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The wasting away of metals as a result of chemical action. In a boiler, usually caused by the presence of O2, CO2, or an acid.
Most of the metal equipment in marine power plants is made up of steel or copper alloys (brass, copper-nickel, bronze and others). All of these metals will dissolve slowly in water unless the water is properly treated. This is called corrosion.

Some of the most important kinds of corrosion damage which can occur in marine power plant equipment are:

  1. Thinning of the tube metal. This is the result of corrosion that is continuous and over a fairly large area of metal. This kind of a damage is also called general corrosion. Thinning can progress to the point at which the metal can no longer contain the internal pressure which may cause the metal to swell and eventually burst.

  2. Pitting - when only a small area of metal is corroded the result is a deep hole called a pit. If pitting corrosion is not controlled, some pits may go all the way through the metal. This causes leaks. When these are many pits close together, they may become connected. The effect on the metal is the same as that of general corrosion.

  3. Corrosion cracking is another form of corrosion which can effect certain materials. In general, alloys, which are mixtures of metals are most susceptible to cracking. Stainless steel and brass such as Admiralty are particularly susceptible to cracking under certain conditions. cracking is a form of corrosion which occurs along a very narrow band through the metal.

  4. Some metal alloys are susceptible to exfoliation or de-alloying. Both of these types of corrosion are associated with the selective reaction of only one of the metals in a metal alloy. Exfoliation generally occurs in feedwater heaters. Nickel is selectively oxidised from the copper-nickel alloy tubing leaving layers of copper metal and nickel oxide. Brasses are mixtures of copper and zinc. When de-alloying occurs, zinc is removed from the metal leaving a spongy mass of copper behind. This is commonly referred to as dezincification.

  5. Embrittlement is an effect of corrosion that changes the physical properties of a metal. Some corrosion reactions cause metals to lose their normal strength and ductility and become brittle and weak. Embrittlement cannot be seen by inspecting a boiler tube that has not failed. However, an embrittled tube that has failed will have a crystallized appearance at the edge of the point of failure and usually there will be no evidence of bulging.

The study of corrosion considers reactions between a material and its environment. From the standpoint of the power plant water chemist, the study of corrosion also includes suppression of corrosion by altering or controlling the environment to which steam power plant materials are exposed. In order to understand the suppression of corrosion, one must first understand its causes.

The earliest studies of corrosion of iron (steel) showed that practically the only factor which limits the life of iron is oxidation. All of the chemical processes by which iron is corroded, eaten away, or rusted are covered by this term.

Oxidation implies the chemical reaction of a substance with oxygen. This is true, but this is a very specific application of the term. It also has a much broader and more important meaning.
The term reduction is often thought of as a reaction that involves the removal of oxygen from a substance. Again, this is true but it is also only a specific application of the term. The reduction also has a much broader and important meaning. The terms are often abbreviated as "REDOX" because they are closely related.

Simply stated oxidation involves the loss of electrons by a substance and reduction of the gain of electrons by a substance. By this definition, the element oxygen does not have to be involved in an oxidation-reduction reaction at all. The reason for this close relationship of the terms should also be apparent: If one substance gives up electrons another substance must gain them.

In experiencing oxidation, uncharged iron atoms pass into solution and become iron ions. This change involves the atoms giving up electrons. The oxidation of iron is therefore electric in nature because of the flow of electrons.

Oxidation and corrosion are therefore electrochemical processes. On this basis, corrosion can be looked at in a similar fashion to other electrical processes.

The basic nature of corrosion is almost always the same. A Flow of electricity occurs between certain areas of a metal surface through a solution capable of conducting an electric current. This electrochemical action causes the eating away of metal at areas where the electric current leaves the metal and the metal atoms enter the solution as ions.



Related Terms

PITTING

A form of extremely localized attack characterized by holes in the metal. Pitting is one of the most destructive and insidious forms of corrosion. Depending on the environment and the material, a pit may take months, or even years, to become visible.
Pitting is an example of the differential aeration effect. The initial depression or pit in the surface may be the result of several factors e.g. a break in a protective film or scale, or the solution of a non-metallic inclusion due to electrolytic action.

Once a pit is formed the corrosion proceeds rapidly since the surface of the metal (Cathode) has a greater access to 'oxygen' than the base of the pit (Anode).

Corrosion is accelerated by the fact that the surface area of the Cathode is considerably greater than that of the Anode.

The corrosion products accumulate at the mouth of the pit and assist corrosion by making oxygen diffusion more difficult.

CORROSION INHIBITOR

Additive for protecting lubricated metal surfaces against chemical attack by water or other contaminants. There are several types of corrosion inhibitors. Polar compounds wet the metal surface preferentially, protecting it with a film of oil. Other compounds may absorb water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Another type of corrosion inhibitor combines chemically with the metal to present a non- reactive surface.

ALUMINIUM

Lightweight corrosion resistant metal used for fittings and hull plating.

METAL

Any of a class of chemical elements that are typically lustrous solids that are good conductors of heat and electricity.

BATCH TREATMENT

A corrosion control procedure in which chemical corrosion inhibitors are injected into the lines of a production system.

OXYGEN CUTTING

Any of several types of cutting processes in which metal is removed with or without a flux by a chemical reaction of the base metal with oxygen at high temperatures.

False brinelling

False brinelling of needle roller bearings is actually a fretting corrosion of the surface since the rollers are the I.D. of the bearing. Although its appearance is similar to that of brinelling, false brinelling is characterized by attrition of the steel, and the load on the bearing is less than that required to produce the resulting impression. It is the result of a combination of mechanical and chemical action that is not completely understood, and occurs when a small relative motion or vibration is accompanied by some loading, in the presence of oxygen.

COUPON

Polished metal strip of specified size and weight used to detect the corrosive action of liquid or gas products or to test the efficiency of corrosion-inhibitor additives. Also known as corrosion coupon.

ACID SOOT

Carbon particles that have absorbed acid fumes as a by-product of combustion; hydrochloric acid absorbed on carbon particulates is frequently the cause of metal corrosion in incineration.

TUBERCLE

A protective crust of corrosion products (rust) which builds up over a pit caused by the loss of metal due to corrosion.

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