• Uniform • Galvanic • Crevice • Pitting • Intergranular • MIC • SCC • HB-HE-HIC • Fatigue • Erosion • Fretting • Stray Current • Index

Different Types of Corrosion
- Recognition, Mechanisms & Prevention

Uniform Corrosion (General Corrosion)

Recognition

What is uniform corrosion? Uniform corrosion or general corrosion, as sometimes called, is defined as a type of corrosion attack (deterioration) that is more or less uniformly distributed over the entire exposed surface of a metal (see illustration below). Uniform corrosion also refers to the corrosion that proceeds at approximately the same rate over the exposed metal surface.

Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance.

The photo on the right showed uniform corrosion (rusting) of a pair of steel nuts used to fasten a galvanized steel clamp on a street lamp post. In sharp contrast, the galvanized steel clamp did not show any signs of corrosion but its surface was discolored by the rust. It is also interesting to note that the surface of the top bolt looked like galvanized but the surface of the bolt below was completely rusted (just like the nut).

The photo (above, left) shows a steel coupon corroded (rusted) uniformly over its entire surface after immersion in oxygen aerated water while the same batch of coupons exposed to deaerated water (above, right) retained their metallic appearance with no visible corrosion (rust).

In natural environment, oxygen is the primary cause of uniform corrosion of steels and other metals and alloys.

What causes uniform corrosion? The anodic reaction in the corrosion process is always the oxidation reaction:

M = M⁺ + e  (1)

In acidic environments, i.e., pH<7,  the cathodic process is mainly the reduction of hydrogen ions:

2H⁺ + 2e = H₂       (2)

In alkaline or neutral environment, i.e., pH=7 or pH>7, reduction of dissolved oxygen is the predominant cathodic process that causes uniform corrosion:

O₂ + 2H₂O + 4e = 4OH^-         (3)

With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2) and/or (3) take place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The cathodes and anodes are located randomly and alternating with time. The end result is a more or less uniform loss of dimension.
 

• Use thicker materials for corrosion allowance
• Use paints or metallic coatings such as plating, galvanizing or anodizing
• Use Corrosion inhibitors or modifying the environment
• Cathodic protection (Sacrificial Anode or Impressed Current -ICCP) and Anodic Protection

General corrosion can be predicted using corrosion modeling and corrosion prediction software such as the following:

• CO2Compass: Modeling and Prediction of Corrosion by Carbon Dioxide (CO2), Hydrogen Sulfide (H2S), Acetic Acid (HAc), Elemental Sulfur (S), and Mercury (Hg) in Oil and Gas Pipelines and Production Tubing

• O2Compass: Modeling and Prediction of High Temperature Oxidation

• CSR-Compass: Modeling and Remaining Life Prediction of Creep and Stress Rupture of Boiler and Heater Tubes

• H2Compass: Modeling and Prediction of Low Temperature Hydrogen Damages (Hydrogen Blistering, Hydrogen-Induced Cracking, Hydrogen Embrittlement) and High Temperature Hydrogen Attack (HTHA)

• S-Compass: Modeling and Prediction of High Temperature Sulfidation/Sulfidic Corrosion/H2-H2S Corrosion and Low Temperature Elemental Sulfur Corrosion

• NAC-Compass: Modeling and Prediction of High Temperature Naphthenic Acid Corrosion

• CCC-Compass: Modeling and Prediction of Caustic Corrosion and Caustic Stress Corrosion Cracking

• CP-Compass: Cathodic Protection Design Calculations, Verification, Assessment and Solutions

• PipelineCompass: Pipeline Corrosion Modeling, Prediction, Assessment and Solutions

• SC-Compass: Stray Current Corrosion and AC Corrosion - Identification, Assessment and Prediction

• GC-Compass: Galvanic Corrosion Prediction and Materials Compatibility Assessment

• CIPAL-Compass: Copper-Induced Pitting in Aluminium Alloys - Modeling, Life Prediction and Process Control

• FAC-Compass: Erosion Corrosion and Flow - Accelerated Corrosion Modeling, Life Prediction and Materials Selection in Water-Steam Systems

• EVS-Compass: Applications of Extreme Value Statistics in Corrosion Modeling and Corrosion Life Prediction of Structures and Plant Assets

• CRA-Compass: Corrosion Resistant Alloys - Corrosion Prediction, Alloy Selection and Application Limits (Temperature Limit and Chloride Limits)

• CUI-Compass: Prediction and Risk Assessment of Corrosion Under Insulation and Fireproofing

• Dew-Point-Compass: Prediction of Dew Point Temperatures of Flue Gases and the Risk of Dew Point Corrosion

• Atmosphere-Compass: Prediction and Modeling of Atmospheric Corrosion of Metals and Alloys

• Shipwreck-Compass: Shipwreck Corrosion Modeling and Corrosion Prediction

• VPC-Compass: Prediction and Modeling of Internal Corrosion in Vapor Phase in Closed Systems

• VPC-Compass-SE: Prediction and Modeling of Corrosion in Microelectronic Packages

• Corrosion Rate Units Converter: Converting Between All Corrosion Rate Units for All Metals and Alloys

• ACE - Apps for Corrosion Engineers: A Collection of Essential Corrosion Software Applications for Corrosion Engineers, Corrosion Researchers, and Corrosion Technicians in Laboratories and in Fields.

Corrosion Control and Its Prevention (5 days)

Corrosion Inspection, Testing and Monitoring: Techniques and Applications (5 days)
Corrosion, Metallurgy, Failure Analysis and Prevention (5 days)
Marine Corrosion, Causes and Prevention (2 days)
Materials Selection and Corrosion (5 days)
Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)

If you require corrosion expert witness or corrosion consulting service on uniform corrosion, our NACE certified Corrosion Specialist is able to help. Contact us for a quote.

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