Whitepaper: Fasteners and Corrosion

Each year, the effects of corrosion on public and private assets represents an equivalent cost of 3.4% of global GDP, according to estimates.¹ This staggering cost affects the infrastructure, energy, manufacturing, transportation, construction, marine, and service industries, costing manufacturers, operating companies, service providers, contractors, investors, engineers, designers, customers, taxpayers, and others involved in local, national, and global economies.

Corrosion engineers are engaged in the study and practice of corrosion management, providing guidance on the selection of cost-effective materials, coatings, and corrosion mitigation strategies. The work performed by these specialists helps to ensure the technical integrity of materials and installations. In determining appropriate materials, coatings, and corrosion management tactics, the specialist must assess each component of a system against a variety of factors, including environmental conditions, application requirements, estimated product/system lifetime, and available mitigation methods.

Fasteners – often the least expensive component in a system’s design – deserve particular attention in that (a) their failure can lead to serious expense and performance issues affecting the entire installation, and (b) this failure can largely be avoided or mitigated through the proper selection of materials, coatings, and platings, to help ensure desired performance over the life of the installation.

This document will discuss some of the important considerations involved in the selection of fastener materials, designs, and coatings, in order to help design engineers, project managers, and purchasing personnel avoid potential problems that can result from fastener corrosion.

1 National Association of Corrosion Engineers (NACE) NACE International has become the global leader in developing corrosion prevention and control standards, certification and education. NACE is a standards-writing organization accredited by the American National Standards Institute (ANSI).

Environmental factors play an overriding role in the selection of materials and coatings to resist corrosion. A review of environmental factors is therefore critical to help avoid potential problems. This section discusses factors to consider in atmospheric, water, and soil environments.

Atmospheric Environments

Contaminants, humidity, wind or water currents, pH level, and temperature are all elements of atmospheric environments that should be considered in the design and selection of fastener products.

Contaminants

Rural environments are generally the least corrosive, with the principal corrodents being oxygen and moisture content. Urban environments, while similar to rural, may present sulfur dioxides and nitrous oxides from vehicle and domestic fuel emissions. Industrial environments may contain sulfur dioxides, chlorides, nitrates, and phosphates from processing facilities, as well as special instances of hydrogen sulfide, hydrogen chloride, and chlorine, which are highly corrosive to most metals.

Humidity and Rainfall

Humidity is a major factor in determining the potential for the corrosion of metals. This is because moisture provides the electrolyte that is required for corrosive processes to occur. In the absence of other electrolytes, the level of relative humidity required for corrosion to occur is 60%.

Wind

The corrosivity of atmospheric environments can be enhanced by wind, as it affects the distance and direction contaminants can be dispersed. Proximity to coastal waters and industrial sites can affect general corrosion rates.

Temperature

As environmental temperature increases, the potential for corrosion increases. Temperature can also affect the form of corrosion that occurs – as temperature changes, the type of corrosion may change from one form to another. High temperatures can even cause a form of corrosion in which gas becomes an electrolyte.

Water Environments

Water environments are divided into natural (fresh) water and seawater (salt) type environments. The factors that determine the corrosivity of water environments include water composition, salinity, pH level, temperature, water velocity, and biological organisms.

Water Composition

Water composition can vary widely, based on atmospheric materials and contaminants picked up from rainfall, soil, and man-made pollutants. Dissolved gases (primarily oxygen and sulfurous gases) and salts are the compounds representing the greatest corrosion risk in water environments. Oxygen is, by far, the biggest concern, with its greatest concentration on the surface of water and in the presence of algae.

pH Level

The normal pH level of water (both natural and seawater) ranges from 4.5 to 8.5. The rate of corrosion of certain metals increases in acidic water.

Temperature

As in atmospheric environments, higher water temperatures generally increase corrosion rates. Even though higher temperatures decrease oxygen solubility, they increase biological growth, which increases overall oxygen content.

Water Velocity and Agitation

Water velocity and agitation can both increase and decrease corrosion rates on certain metals. For most metals, there is a critical velocity beyond which serious corrosion occurs.

Biological Organisms

Certain biological organisms alter the composition of water, which may lead to increased or decreased corrosion rates. Some organisms increase oxygen or sulfide content, increasing the potential for corrosion.

Soil Environments

Factors contributing to the corrosivity of soil includes soil particle size, water, aeration, pH level, temperature, salt content, and biological activity.

Water

Soils comprised of large particles will retain less water and exhibit more oxygen content, thereby increasing the potential for corrosion. (Oxygen content is highest at or near the surface.)

pH Level

The pH level of soil is typically 5 to 8, a range in which corrosion rates are not significantly affected. However, acidic soils will increase corrosivity.

Salt

A soil’s electrical resistivity provides a general indication of the potential for corrosion. (Lower resistivity signals higher corrosivity.) Salt content has an effect on corrosivity, with sulfites and chlorides being the most aggressive corrosive agents.

The majority of observed corrosion-related fastener problems are caused by well known, easily identifiable forms of corrosion. These include uniform and galvanic corrosion, as well as pitting corrosion, fretting fatigue, stress corrosion, erosion corrosion, and other forms. In specific environments (e.g., where specific chemicals are present in the atmosphere), lesser and more specialized forms of corrosion occur. This section will focus on the most common types of corrosion affecting fastener performance. 

Uniform Corrosion

Uniform corrosion (also referred to as “general” corrosion and “general attack” corrosion) tends to develop uniformly over an exposed surface, resulting in the thinning of materials, until failure occurs. Uniform corrosion is dependent upon two factors: the composition of the material and the characteristics of the environment.

Uniform corrosion occurs due to an electro-chemical process that takes place on the surface of the material. During this process, anodes and cathodes are created that facilitate the corrosion process.

General corrosion occurs predictably. It is normally the result of placing inappropriate materials in corrosive environments.

Galvanic Corrosion

Galvanic corrosion is a process that occurs when two metals of differing electrical potentials are physically connected, or electrically connected through a conductive electrolyte such as rain-water or groundwater. An electrical current can be formed that attracts electrons away from the more active metal (anode), thereby causing corrosion, while boosting the corrosion resistance of the passive metal (cathode).

Galvanic corrosion is a major consideration in the selection of fastener materials to be used in a given application. Due to the fact that fasteners have a much smaller surface area than the materials they fasten, they are at the risk of rapid corrosion if subject to galvanic corrosion.

Other Forms of Corrosion

In addition to uniform and galvanic corrosion, other forms of corrosion can play a role in fastener material selection (and potential mediation efforts in a given application), depending on specific characteristics of the application and/or environment.

Pitting Corrosion

Pitting corrosion (also known as “pitting”) requires that only one metal is present, where an electrolyte in the environment sets up an attack system. Pitting is an extremely localized corrosive process, typically caused by a breach of a protective coating or oxide film due to mechanical damage or chemical degradation.

Uniform corrosion occurs due to an electro-chemical process that takes place on the surface of the material. During this process, anodes and cathodes are created that facilitate the corrosion process.

General corrosion occurs predictably. It is normally the result of placing inappropriate materials in corrosive environments.

Fretting Fatigue

Fretting fatigue (also called “fretting corrosion”) can occur when there is relative motion (often the result of vibration) between two metals that are in contact and under load. Such conditions can cause the surface of one or more of the metals to physically wear away, releasing particles that further accelerate the speed of fretting over time.

Galvanic corrosion is a major consideration in the selection of fastener materials to be used in a given application. Due to the fact that fasteners have a much smaller surface area than the materials they fasten, they are at the risk of rapid corrosion if subject to galvanic corrosion.

Stress Corrosion

Stress corrosion and stress corrosion cracking occur when a metal is subjected to both corrosion and static tensile stress at the same time. Stress corrosion and stress corrosion cracking can be difficult to detect.

Corrosion Fatigue

A similar type of corrosion to stress corrosion, corrosion fatigue involves a cyclic or dynamic stress in combination with corrosion, rather than static tensile stress.

Erosion Corrosion

Erosion corrosion occurs as the result of the interaction of an electrolyte solution in motion relative to a metal surface. The fluid motion results in wear and abrasion. This type of erosion is commonly found in pipelines, valves, cooling and boiler systems, propellers and impellers, and other components.

Additional Forms of Corrosion

Exfoliation corrosion, microbiological corrosion, liquid and solid metal embrittlement, molten salt corrosion, filiform corrosion, stray-current corrosion, and grooving corrosion (affecting carbon steel) are additional forms of corrosion that can have adverse effects on metal installations.

Other Types of Coating

Aluminum, lead, copper, chromium and tin are additional types of metallic coatings that can enhance corrosion resistance. In addition, organic coatings (such as paint, varnish, and lacquer), can be considered for use in any given application.

Specialty Coating

A wide range of specialty coatings are available to provide additional performance characteristics when needed for given applications, including lubrication, hydrophobic perfor­mance, flexural strength, and other properties.