CORROSION CASE STUDY ,PRINCIPLES AND TYPES

CORROSION CASE STUDY ,PRINCIPLES AND TYPES 

INTRODUCTION
Corrosion removal deals with the taking away of mass from the surface of materials by their environment and other forms of environmental attack that weaken or otherwise degrade material properties.

Principles of Corrosion

Corrosion of metals and alloys is an electro chemical process. The scientific basis of corrosion in aqueous systems is well understood. In these electrochemical corrosion processes, the following steps take place:
· A difference exists between the oxidation reduction potential for two materials or sites that are connected electrically. The material with the most negative oxidation reduction potential is the anode and the material with the most positive oxidation reduction potential is the cathode.
· The anode loses positively charged ions and electrons flow away from the anode. This process is referred to as oxidation. The anode is the portion of the metal surface that is corroded.
The general form of the anodic reaction is:

M → Mⁿ⁺ + ne⁻

Where M is the substance being oxidized, and n is the number of electrons (e⁻) given up by M. Iron, zinc, nickel, tin, and copper typically release two electrons.
· The electrons from the anode flow through a conductor to the cathode and react with positive ions in the electrolyte at the cathode.
· The electrons react with positive ions in the electrolyte at the cathode. This process is referred to as reduction. The cathode does not have to be another component, but can be hydrogen ions, dissolved oxygen or metal ions.
There are several cathodic reactions that may occur:
Mⁿ⁺ + ne⁻ → M (Metal deposition)
O2 + 4H⁺ + 4e⁻→ 2H2O (Oxygen reduction in an acidic solution)
O2 + 2H2O + 4e⁻ → 4OH⁻ ( Oxygen reduction in a neutral or alkaline solution)
Mⁿ⁺ + e⁻ → M(ⁿ¹)⁺ (Metal ion reduction)
2H⁺ + 2e⁻ → H2 (Hydrogen ion reduction, causing hydrogen gas evolution)
2H2O + 2e⁻ → 2OH⁻+ H2 (Reduction of water)

Corrosion is classified as general or localized. For general corrosion of a metal surface, the whole surface consists of continually shifting anodic and cathodic sites, so that any point on the surface is alternately anodic and cathodic during the time corrosion is taking place. For localized corrosion, the anodic and cathodic sites are permanently separated from each other.

The slowing down or retardation of the corrosion process
is called polarization. There are three types of polarization that can retard corrosion:
activation polarization, resistance polarization and concentration polarization.
· Activation polarization occurs when activation energy is required to enable the corrosion reaction to occur.
· Resistance polarization occurs when the electrolyte resistance is sufficiently high that the current resulting from corrosion polarizes the anodes and the cathodes.
· Concentration polarization occurs when the reaction is controlled by the diffusion of a reactant (such as Fe²⁺, H⁺ or oxygen) through the electrolyte to the metal surface.
types of corrosion
The major types of corrosion that may occur at a nuclear power station include:
· General or Uniform Corrosion

· Localized Corrosion
· Galvanic Corrosion
· Intergranular Corrosion
· Dealloying Corrosion
· Cracking Phenomena

General or Uniform Corrosion
General or uniform corrosion is a corrosion that occurs without significant localized degradation. Although rapid general corrosion can occur if improper materials are selected, this type of degradation is normally the least dangerous type due its predictability. The susceptibility of a given metal or alloy to general corrosion in a given environment can be classified in one of three ways:
· General corrosion occurs and corrosion product dissolves.
· General corrosion occurs, but a passive protective coating develops and significantly reduces the corrosion rate.
· The metal or alloy does not corrode in the given environment.

A common form of general corrosion is the attack of oxygenated water on iron, which
produces rust. Rust is a combination of three compounds:
· Ferrous hydroxide (Fe(OH)2)
· Ferric hydroxide (Fe(OH)3)
· Magnetite (Fe3O4)
Acid and alkaline corrosion are other forms of general corrosion.

Localized Corrosion
Localized corrosion is a general term that describes corrosion mechanisms that preferentially attack specific areas of a SSC. In some instances, localized corrosion can appear as an area where the rate of attack of general corrosion is greater in a specific part of the SSC.

Many examples of localized corrosion arise from conditions that establish differences in the environments present at different regions on a metallic surface. The environmental differences develop anodes and cathodes, creating local corrosion cells.
Service water systems are susceptible to localized corrosion arising from oxygen concentration cells. The regions of higher oxygen concentration become cathodic, gaining electrons through the reaction:
O2 + 2H2O + 4e⁻ → 4OH⁻ 
The service water piping and components, normally carbon steel, act as the anodes in the oxygen depleted areas through the reaction:
Fe → Fe²⁺ + 2e⁻
The Fe²⁺and OH⁻ ions combine to form ferrous hydroxide (Fe(OH)2).

Pitting Corrosion
Pitting occurs when corrosion selectively removes small volumes of metal, creating cavities in the metallic body of a structure or component. Generalized pitting usually arises from defects or weak areas in the protective film or coating on a metallic surface. The rate of corrosion will depend
upon the specific electro chemical reaction, local environmental conditions and the number of pits. Pit growth rate will generally decrease as the number of pits increases.

Crevice Corrosion
Crevice corrosion occurs when a liquid filled crevice (or crack) undergoes corrosion. If the opening of the crevice is tight enough, the liquid in the crevice will be stagnant. Corrosion will deplete the oxygen in the crevice faster than it can be replaced through diffusion. The depletion of the oxygen in the crevice creates a concentration cell relative to the liquid outside of the crevice.
The crevice region becomes an anode and corrosion rates in the crevice accelerate. The accelerated corrosion can result in acidic conditions within the crevice, further increasing the corrosion rates. Places where crevice corrosion is commonly seen include flanges, bolt holes, heat exchanger tube sheets, threaded connections and unsealed joints

Tuberculation
Tubercles are lumps or mounds of corrosion products and deposits that form on carbon steel pipe exposed to oxygenated water. Tubercles cover regions of metal loss and create oxygen concentration cells. Besides contributing to pipe wall degradation, tubercles also reduce flow by clogging pipes.
Tubercle formation is started when carbon steel corrodes due to contact with oxygenated water. The corrosion products coalesce into a structure that reduces oxygen diffusion to the area underneath. The corrosion beneath the structure depletes the oxygen and an oxygen concentration cell is established, causing accelerated corrosion and tubercle growth. The tubercle’s growth is enhanced by deposits from the water flowing through the pipe. Tubercles are most commonly found in service water piping, but can also be found in heat exchangers, pumps and storage tanks.

Underdeposit Corrosion
Underdeposit corrosion is the result of the shielding of surfaces by deposits, creating the conditions necessary to establish concentration cells. Deposits containing corrosive substances will directly attack the surrounding metal. Substances that can lead to underdeposit corrosion include silt, sand and shells.

Microbiologically Influenced Corrosion (MIC)
MIC is of particular interest, as it gives the appearance that microorganisms are “eating” metal. In actuality, MIC arises from a variety of normal electrochemical reactions that are influenced by the presence of living microbial organisms on metallic structures or components.
MIC can attack both carbon and stainless steels. In carbon steel, MIC may result in random pitting, general corrosion and formation of tubercles or corrosion product deposits. MIC of stainless steels is characterized by pitting, most commonly at weldments. These pits often exhibit very small entrance and exit penetrations with very large subsurface cavities. Pits in stainless steels caused by MIC can also tunnel, where the corrosion changes direction. MIC can also attack commonly used copper
alloys (coppernickels, brasses, aluminum bronzes and admiralty brass) and high nickel alloys (Monels, Incoloys and Inconels).
Microbes can cause crevice corrosion due to the formation of corrosion cells beneath areas of microbial growth. Other microbes can produce substances (metabolites) that are corrosives, such as acids, ammonia or hydrogen sulfide. Some microbes can concentrate halides, causing severe localized corrosion. Other microbes can influence corrosion rates by attacking protective coatings.

Galvanic Corrosion
Galvanic corrosion is one of the most familiar types of material degradation. Galvanic corrosion occurs when two materials with sufficient galvanic potential difference are immersed in an electrolyte and have a separate electrically conductive pathway. A sample of relative galvanic potential differences is given in Table 1 below. A metal that is lower on the chart (more active) will undergo galvanic corrosion in the presence of a metal higher on the chart (more noble)

table 1

One important aspect of galvanic corrosion is the area effect. If the surface area of the more noble metal is relatively large in comparison to the surface area of the more active metal, very rapid galvanic corrosion can occur due to the greater anodic current density. This is important when protective coatings are only used on the anodic surface to prevent galvanic corrosion.

Intergranular Corrosion
Intergranular corrosion takes place when the corrosion rate of the grain boundary areas of an alloy exceeds that of the grain interiors. For example, sensitized austenitic stainless steels are very susceptible to intergranular corrosion. Sensitization of austenitic stainless steels occurs when chromium carbides precipitate at the grain boundaries. This precipitation depletes the matrix of chromium adjacent to the grain boundaries, causing a wide difference in corrosion rates between the grain boundaries and the remainder of the matrix. Welding is one of the primary causes of sensitization of austenitic stainless steels. The welding process can bring the weld's heat affected zone (HAZ) into the temperature range for chromium carbide precipitation. Carbides will precipitate and a zone somewhat removed from the weld will become susceptible to intergranular corrosion. Some plant examples are the thermal sleeve and safe end of the reactor at Boiling Water Reactor nuclear plants.

Dealloying Corrosion
Dealloying corrosion occurs when one constituent of an alloy is preferentially removed, leaving a different structure. Dealloying has been observed in copperbased alloys and
cast iron. Terms to describe dealloying include dezincification (for brass), graphitic corrosion (for cast iron), selective leaching and parting.

Cracking Phenomena
Cracking phenomena, or environmental cracking, is the result of tensile stress and corrosion. The most common cracking phenomenon at nuclear power plants is fatigue type cracking and stress corrosion cracking (SCC), which occurs when three requirements are met:
· Susceptible material (normally ductile) involved.
· Sufficient tensile stress exists, either residual or applied. The amount of stress required is dependant upon the specific alloy, corrodent concentration and temperature.
· Initiating corrodent is present. Specific alloys are susceptible to SCC only if the correct initiating corrodent is present.

Stress corrosion cracking is a combined electrochemical and mechanical phenomenon. The tensile stress ruptures the existing oxide film, exposing the base metal to corrosive attack. The film-free base metal surface is anodic compared to the film covered surface and corrosion occurs, causing grooves to form. The tensile stresses concentrate at the tips of the grooves, causing further cracking and preventing the formation of a new protective oxide film.