
Corrosion Fatigue - Fatigue resistance of most materials is reduced in the presence of a corrosive environment; corrosion processes that occur at crack tips increase the rate of fatigue crack growth; and, it occurs in structural materials such as aluminum and steel
Alkali Silica reaction on concrete occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture. This reaction causes the expansion of the altered aggregate causing spalling and loss of strength of the concrete, finally leading to its failure
Mold requires four factors to be present (i.e. mold spores, organic matter (like wood, paper, and drywall), moisture, and warmth) in order to grow or become established
Soil resistivity and moisture occurs when:
More acidic soils (pH 0 - 4.5) may represent a serious risk to common construction materials and metal (Soils usually have a pH range of 4.5 - 8.0)
Soils acidity is produced by mineral leaching, decomposition of acidic plants, industrial wastes, acid rain and certain forms of micro-biological activity
Chlorides, Sulphides, & Bacteria:
Chloride ions are harmful, as they participate directly in pitting initiation of metals and their presence tends to decrease the soil resistivity. They may be found naturally in soils as result of brackish groundwater and historical geological sea beds or from external sources such as de- icing salts
Sulphates are generally considered to be less aggressive, however sulphates can be converted to highly corrosive sulphides by anaerobic sulphate reducing bacteria. These microorganisms normally operate in temperatures from 20 to 30 °C, pH from 6 to 8, and soil resistivities from 500 to 20,000 Ω
It is important to recognize the types of corrosion as well as the susceptibility of the materials where corrosion can occur. Knowledge of the Corrosion Science basics, materials and the ability to recognize the signs of corrosion will assist in the development of a mitigation strategy and pursuit of associated actions that will serve to reduce life cycle costs, extend facility durability and sustainability.
Here are a few examples of associated actions that will reduce life cycle costs and help improve facility durability:
- Aluminum alloys do poorly in alkaline and acidic environments
- Steels must be protected from chloride containing media such as seawater and deicing materials
- Applying "cathodic protection" wherein zinc or magnesium anodes are installed to corrode sacrificially to protect a structure
- Organic materials may need to be specified with UV blockers in solar exposures
- Moisture control may be important in marine and tropical areas
- Failure to consider the impacts of heat and chlorides in association with other corrosion impacting factors will accelerate material loss
- Wood products in contact with soils and in waterfront environments can be expected to deteriorate unless some treatment is applied to slow down chemical reaction
- Coatings that are not appropriate for the intended use will not protect the surface intended and will require re-application at a faster rate
- Ultra-violet degradation will accelerate the loss of asphaltic materials in pavements and increase the rate of deterioration
- Failure to protect bridge or concrete structures will increase the rate of corrosion of reinforcing steel along with the associated surface spalling and structural failure
The CPC Source provides additional knowledge and practical information to help the Architect, Engineer, Facility Management and Acquisition professionals make the right corrosion related decision.
Relevant Codes and Standards
- ISO 9223:2012 Corrosion of metals and alloys - Corrosivity of atmospheres - Classification, determination and estimation
- ISO 9226:2012 Corrosion of metals and alloys - Corrosivity of atmospheres - Determination of corrosion rate of standard specimens for the evaluation of corrosivity
- ISO/TC 156/WG4 ISOCORRAG International Atmospheric Exposure Program: Summary of Results
- A Cumulative Damage Approach to Modeling Atmospheric Corrosion of Steel by David H. Rose, Dissertation - University of Dayton. December 2014.
- Facilities Environmental Severity Classification Study Report by Leidos. February 16, 2017