SCC

Prepared By- Pranav Sharma

SCC• Stress-corrosion cracking (SCC) is a cracking phenomenon that occurs in

susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment.

Types

Factors

Environmental Factor

• Temperature• pH• Electrochemical potential• Solute species• Solute concentration• Oxygen concentration

Special Agents

Microrganisms• Temperature is important factor affecting microbial growth.Microbial life is

possible within the range of 21 to 230°F (–5 to 110°C). Microorganisms are also classified as to the temperature range in which they thrive, as in the following table:

Contd..

• The methods by which microorganisms increase the rate of corrosion of metals and/or their susceptibility to localized corrosion in an aqueous environment are:

1. Production of metabolites. Bacteria may produce organic acids, inorganic acids, sulfides, and ammonia, all of which may be corrosive to metallic materials.2. Destruction of protective layers. Organic coatings may be attacked by various microorganisms, leading to the corrosion of the underlying metal.3. Hydrogen embrittlement. By acting as a source of hydrogen and/or through the production of hydrogen sulfide, microorganisms may influence hydrogen embrittlement of metals.4. Formation of concentration cells at the metal surface and, in particular, oxygen concentration cells. A concentration cell may be formed when a biofilm or bacterial growth develops heterogeneously on the metal surface. Some bacteria may tend to trap heavy metals such as copper and cadmium within the extracellular polymeric substance, causing the formation of ionic concentration cells. These lead to localized corrosion.

Contd..

5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite corrosion inhibitors used to protect aluminum and aluminum alloys from nitrate and ammonia.6. Stimulation of electrochemical reactors. An example of this type is the evolution of cathodic hydrogen from microbially produced hydrogen sulfide.

Mechanical Factors• Threshold stresses and stress-intensity factors, the presence of a stress-

independent crack-growth regime, and the dependence of cracking to strain rate are important features in determining the susceptibility of alloys to SCC.

• The stress-intensity factor, K, is a parameter that describes the relationship between the applied stress and crack length for specific specimen geometries

• Figure shows the stress-intensity factor, K, as a function of the crack propagation rate, da/dt. The threshold is defined in this figure by the minimum detectable crack growth rate. The threshold stress intensity is generally associated with the development of a plastic zone at the crack tip

• The slow-strain-rate technique provides an excellent way to determine the susceptibility of an alloy to SCC (Ref 13, 14). However, the strain-rate behavior strongly depends on the alloy/environment combination.

• For example, for most materials, the critical strain rate, at which the maximum susceptibility is obtained, is 10–6/s. This critical strain rate points to a cracking mechanism whereby the rate of anodic dissolution is equal to the rate of protective film formation. If a higher strain rate is applied, the mechanical fracture will be more rapid than the rate of anodic dissolution

• The fracture toughness of most materials is determined by a parameter called ‘stress intensity factor,’ Ki. It is a measure of the concentration of stresses at the tip of the crack. It is given by

Fracture Role

Zones of Transition

Metallurgical Factor• Strong influence of alloy composition and microstructure on the susceptibility to

SCC is given by austenitic stainless steels, where chromium and molybdenum promote the formation of passive films on the surface. Trace elements such as carbon at concentrations greater than 0.03 wt%, may cause sensitization by forming chromium carbides at the grain boundaries and depleting zones around the carbides of chromium, thereby rendering the steel susceptible to intergranular SCC (IGSCC). Austenitic stainless steels will fail transgranularly in high-temperature chloride solutions.

Mechanism

Mechanism• The mechanism of further development and growth depends on the combination

of material and environment. The cracks may be transgranular, intergranular or a mixture of these. In particular, two mechanisms seem to dominate transgranular crack development and growth:

1.Accelerated anodic dissolution at the crack tip where some material is subject to continual plastic deformation.

2. Hydrogen induced or hydrogen-assisted crack formation (often denoted hydrogen embrittlement). In addition, a mechanism involving stepwise crack growth due to fracture of a very thin corrosion product film has been proposed.

Contd..• For some material–environment combinations it has been shown that accelerated

anodic dissolution of yielding metal is the significant mechanism. Example-• austenitic stainless steels in acidic chloride solutions. In these steels, plastic

deformation is characterized by a dislocation pattern giving wide slip steps on the surface.

• Scully has proposed a model for initiation and development of stress corrosion cracks, which has been supported by other scientists

• The model in its simplest form is illustrated in Figure A necessary condition is that the surface from the beginning is covered by a passivating film (A).

Contd..• Plastic deformation gives a slip step with a surface that is metallically clean and very

active (B). • The further development depends on the repassivation rate: a) In an intermediate

range of this rate, most of the new surface is passivated before significant corrosion occurs, and the attack is concentrated on a narrow region where the dislocation density is highest (C).

• This gives initiation and growth of cracks. b) If the repassivation rate is higher, the critical region is passivated before significant corrosion occurs. c) If the repassivation rate is low, corrosion is spread over a larger part of the new surface, which will result in a kind of pit

• The crack growth rate is controlled by the rate of dissolution at the crack tip, then the following equation can be used to estimate the crack growth rate.

Mechanism

METALLURGIACL MECHANISM-Dislocation Coplanarity

-Stress Aging And Microsegregation

-Adsorption

DISOLUTION MECHANISM-Stress accltd Disolution

-Film formation at cracking wall

- Noble Elment enrichment

- Film Rupture

- Chloride ion migration

Hydrogen Mechanism

Mechanical Mechanism

Metallurgical Mechanism

Dislocation Coplanarity- Resistance to cracking on accnt of disloctions.Pattern of dislocations in SS are planar arrays where in other alloys its tangeled or cellular.

Stress Aging and Microsegregation- Microsegregation of solute atoms to dynamic effects in crystal structure. Results in Transgranular SCC.Crack propagation is limited to solute diffusion rate and electrochemical polarisation.

Adsorption-Surface Active species adsorb and interact with strained bonds at the crack tip, causing reduction in bond strength and leading to crack propagation.

Dissolution Mechanism

Other Mechanism

Hydrogen Embrittlement

Hydrogen Embrittlement

Prevention

• Substitution Of alloys By using Ni and Mo Because of there low hydrogen diffusion rates

• Coating

General Conditions• Alloys that are normally very passive often require an agent in the environment

that can promote the breakdown of that passivity. An example is exposure of 316ss to chloride-containing fluids with dissolved oxygen

• Alloys that are normally very active often require an agent in the environment that can promote the formation of passivity. An example is exposure of carbon steel to more concentrated hydroxide-containing or nitrate-containing fluids

• The open circuit potential is often required to lie within a certain range that is determined by the alloy-environment interaction. Determining that potential range can be difficult. For example, for some alloys-environment combinations the potential has to be in the region of hydrogen evolution, the absorption of which by the alloy leads to modification of the structure and increased internal stress. In other alloy-environment systems the potential needs to be above that potential which causes localized corrosion to initiate.

Open Potential Condtn.

• The measured threshold stress is a stress above which stress corrosion cracking will usually occur. This stress is a function of the alloy and environment. It is not a property of the alloy. Cracking has been found at stress levels below the threshold stress

• Alloy constituents can influence initiation of stress corrosion cracking. Specialized heat treatment (tempering) can eliminate the problem. This characteristic is especially true of aluminum alloys.

• Temperature is an important variable. If stress corrosion cracking is a possibility, often the higher the temperature the greater the risk. But the threshold temperature at which stress corrosion cracking occurs depends on alloy and environment.

• Stress corrosion cracking usually occurs on materials that exhibit low corrosion rates, i.e. are normally passive, in the environment. The implication is that in many, but not all cases a passive surface layer, e.g. chromium-oxide rich surface region on stainless steel, is required for stress corrosion cracking. An exception to that rule is stress corrosion cracking of copper in certain non-oxidizing environments

Prevention

Contd..

Inhibitors