JABIN MATHEW BENJAMIN13MY04

M.E. INDUSTRIAL METALLURGY

STRESS CORROSION CRACKING

(SCC)

Flixborough explosion, UK : June 1974 

28 fatalities as well as the near complete destruction of the NYPRO plant in north Lincolnshire by blast and then fire

The failure of a bypass assembly introduced into a train of six cyclohexane oxidation reactors after one of the reactors was removed owing to the development of a leak.

Resulting from the presence of nitrates that had contaminated river water being used to cool a leaking flange.

Mild steel exposed to hot nitrate solution and to stresses which in extreme cases may be well below the yield strength, and may remain in the structure from the fabrication procedure or may derive from operating stresses developed intergranular cracks.

WHAT IS SCC?

Failure of metal resulting from joint action of Stress Chemical attack

Initiation and propagation of cracks without any outside evidence of corrosion

Sudden and unpredictable failure: Catastrophic

Requirements for SCC

Susceptible metal

Specific environmentTensile stress

METAL-ENVIRONMENT FOR SCC

Cu alloys in ammoniacal solutionAustenitic Steels in oxide and chloride

environment at high temperature

Carbon steels in hydroxide and nitrate environment

Very high temperature water is aggressive to wide variety of materials

Stresses!!

SCC is caused by Static stresses

Residual : Cold working, welding, heat treatment, machining

Thermal stresses Applied stresses Build-up of corrosion products in confined spaces 

Compressive residual stress Used to prevent or delay onset of SCC

Stress Corrosion Cracking

Crack produced by SCC in an aircraft part

SCC sites

SCC initiates at any pre-existing flaws

Surface discontinuities Grooves, cracks

Corrosion pits

Grain boundaries Intergranular cracking: sensitization of impurities at

grain boundaries making them very reactive

Mechanisms of SCC

Pre existing active path mechanism Suggests that there are pre existing paths that are

susceptible to anodic dissolution For example

A grain boundary precipitate anodic to the grain boundary Segregated impurities that behave anodic.

Area adjacent to depleted grain boundary will be preferentially attacked.

Creates localized galvanic cells, segregate being generally anodic

Predominant : SCC governed by electrochemical factors than stress.

Grain boundary precipitation

E.g.: formation of chromium carbide on the grain boundary and depletion of Cr adjacent to grain boundary in stainless steels.

Precipitation of carbide in Ni alloys and Mg2Si in Al alloys.

Grain boundary segregation

Impurities, such as P, S, C and Si, segregate at the grain boundary and cracking

Mechanisms of SCC

Strain generated active path mechanism Strain is the controlling influence Initially a passivating film is formed on the metal surface

Film is ruptured by plasticstrain, exposing bare metal.

Film forms again andprocess continues

Crack propagates by successive dissolution of crack tip in the process

Emergence of slip steps also considered to cause film rupture

Formed brass product with SCC caused cracks

Inside of Cu pipe showing crack

Intergranular cracks in Cu

PREVENTION

Choosing a material that is not susceptible to SCC in the service environment

Residual stresses can be relieved by stress-relief annealing

Similarly shot-peening or grit-blasting tend to introduce a surface compressive stress.

Neutralizing the environment whenever possible, e.g. reducing chloride ion concentration.

Reference

Zaki Ahmad, Principles of corrosion engineering and corrosion control, Sept 2006, Elsevier science and technology books.

www.corrosion-doctors.org/forms-scc

THANK YOU