Part A Corrosion of metals This section on the corrosion of metals is greatly simplified for purposes of clarity. It is not meant to train the student with the intention of making him/her an expert in this area but rather to give a sense of the depth and scope of the problem.
Why theory? If we are to be effective in controlling corrosion, and in making an intelligent selection and recommendation of protective coating systems, we need to understand clearly the actual processes by which a clean, useful iron or steel structure can eventually be reduced to a collection of rusty scrap.
What is Corrosion? NACE definition: The deterioration of a substance, usually a metal, or its properties, because of a reaction with its environment.
Corrosion of Iron Electrochemical process, involving: a chemical change of iron to iron oxide, and an electrical process involving current flow.
Five things must be present for normal atmospheric corrosion to occur: Oxygen Electrolyte (moisture and ions) Anode Cathode Metallic pathway
Oxygen Oxygen must be present for normal atmospheric corrosion to occur. Without oxygen, iron can not be oxidized to iron oxide (rust), and corrosion will not occur. There is more than enough oxygen available in the surrounding atmosphere, and dissolved in fresh or salt water, to support corrosion.
Electrolyte The electrolyte is a solution of salts, minerals, acids, alkalis, industrial soot and fumes, or other chemical compounds in water or atmospheric moisture, which is capable of conducting electrical current.
Electrolyte The current carrying capacity of the electrolyte is due to the presence of tiny, electrically charged particles, called IONS, derived from salt or other chemicals dissolved in the water.
Ions Formed by dissociation (upon mixing with water) of salts, acids, and caustic chemicals into electrically charged fragments of the original chemical. The greater the concentration of ions in the electrolyte, the greater the current carrying capability and resulting corrosion rate. This is why salt-laden marine environments are more corrosive than dry, desert environments. Purified water, containing very few ions, is a poor current conductor.
The Anode The anode is a microscopic area of the corroding metal where the metal actually goes into solution and where the actual metal loss takes place. A piece of corroding steel contains millions of microscopic anodic areas.
The Cathode The cathode is a microscopic area of the corroding metal adjacent to the anode. The cathode area accepts current flow from the anode, completing the corrosion circuit. The cathodic area is not attacked and does not corrode.
In the corrosion of a metal such as a steel pipe exposed to atmospheric elements, millions of microscopic anodic and cathodic areas are located on the same metal. The overall effect is eventually complete corrosion of the metal.
Metallic Pathway S-T-E-E-L A-L-U-M-I-N-U-M Z-I-N-C The metallic pathway is provided by the metal actually undergoing corrosion. Adjacent anodic and cathodic areas are located on the same metal surface. This metal surface, a good conductor of electric current, provides the internal current path necessary to complete the corrosion circuit.
A Really Simple Corrosion Cell
Corrosion can be greatly influenced by: Oxygen aka air Gases such as carbon dioxide, hydrogen sulfide, sulfur dioxide Ingredients in a particular metal i.e. carbon steel vs. stainless steel Ion concentration in an electrolyte
Why do some metals corrode faster than other metals?
Refining / Corrosion Process
Forms of Corrosion Uniform Attack Galvanic Attack Pitting Erosion Stress MIC
Uniform Corrosion The corrosion process appears to be evenly distributed across the metal that is corroding.
Galvanic Attack Occurs when 2 dissimilar metals are in electrical contact with each other & exposed to a suitable electrolyte.
Severe Galvanic Corrosion Mild Galvanic Corrosion Big Cathode Little Anode Big Anode Little Cathode
Galvanic Corrosion
Galvanic Corrosion Dissimilar Metals
Galvanic Corrosion Stainless steel cabinet Painted carbon steel pad Galvanized grating Blisters on painted pad
The Galvanic Setup Discoloration at strap buckle Blisters forming at interface of stainless band and vessel
The Big Galvanic Setup Carbon Steel Floor of a raw sewage holding tank Stainless Steel
Pitting Corrosion A form of extremely localized attack that results in either excessive metal loss or holes in the metal.
Pitting Corrosion
Pitting Corrosion Sludge on pipe wall
Pitting Corrosion
Pitting Corrosion