Corrosion of Steel Part A

Part ACorrosion of metals

Part ACorrosion 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.

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.

Why theory?Why theory?

What is Corrosion?What is Corrosion?

The deterioration of a substance, usually a metal, or its properties, because of a

reaction with its environment.

NACE definition:

Corrosion of IronCorrosion 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:

OxygenElectrolyte (moisture and ions)

AnodeCathode

Metallic pathway

Five things must be present for normal atmospheric corrosion

to occur:

OxygenElectrolyte (moisture and ions)

AnodeCathode

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.

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.

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.

ElectrolyteElectrolyte

ElectrolyteElectrolyte

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.

IonsIons 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 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 AnodeThe Anode

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.

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.

The CathodeThe Cathode

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-LA-L-U-M-I-N-U-M

Z-I-N-CThe 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.

Metallic Pathway

S-T-E-E-LA-L-U-M-I-N-U-M

Z-I-N-CThe 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

A Really Simple Corrosion Cell

Corrosion can be greatly influenced by:

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?

Why do some metals corrode faster than other metals?

Refining / Corrosion ProcessRefining / Corrosion Process

Forms of CorrosionForms of Corrosion

Uniform AttackGalvanic Attack

PittingErosionStressMIC

Uniform CorrosionUniform Corrosion

The corrosion process appears to be evenly distributed across the metal

that is corroding.

Galvanic AttackGalvanic AttackOccurs when 2 dissimilar metals are in electrical contact with each other & exposed to a suitable electrolyte.

Severe Galvanic

Corrosion

Severe Galvanic

Corrosion

Big Cathode Little Anode

Mild Galvanic

Corrosion

Mild Galvanic

Corrosion

Big Anode Little Cathode

Galvanic CorrosionGalvanic Corrosion

Galvanic CorrosionGalvanic Corrosion

Dissimilar Metals

Galvanic CorrosionGalvanic Corrosion

Stainless steel cabinet

Painted carbon steel

pad

Galvanized grating

Blisters on painted pad

The Galvanic SetupThe Galvanic Setup

Blisters forming at interface of

stainless band and vessel

Discoloration at strap buckle

The Big Galvanic SetupThe Big Galvanic Setup

Carbon Steel

Stainless Steel

Floor of a raw

sewage holding

tank

Pitting CorrosionPitting Corrosion

A form of extremely localized attack that results in either excessive metal

loss or holes in the metal.

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Pitting CorrosionPitting Corrosion

Pitting CorrosionPitting Corrosion

Sludge on pipe wall

Pitting CorrosionPitting Corrosion

Pitting CorrosionPitting Corrosion

Part ACorrosion of metalsFive things must be present for normal atmospheric corrosion to occur:OxygenElectrolyte (moisture and ions) Anode CathodOxygenOxygen must be present for normal atmospheric corrosion to occur. Without oxygen, iron can not be oxidized to iron oxThe electrolyte is a solution of salts, minerals, acids, alkalis, industrial soot and fumes, or other chemical compounds in wElectrolyteIonsThe anode is a microscopic area of the corroding metal where the metal actually goes into solution and where the actual metalThe cathode is a microscopic area of the corroding metal adjacent to the anode. The cathode area accepts current flow from Metallic Pathway S-T-E-E-LA-L-U-M-I-N-U-MZ-I-N-CThe metallic pathway is provided by the metal actually undergoing cCorrosion can be greatly influenced by:Why do some metals corrode faster than other metals?Refining / Corrosion ProcessForms of CorrosionGalvanic AttackGalvanic CorrosionGalvanic CorrosionGalvanic CorrosionThe Galvanic SetupThe Big Galvanic SetupPitting CorrosionPitting CorrosionPitting CorrosionPitting CorrosionPitting Corrosion