Metal Corrosion: Selective Leaching, Erosion and Stress Corrosion Cracking

This article is intends to discuss:
  • Selective leaching occurs when selective elements are removed from the alloy through corrosion process
  • Erosion corrosion occurs when there is relative motion of corrosion fluid and metal surface
  • Stress corrosion cracking (SCC) is cracking which is induced from a combination of tensile stress & a corrosive environment

Corrosion most often occurs in aqueous environments. Let’s explore the different types of degradation a metal can experience in such conditions:

Selective Leaching

Selective leaching, also known as dealloying, can occur in metals in which the alloying elements are not uniformly distributed. It is considered to be an unusual form of corrosion, in which one element is targeted and removed from a metal alloy, leaving a changed structure behind.
Illustration of Selective Leaching of Brass
Typical examples of selective leaching (dealloying):
  • Dezincification of brass:
    • Zinc is extracted from the surface of brass alloy and leaves behind the weakened copper
  • Graphitization of cast iron:
    • Iron is removed, leaving behind a network of graphite with low mechanical strength
These processes are subjected to corrosive environments like moisture soil, high temperature steam of acidic surroundings.

Erosion Corrosion
  • Erosion corrosion is a form of attack with the combination of electrochemical corrosion and the action of a high velocity fluid eroding the metal. As high velocity fluids flow along the metal surface, it causes wear and abrasion. This causes the metal’s passive oxide layer to be removed, resulting in metal damage.
Schematic representation of erosion corrosion caused by turbulent eddy in downstream.
[Image from Corrosion Lab]

Erosion corrosion is susceptible to locations such as:
  • Piping of downstream flow
  • PIping bends
  • Downhole flow restriction devices (safety valves, emergency shutdown valves, inflow control devices)
  • Cooling systems
  • Impellers associated with high velocity fluid

Example of erosion corrosion in pipes
{image from The Constructor]

In many of these fluid handling systems, corrosion caused by cavitation can be reduced by applying suitable protective coatings. Coatings act as an energy absorbent here to handle the pressure by collapsing bubbles.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking occurs when a metal is subjected to a tensile stress and a corrosive environment simultaneously. Most of these are promoted by an applied tensile, thermal or welding stress, along with the attack of corrosive agents from the environments.

[Image from Sneider & JJSINA/]
The corrosion takes place within a material where the cracks are present and that it propagates through the interior but leaving the surface unharmed. This form of corrosion is hard to detect as fine cracks normally appear inside the structure and can be classified as a dangerous form of attack. If proper protective coating is applied as a barrier layer, stress corrosion can be discouraged.

Click here for more information on metal corrosion and Dritech’s range of anti-corrosion metal coatings.

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by Oh Hui Xuan
[15 May 2022]

15 Dec 2022