Corrosion is a naturally occurring phenomenon commonly defined as the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. Corrosion can cause dangerous and expensive damage to water and wastewater systems, pipelines, bridges and public buildings costing the U.S. according to a current corrosion study approximately $276 billion on an annual basis. Fortunately, there are time proven methods to prevent and control corrosion that can reduce or eliminate its impact.
Production and manufacturing industries are widely affected by corrosion with pulp and paper corrosion costing approximately $6 billion per year followed by oil and gas exploration at $4.1 billion, food processing at $2.1 billion and mining at $0.1 billion.
The science of corrosion prevention and control is highly complex, exacerbated by the fact that corrosion takes many different forms and is affected by numerous outside factors. Corrosion professionals must understand the effects of environmental conditions, type of product to be processed or handles, required lifetime of the structure or component, proximity to corrosion causing phenomena, appropriate mitigation methods and other considerations before determining the specific corrosion problem and specifying an effective solution.
There are ten primary forms of corrosion, but it is rare that a corroding structure or component will suffer from only one. The combinations of metals used in a system and the wide range of environments encountered often cause more than one type of attack. Even a single cell alloy can suffer corrosion from more than one form depending on its exposure to different environments at different points within the system. All forms of corrosion with the exception of some types of high temperature corrosion occur through the action of the electrochemical cell.
The elements that are common to all corrosion cells are an anode where oxidation metal loss occur, a cathode where reduction and protective effects occur, metallic and electrolytic paths between the anode and cathode through which electronic and ionic current flows, and a potential difference that drives the cell. The driving potential may be the environment, including chemical concentrations. There are specific mechanisms that cause each type of attack, different ways of measuring and predicting them, and various methods that can be used to control corrosion in each of its forms.
There are three main groups of corrosion:
1. Corrosion that is readily identifiable by ordinary visual examination. Examples of this type of corrosion include:
- Uniform Corrosion
- Crevice Corrosion
- Filiform Corrosion
- Pack Rust
- Galvanic Corrosion
- Lamellar Corrosion
2. Corrosion that may require supplementary means of examination to identify. This type of corrosion includes:
- Erosion Corrosion
- Fretting Corrosion
- Intergranular Corrosion
3. Corrosion identification which may require microscopy either visually or electron etc. Examples include:
- Environmental Cracking
- Stress Corrosion Cracking
- Corrosion Fatigue
- Hydrogen Embrittlement
Many types of corrosion exist, but for the purposes of this article, only the most common forms will be discussed in detail. Check back next week for Part 2 of The High Cost of Corrosion: Is Your Plant at Risk where we do just that.
For a complete list and detailed description of the various forms of corrosion, please visit www.nace.org.