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Design measures for corrosion prevention

This article provides an overview of the types of corrosion that occur and possible constructive measures to prevent or reduce corrosion.

In practice it is sometimes difficult to foresee the expected corrosion phenomena completely and precisely for the first product design. This can be remedied by the guidelines and regulations that may exist for the respective product, into which corresponding empirical values, such as additional values ​​for the material thickness, have already been incorporated. If there are no such regulations, the following list is helpful for constructive measures against corrosion.

Types of corrosion

There are different types of corrosion, but what they have in common is that an electrolyte is usually required to start the corrosion. The following types can be distinguished:

Corrosion of free surfaces

  • Uniform surface corrosion
  • Peptic corrosion
  • Pitting corrosion
  • Crevice corrosion

Corrosion of contacting surfaces

  • Contact corrosion
  • Sediment corrosion
  • Corrosion at phase interfaces

Corrosion from stress

  • Stress corrosion cracking
  • Vibration corrosion cracking
  • Corrosion induced by strain
  • Erosion and cavitation corrosion
  • Friction corrosion

Selective corrosion

  • Intergranular corrosion
  • Graphite corrosion
  • Dezincification

The following describes the individual types of corrosion, their causes and the respective design measures against corrosion.

Corrosion of free surfaces

Uniform surface corrosion

Root cause

A metallic surface is exposed to moisture (weakly basic or acidic) and oxygen (normal contact with the atmosphere). These environmental conditions lead to material erosion over the entire surface due to corrosion.

Design countermeasures
  • use stainless steel
  • plan additional material thickness
  • design to minimal surface area
  • Allow areas to be ventilated to dry
  • avoid the accumulation of liquids (e.g. plan inclined surfaces for drainage)
  • avoid falling below the dew point (formation of condensate)
  • use an anti-corrosive surface coating
  • use cathodic protection

Peptic and pitting corrosion

Both peptic and pitting corrosion are locally increased free types of corrosion that are caused by inhomogeneities in the material. The design countermeasures apply as described above.

Crevice corrosion

Root cause

Electrolytes can accumulate in crevices and accelerate corrosion. In addition, the accumulation of electrolytes leads to a lack of oxygen, which can also cause damage to corrosion- and acid-resistant steels.

Design countermeasures
  • weld overlapping material on both sides
  • apply sealing runs to weld seams
  • do not use interrupted weld seams
  • seal the gap
  • increase the gap to allow ventilation
Bad: If material overlaps are welded on one side, electrolyte can collect in the gap
Good: When welding on both sides, the gap is sealed so that no electrolyte can accumulate
Bad: Partially welded seams lead to gaps in sections that are prone to corrosion
Good: A welded-through weld seam closes all gaps

Corrosion of contacting surfaces

Contact corrosion

Root cause

Use of metals with different potentials (electrochemical series) with the simultaneous presence of an electrolyte.

Design countermeasures
  • use materials with a lower potential difference
  • isolate the two materials
  • avoid electrolyte
  • use of sacrificial anodes

Sediment corrosion

Root cause

The origin of sediment corrosion is also in the electrochemical series, i.e. the potential difference. Sediments can accumulate during production (e.g. extraneous rust) as well as during operation due to abrasion or other effects and lead to corrosion.

Design countermeasures
  • avoidance of deposit products/sediment through filtering or cleaning
  • in the case of machine parts with flow: avoiding “dead” flow zones in which particles can settle
Bad: Deposits/Sediments or electrolytes can collect on or in profiles during operation.
Good: The profiles should be arranged in such a way that as few sediments as possible can accumulate. For example, drainage holes can also be provided.

Corrosion at phase interfaces

Root cause

In the transition area between, for example, the liquid phase and the vapor phase of a medium, increased corrosion can occur. This can occur both with stationary liquids and with “changing” media, e.g. in the transition zone from water to water vapor in a heat exchanger.

Design countermeasures
  • minimize the component area in the area of ​​the phase interface
  • slow down the phase change (with flowing media)
  • vary fill levels

Corrosion from stress

Stress corrosion cracking

Root cause

There are materials that are susceptible to stress corrosion cracking. These are:

  • unalloyed carbon steels
  • austenitic steels
  • Brass
  • Magnesium alloys
  • Aluminum alloys
  • Titanium alloys

Stress corrosion cracking can be triggered by external tensile stresses or the internal stress state of a component.

Design countermeasures
  • avoid susceptible materials
  • lowering the tensile stress (in the area of ​​the surface)
  • application of compressive stress
  • stress relief heat treatment
  • avoid electrolytes
  • use cathodic coatings

Vibration corrosion cracking

Root cause

The vibration corrosion cracking is difficult to distinguish from fatigue failure because it shows almost the same characteristics.

Design countermeasures
  • avoid alternating stress
  • avoid grooves
  • avoid electrolytes
  • Introduce compressive prestress into the surface of the component (e.g. by shot peening)
  • use surface coating

Strain corrosion

Root cause

The expansion or compression of a component can damage a protective surface layer or surface coating.

Design countermeasures
  • reduce strain or compression

Erosion

Root cause

Surface damage caused by particles contained in a flowing fluid.

Design countermeasures
  • filtering the particles
  • decrease the flow velocity
  • redirecting the flow
  • apply additional surface layers, such as build-up welds, nickel layers, hard chrome or stellite

Cavitation

Root cause

Cavitation leads to local overstressing of the surface, so that corrosion is favored at these points.

Design countermeasures
  • decrease the flow velocity
  • apply additional surface layers, such as build-up welds, nickel layers, hard chrome or stellite

Friction corrosion

Root cause

Friction from moving, adjoining components leads to both abrasion and higher surface stress due to surface pressure.

Design countermeasures
  • use of interchangeable friction materials
  • use of lubricants
  • avoid the motion
  • avoid the contact

Selective corrosion

Root cause

Selective corrosion is material corrosion on the crystal lattice level. The material can be more susceptible to corrosion at the grain boundaries, so that e.g. in cast iron, the iron components dissolve (spongiosis or graphite corrosion).

Intracrystalline corrosion particularly affects austenitic CrNi steels after unsuitable heat treatment.

Design countermeasures

  • apply suitable heat treatment measures for the materials used