Corrosion can be defined as the degradation of a material due to a reaction with its environment. Metals corrode because we use them in environments where they are chemically unstable. Degradation implies deterioration of physical properties of the material and can cause severe failures. We have the knowledge and products to give you a safe, reliable and long lasting fastener solution for every application and corrosive environment.

Corrosion resistance

The corrosion properties of a stainless steel are defined mainly by the ability to form a protective passive layer of Chromium Oxide, and to remain in the passive state in the actual environment. When passivity cannot be maintained, due to a too aggressive environment, the metal will be exposed to the surrounding environment and corrosion will occur.

The Pitting Resistance Equivalent number (PRE1)) gives a good indication of the pitting and crevice corrosion resistance as a function of the alloying content. The corrosion resistance of a stainless steel fastener is not only defined by the chemical composition of the steel, but by many other factors such as, fastening design, surface quality, stresses and presence of crevices. Environmental factors as concentration of chloride ions, chemical composition of the corrosion environment, temperature, pH, pressure and oxidizing agents is important information to determine the material resistance and selecting the right material.

Many forms of corrosion exists, the most common types are described below.

General corrosion: Characterized by a uniform corrosion over the surface. It is therefore common to define a corrosion rate as mean metal lost per unit time, mm/year or milli-inch/year (mpy).

Pitting corrosion: Localized to a small area and creates pits in the metal. Pitting corrosion is often much more insidious than general corrosion as a single pit can be starting point of a failure.

Crevice corrosion: Same corrosion principle as pitting corrosion but occurs in crevices. Crevice corrosion can occur in concealed places such as small gaps and contact areas between parts and spaces where corrosive deposits can be retained. Correctly selected material in combination with a good design and cleaning will reduce the risk of crevice corrosion.

Galvanic corrosion: Two different metals in the same corrosive environment, in which the less noble metal corrodes. The potential difference between the two metals produces a flow of electron from the less noble metal (anodic) to the noble metal (cathodcic). Several factors determine galvanic corrosion potential, the electrochemical potential difference between the two metals, the presence of moisture and the relative surface area ratio. The ranking of materials with regard of potential can be found in galvanic series. The relative surface area of the two metals is very important. When the surface of the noble metal is large relative to the less noble metal, there is an increase in corrosion rate of the less noble material. For an example, carbon steel bolt in stainless steel sheet has a higher corrosion rate than stainless steel bolts in carbon steel sheet, when used in the same environment. Coupling different stainless steel grades is rarely a problem as the potential difference is too small.

Stress corrosion: Stress Corrosion Cracking (SCC) can occur when the metal is exposed to tensile stresses in a corrosive environment, often at elevated temperatures above 60°C. It is an insidious corrosion type as it can lead to unexpected sudden failure of normally ductile metals. The most common media type where SCC occurs is chloride containing solutions. Grades such as Bumax Super Austenite, Super Duplex or Hyper Duplex is highly recomended in severe condition such as swimming pool suspended ceilings and corrosive petrochemical, industrial environments.

Practical guidelines

• Evaluate the environment and the likelihood of accumulated deposits.
• Use a design that minimizes crevices and allows rain to rinse away deposits
• Use a stainless steel fastener with equivalent or higher corrosion resistance than the component being fastened
• Avoid mixing materials with large difference in electrode potential

1) PRE = % Cr + 3.3 x % Mo + 16 x % N

Corrosion resistance table

Material selection guide, contact your local Bumax sales representative for more information

O   No corrosion under normal conditions

⊗   Possible risk of corrosion, but the steel grade might be suitable depending on requirements, Environment, design and maintanence.

•   Not suitable, corrosion is likely to occur

Low: Mild condition, such as low concentrartion at low temperatures.

High: Severe condition, such as high concentration at elevated temperature.