Corrosion Monitoring
CORROSION PRINCIPLES

Corrosion is defined as the damage or deterioration of a material (usually a metal) due to a reaction or interaction with the environment.

The basic cause of corrosion is the instability of metal in its refined form. The process of corrosion is the tendency of a metal to revert to its natural state. What dictates the level of corrosion is the combination of the material type and the environment it is exposed to.

All environments are corrosive in some manner. Understanding the environment helps to determine what factors contribute to corrosion activity and what the appropriate control methods could be.

Corrosion environments can be placed into four major categories: liquid, underground, atmospheric and high temperature. In most industrial applications, the process system is exposed to many, if not all of these environments.

A material that is inert in one environment may not be in another. It is for this reason that material selection is important to ensure that adequate performance characteristics, especially life span, are obtained. Cost and availability dictate the materials that are used in industrial processes. This trade-off is what causes most corrosion problems.

With the exception of some forms of high-temperature corrosion, all forms of corrosion occur through the action of the electrochemical cell. This cell contains what is known as an oxidation/reduction reaction. In this reaction, an exchange of electrons (due to a difference in potential) occurs, where an anode is the site of oxidation and a cathode is the site of reduction. The electrons given off at the anode travel through the metal to the cathode, where they are consumed in a reduction reaction.

Corrosion is often classified as wet or dry. Wet corrosion occurs when a liquid phase is present and dry corrosion occurs in the absence of a liquid phase or above the dew point of the environment.

In most cases, the combination of the metals found in equipment and structures, combined with the wide range of possible environments, will result in more than one form of corrosion within a system.

TYPES OF CORROSION
The most common industry classifications of corrosion are as follows:
 
1. General Attack (Uniform) Corrosion

This is the most common form of corrosion. It occurs when a chemical or electrochemical attack occurs over a large area in a uniform manner. This is often referred to as a general wall loss or thinning.
 
2. Galvanic Corrosion
This occurs through the electrochemical cell. It requires electron flow, and is characterized by the presence of an anode (negative), cathode (positive) and an electrolyte. Most corrosion occurs at the anode, although some corrosion will occur at the cathode. Depending on the cell configuration, the corrosion may be localized or uniform.
 
3. Localized Corrosion
  • Crevice Corrosion
    Crevice corrosion is a highly localized attack occurring in a crevice or an otherwise shielded area when a material is exposed to a stagnant corrosive media. Common locations for crevice corrosion are:
    • Crevices(such as under bolt or rivet heads)
    • Gasket surfaces
    • Holes
    • Lap joints
    • Surface deposits
  • Pitting
    A highly localized corrosion attack that results in holes is referred to as pitting. Pits may be isolated or localized and of virtually any configuration. They occur at defects or imperfections in a protective or passive film.
  • Filiform Corrosion
    Filiform corrosion is a special form of oxygen cell corrosion occurring beneath organic or metallic coatings on materials. The attack results in a fine network of random “threads” of corrosion product developed beneath the coating material. Its cause is associated with mild surface contamination of solid particles or residue deposited on the metal surface after processing.
4. Intergranular Corrosion
All materials, with the exception of amorphous materials (such as plastic), are composed of grains and grain boundaries. Intergranular corrosion occurs when the grain boundaries are attacked in preference to the material matrix. The only difference between this and uniform corrosion is that the grains remain undamaged. Metallographic examination is usually the only way to identify this corrosion mechanism.
 
5. Dealloying (Selective Leaching)
Most materials are made up of a combination of several elements. Dealloying occurs when one of the elements is removed from the metal matrix, leaving an altered residual structure. It is commonly identifiable by a color change or a drastic change in mechanical strength.
 
6. Velocity Effects Corrosion
 
  • Erosion
    Erosion occurs when the velocity of the fluid is sufficient to remove protective films from the metal surface. It is often a combination of erosion (mechanical damage) with corrosion (electrochemical damage). It is characterized by patterns that are shaped by the flow of a fluid past the material.
  • Impingement
    Impingement is localized erosion/corrosion caused by turbulence or impinging flow. Entrained air bubbles and suspended solids tend to accelerate this action. It is commonly found in pumps, valves and at elbows and tees in pipelines.
  • Cavitation
    This is a mechanical damage process caused by collapsing bubbles in a flowing liquid. Cavitation usually results in the formation of deep aligned pits in areas of turbulent flow
7. Environmental Corrosion Cracking
  • Stress Corrosion Cracking
    Stress corrosion occurs when a material is exposed to a corrosive media while a force (stress) or pressure is applied. The material usually remains undamaged with the exception of cracks that grow through the material matrix. These cracks are usually very fine, visible only under microscopic conditions, but often network through the material, ultimately causing failure.
  • Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC)
    Hydrogen-induced cracking results from the combined action of a tensile stress and hydrogen in the metal. It results in the brittle failure of otherwise ductile materials when exposed to an environment where hydrogen can enter the metal.
    Sulfide stress cracking is a type of HIC in which sulfide is the primary poison for the hydrogen evolution. SSC of medium-strength steels has been a continuing source of trouble in oil fields.
  • Liquid Metal Embrittlement (LME)
    This is defined as the decrease in strength or ductility of a metal or alloy as a result of contact with a liquid metal. Unlike fracture by SSC, cracking begins immediately upon the application of stress if the liquid metal has wetted the solid material.
8. Fretting Corrosion 
Fretting occurs when motion between surfaces either removes protective films or mechanically removes material from surfaces in relative motion. It usually involves the motion of two surfaces that were not intended to move in that fashion.
 
9. High Temperature Corrosion

High-temperature corrosion is a form of material degradation that occurs at elevated temperatures, often beginning with temperatures in the range of 30-40% of the melting point of the alloy. Direct chemical reactions, not the electrochemical cell reaction, are responsible for the corrosion.

Biological Corrosion
In itself, biological corrosion is not a mechanism but a cause. The presence of microorganisms can lead to any of the above mechanisms occurring. Corrosion caused by microorganisms is usually indistinguishable from other sources; it is often determined by sampling the process condition for evidence of microbiological activity.
 
References: Corrosion Engineering. Mars G. Fontana, McGraw-Hill Book Co., 3rd Ed.
NACE Basic Corrosion Course. NACE ETC-10 Committee, 1996 Rev.