Principles of Corrosion

Corrosion is defined as the deterioration of metal or concrete by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals or other agents in the environment in which it is placed.

The result of corrosion

Corrosion is an ever-present fact of life. It’s a relentless natural force of deterioration.  Corrosion results in:

  • Destruction of equipment or high operating cost
  • High maintenance cost of equipment or structures
  • Product contamination
  • Loss of mechanical properties, heat or electrical conductivity
  • Safety problems
  • Unsatisfactory appearance of equipment and structures

The direct financial impact of losses due to corrosion is estimated at $50 billion a year in the US alone. This figure does not include the cost of service, the cost of production interruptions, safety losses and customer goodwill lost when corrosion interferes with a process.

Different materials are affected by corrosion to varying degrees.  Corrosive attack may be by general tarnishing or rusting, with occasional attack in highly affected areas like pitting and cracking in crevices, or under gaskets.  Corrosion is capable of removing one of the constituent metals of an alloy or components of concrete leaving a weak and deteriorating residue.

How does corrosion work?

Both metal (primarily steel) and concrete are used widely for construction purposes.  The corrosion or destruction of concrete is caused by acid reacting with lime in the concrete or strong alkalies attacking the aggregate.  The corrosion of metals is more complex, and of primary importance in this discussion because of the wide use of metals for equipment, tanks and construction purposes.  In our discussion, we will concentrate on the corrosion of steel and try to give you a good working knowledge of its mechanism.

Corrosion causes instability of metals in their refined forms.  Because of free energy relationship, refined metals return to their original form, their natural state as a product mined from the earth. If we analyze rust, we find that rust is iron oxide.  An analysis of mined iron ore reveals that it too is iron oxide.  Corrosion can be explained as metallurgy in reverse.

Steel is an alloy of iron and carbon and one or more alloying metals.  Iron oxide (ore) is reduced in a blast furnace in the presence of limestone to form pig iron.  The molten pig iron is then heat processed to oxidize out the excess carbon and other impurities.  Mild steel contains less than 0.3% carbon; others contain up to 1.5% carbon.  Other alloying metals are then added to enhance the properties of the steel: cobalt and zirconium are used for construction steels, chromium and nickel in stainless steel and tungsten, molybdenum and manganese in other alloys.

As a result, steel at a microscopic level, is very diverse in composition and grain structure.  This is further complicated by various impurities and defects.  These surface differences from one area to an adjacent area create small but significant differences in electrical energy levels and are called the electromotive force (EMF).  This electromotive force drives the corrosion process across the steel surface.

Electrochemical Corrosion

The driving force, called electromotive force or corrosion potential, inherent in a metal (or metals) and influenced by the environment, determines the tendency to corrode.  The resistance of a metal corrosion determines the rate of corrosion.

The basic theory of electrochemical corrosion requires an anode, a cathode, an electrolyte and a flow of electricity between the anode and the cathode.  The anode always corrodes in preference to the cathode.  The smaller the anode area in relation to the cathode area, the faster the corrosion rate.  An electrolyte is a solution containing ions which are particles bearing an electric charge.  Ions are present in solutions of acids, alkalies and salts.  Anodes and cathodes exist on all iron and steel metals.  They are caused by surface imperfections, grain orientation, lack of homogeneity of the metal, variation in environment, localized stresses, mill scale, and existing red iron oxide rust.

In the presence of an electrolyte, metal ions go in solution.  At the same time the cathode, hydrogen ions, are released which react with electrons to form hydrogen gas.  The hydrogen forms a film at the cathode, which slows the rate of corrosion.  Introduction of oxygen will break the hydrogen film and the corrosion rate will increase (depolarization of the cathode).  Oxygen is available from the atmosphere in abundant quantities.

Reaction at the anode: Fe-> Fe** + 2e

Reaction at the cathode: 2H* + 2e -> H2

The formation of rust by electrochemical reactions may be expressed as follows:

4Fe -> 4Fe** 1 + 8e

4Fe + 3O2 + 2H2O 2Fe2O3 H2

(hydrated red iron rust)

4Fe + 2O2 + 4H2O 4Fe (OH)2

4Fe (OH)2 + O2Fe2O3 H2O + 2H2O

(hydrated red iron rust)

The most stable form of rust is Fe3O4, which is black iron oxide (magnetite).  Magnetite is formed by reducing Fe2O3 to Fe3O2.  The formation of FeO can occur either by heating the rusted steel to high temperatures or by a reduction operation.  In an acid environment, even without the presence of oxygen, the metal at the anode is attacked at a rapid rate while at the cathode the hydrogen film is being disrupted continuously, forming hydrogen gas.  When corrosion by an acid results in the formation of a salt, the reaction is slowed down considerably because a salt film forms on the surface being attacked.

The electromotive force theory of metals states that metals located higher in the EMF series will corrode and protect metals lower in the scale (if coupled to a metal lower in the EMF scale).  A partial list of metals in the electromotive series is shown below with the more active metals on top of the list.  The higher the position on the list, the greater the tendency of that metal to corrode in preference to another metal when the two are connected together in the presence of an electrolyte.

Electrical Potentials

Standard Potential (Volts)
Magnesium-2.34
Aluminum-1.67
Zinc-0.763
Iron-0.44
Hydrogen0
Copper+0.345
Platinum+1.20
Gold+1.68

This tendency of preferential corrosion of metals is commonly referred to as galvanic corrosion or dissimilar metal corrosion.  We take advantage of this property when we attach zinc or magnesium anodes to buried steel tanks or submerged steel marine equipment.  Zinc-rich coatings rely on galvanic corrosion theory and the zinc corrodes to prevent corrosion of the steel substrate.  For galvanic corrosion to take place, the two metals must be in contact with each other and there must also be an electrolyte to allow current flow.

Corrosion of Iron and Steel

Uniform or General – Electrochemical attack over the entire surface due to the constant shifting of anodes and cathodes.

Pitting – Localized electrochemical corrosion caused by a small anode surface in relation to a large cathode area.  Example: pinholes in coated steel surfaces allow an oxygen concentration difference between anode and cathode. Pitting can also be caused by biological activity, also referred to as MIC (learn more)

Galvanic (Dissimilar Metal Corrosion) – Dissimilar metals coupled together in an electrolyte cause strong corrosion cells with the anodic metal corroding away in preference to the cathodic metal as determined by their potential differences in the galvanic series.  If rust or mill scale is present on the surface of the steel, galvanic corrosion will occur because of the dissimilarity of the metals with the base metal being the anode.

Concentration Cell (Crevice Corrosion) – Caused by differences in oxygen levels, usually at air-liquid boundaries.

Erosion Corrosion – Abrasive action of solids, liquids, and gases causing a mechanical effect.

Embrittlement – Occurs by diffusion of hydrogen gas into the metal where stresses have been built up.  May be severe in concentrated alkalies.

Stress Corrosion – Stresses in the steel causes the steel grains under stress to become anodic to the adjacent areas, which leads to cracking.

Corrosion Fatigue – Caused by cyclic stresses – usually causes a fracture.

Intergranular – Selective corrosive attack at grain boundaries in the metal.  Most common example is heat affected (welding) of austenite stainless steel.

Cavitation – Erosion corrosion caused by the collapse of bubbles on the steel surface.  Example: Ships screws, impellers in pumps, turbine blades, leaves depression and pack marks.

Fretting – Erosion corrosion at highly loaded moving interfaces.  Corrosion caused by oxidation due to heat build up and constant removal of the corrosion product.  Example: Ball bearings wearing on surface.

Impingement – Caused by the movement of high velocity liquids, a mechanical chemical effect.

Dezincification – Selective removal of zinc from brass.

Graphitization – Selective removal of iron from cast iron; leaving carbon.

Stray Current – Occurs in moist soils where metals are exposed to a direct current source such as electric railways or welding equipment.

Controlling Corrosion

The battle against corrosion is tough and demanding, but many weapons are available and effective.  Corrosion problems may be reduced or eliminated by using good engineering approaches at the design stage and also good maintenance procedures after the plant is placed in operation.

Designing Considerations

There are several methods that offer excellent possibilities of reducing or completely eliminating corrosion problems at the design stage.  These recommendations include particular consideration of the following:

  • Plant location (coastal environments are normally troublesome in maintenance)
  • Plant layout (consider prevailing winds)
  • Proper fume removal and control
  • Treatment of effluents
  • Design of equipment to avoid – galvanic corrosion, crevice corrosion

This is important especially when designing steel tanks.  Crevice corrosion (or concentration cell corrosion) may be eliminated by designing the tank properly. If lap welding is used, the laps should be filled with fillet welding or a suitable caulking compound to prevent crevice corrosion. The use of proper butt welding will eliminate the need for further maintenance.

When the tank is to be lined with a protective lining, butt welding is highly desirable to assure proper coverage at the weld.  And it is recommended that weld splatter protrusions be ground smooth.

Use chemical resistant metal, alloys and plastics whenever required. Naturally, economics play a big part in designing for corrosion control and proper protection is not always possible at that stage.  However, methods may be incorporated to allow future additions that will take care of corrosion problems.  In existing plants it may be feasible to alter the environment to reduce corrosion problems.

Use of Inhibitors

Inhibitors are chemical substances used to reduce or eliminate corrosion.  Inhibitors are used widely in the oil industry to prevent corrosion of steel by crude oils.  Inhibitors based on chromates, phosphates, silicates, etc. are used to decrease corrosion of steel in aqueous media.  Inhibitors have to be replenished at regular intervals in stagnate environments and added continuously to solutions that are in motion, such as a pipeline.  Adding inhibitors to a solution does not normally prevent corrosion by its vapors.

Use of Cathodic Protection

As mentioned earlier cathodic protection is used to reduce or eliminate corrosion by electrically connecting a more active metal to the metal that must be protected.  For example, zinc and magnesium anodes are used to protect steel in marine environments.  Cathodic protection is also achieved with the utilization of an impressed current (dc) and using a relatively inert anode.  An electrolyte is, of course, needed for proper electron flow.  This method has found wide use in underwater pipelines, marine structures and equipment and water tanks.  Nevertheless, this method does not work with many chemicals or where an electrolyte is not present.

Use of Protective Coatings

The most widely used method of corrosion protection is the use of protective coatings that reduce or eliminate contact of the chemicals and oxygen with the metallic or concrete surfaces.

Protective coatings vary in thickness from thin paint films of only a few mils, to heavy mastics applied at about 1/4 inch thickness.  All industries use protective coatings of one form or another because of their excellent versatility and relatively low cost.

Inhibitive Coatings – Inhibitive coating systems are based on a prime coat containing a pigment known to chemically react at the surface of the metal.  Red lead primers are probably the oldest, followed by the chromates, molybdates, lead suboxide and barium metaborate.  Many of these are losing their position in corrosion control due to the personal and environmental concerns being raised over the last decade.  Inhibitive pigments release soluble ions into the water that penetrate the coating film.  These ionic species are carried to the metal surface and increase the polarization of the anode or the cathode.  This process encourages the development of microscopic protective surface layers.

Sacrificial Coatings – Sacrificial coatings, usually limited to zinc rich coatings, rely on the incorporation of metallic zinc that will preferentially corrode or sacrifice itself to protect the steel substrate. These coatings take advantage of galvanic (dissimilar metal) corrosion as previously discussed.  When in contact with the steel substrate, the zinc film serves as the anode of a large corrosion cell, minimizing any small electrical differences on the steel surface.  The steel thus becomes totally cathodic to the zinc and is protected.

Barrier Coatings – The barrier concept of corrosion protection is not based on the action of a particular pigment in the prime coat engaging in a reaction with the metal substrate.  Barrier coatings function as primers, intermediates, topcoats or in thick-film formulations, as single coat “systems”.  Barrier coatings produce a tighter, more cohesive film, with lower permeability to water, oxygen and ions than inhibitive or sacrificial films.  This properly insures corrosion control even under the most demanding conditions including immersion in both fresh and salt water, burial in soils and highly corrosive chemical environments.  Examples are coal tars, epoxies, multi-coat vinyls and aliphatic urethanes.

Protective barrier coatings vary considerably in composition, performance and cost.  The corrosion engineer has a wide choice of materials for most all applications and his selection will depend on the important requirements of the job.  Many leading companies emphasize the importance of obtaining protection based on the lowest cost per square foot per year of service.  In some cases, where short-term protection is needed, a barrier that would provide good performance for only that period may be quite adequate.

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