Mechanical stresses can increase corrosion rates by creating sites of localized corrosion. Stress corrosion cracking occurs when three factors are present: a susceptible alloy, a corrosive environment, and tensile stress. It can cause sudden failures. Cold work increases corrosion by segregating impurities. Heat treatment can reduce this effect. Hydrogen embrittlement and corrosion fatigue are additional failure modes involving the combined effects of corrosion and stress. Proper material selection, coatings, inhibitors, and heat treatments can mitigate these issues.

1. Influence of Mechanical Factors
and Stress Corrosion
Chapter 12
Instructor:
Dr. Md. Easir Arafat Khan
Associate Professor
Department of Chemical Engineering, BUET,
Dhaka-1000
July 2022
ChE 409

2. Introduction
• Mechanical stress, both internal and external often increase the rate of
deterioration of a metal/alloy in a corroding atmosphere
• Mechanical forces alone will have little, if any, effect on the overall
corrosion of metals/alloys as measured for example in mils per year (mpy)
corrosion penetration
• Conjoint action of stresses and corrosive environment produce far more
damage than when they are acting separately
• The combined effects of stress and corrosion can cause a special type of
failure known as Stress Corrosion Cracking (SCC).

3. Introduction
• The effects of residual and applied stresses and corrosive
environments in service are closely interrelated.
• The more highly stressed (higher energy) regions of a metal
will become anodic, and corrosive cells will emerge from
differences in local stress levels.
• Cold-worked regions, for example, tube or sheet bends and
cut edges, will be corroded in preference to uniform sections
just as grain boundaries are attacked more than grain
interiors on the microscopic scale.
Stress corrosion cracking (SCC)

4. Introduction
• SCC occurs under specific circumstances for a given
alloy: specific alloy condition, plus specific corrosive
media, and sufficient local tensile stress.
• Chloride induced cracking of stainless steels, caustic
cracking of plain carbon steels, and ammonia damage to
copper alloys are typical examples of this problem
• SCC can occur in environments that are only mildly
corrosive to the material. The damaging concentration
of the harmful ions in that environment may be quite
small and difficult to detect.
Schematic view of Stress
Corrosion Cracking (SCC)

5. Cold Working and Corrosion
• Cold worked vs hot worked metal/alloy
• Higher corrosion for cold worked metal/alloy:
– the segregation of atoms of impurities in a metal/alloy (e.g. C and N in iron) at
imperfection sites or defects in crystal lattice sites produced by plastic
deformation
– Anodic sites are produced and the rest of metal specimen acts as cathode
– Thus within the metal body small anodic and large cathodic areas are produced
– in a corrosive environment the anodic areas obviously corrode very quickly
resulting in a corrosion rate much higher than when those impurities were
uniformly distributed (before cold working ) in the metal/alloy

6. Effect of heat treatment of Cold-worked steel
0.076% C

7. Effect of Carbon

8. Stress Corrosion Cracking
• Stress Corrosion Cracking (SCC) refers to cracking caused by
conjoint action of tensile stress and a specific corrosive
environment
• Compressive stresses do not produce stress corrosion
cracking
• During SCC the metal/alloy is virtually unattacked over
most of its surface
• Fine cracks progress through it and cause sudden
equipment failure resulting in severe damage and even
explosions in pressure vessels.

9. When does SSC occurs?
• Stress corrosion usually occurs under only very mildly corrosive conditions. The new
surfaces resulting from a stress corrosion crack may show evidence of corrosion like blue
color in brass, rust on steel, etc. but other surfaces of the metal usually do not appear to be
corroded
• Stress corrosion cracking occurs in metals exposed in an environment where, if the stresses
were nonexistent or even much lower, there would be no damage. On the other hand, if the
structure, subject to the same tensile stresses, were in a different environment, that is, one
that did not contain the specific corrodent or corrodents for that material, there would be
no failure.
• The general appearance of an equipment undergoing stress corrosion cracking may appear
smooth and corrosion product free. However, since cracks have formed due to SCC, sudden
mechanical failure may occur when the crack proceeds right across the cross section of the
metal. This is why SCC is often unpredictable during service life of an equipment.

10. SCC Occurs in Specific Environment

11. SCC Cracks/Fractures
• Stress corrosion cracks in early stages of their development are
microscopic. In many cases the cracks cannot be seen with naked eyes. As
the cracking penetrates further into the material it eventually reduces the
supporting cross section to the point where the structure fails from
overload.
• The SCC cracks can be either intergranular or transgranular.
– The intergranular crack follows the grain boundary in a metal. (Aluminum alloys ;
low carbon steel and brasses).
– In other metals/alloys the cracks do not necessarily follow the grain boundaries
and are transgranular. (Cr-Ni stainless steels). Some times cracks may be mixture
of two.

12. Characteristics of SCC
1. Stress corrosion occurs only under specific chemical environments.
2. Pure metals are generally immune to SCC. Alloys (e.g., Cu-Zn, Cu-Au, Mg-
Al) are susceptible.
3. Cathodic polarization has been successfully used to avoid the initiation of
SCC.
4. Some extraneous anions added to specific environments inhibit SCC.
5. Metallurgical structure of metals and alloys have significant effect.

13. Mechanism of SCC: Electrochemical Theory

14. Flixborough Disaster 1974

15. Mechanism of SCC: Stress Sorption Cracking
• Given the specificity of SCC, Uhlig proposed that such failure proceeds not by
electrochemical dissolution of metal, but by weakening of the cohesive bonds
between surface metal atoms.
• The cohesive bonds are weakened due to adsorption (chemisorption) of
damaging components of the environment
• Uhlig suggested the name for this mechanism as “stress-sorption cracking“
• Chemisorption is specific, corroding components for SCC are also specific
• Chemisorption reduces surface energy which helps crack propagation

16. One Example of SCC
• Low carbon steels: Riveted steam boilers

17. SCC – Remedial measures
• Severe cold working: Cold rolling to >50% reduction of thickness is found to
impart relative immunity to a stressed mild steel in boiling nitrate solution
• Heat treatment at higher temperature provide SSC resistance.
• Surface peening or shot blasting: compressive stresses produced at surface
of the metal are effective
• Cathodic protection
• Special alloys: steel containing small amount of aluminum, titanium
exhibits improved resistant to SSC.
• Use of inhibitors: Sodium nitrate inhibits SSC for steel exposed to boiling
waters

18. SCC – Remedial measures
• Severe cold working: Cold rolling to >50% reduction of
thickness is found to impart relative immunity to a stressed
mild steel in boiling nitrate solution
• Heat treatment
• Surface peening or shot blasting
• Cathodic protection
• Special alloys

19. Corrosion Fatigue
• Metallic material fails due to conjoint action of repeated cyclic
stresses and a corrosive environment
• Like SCC, the total damage due to corrosion fatigue is greater
than the sum of mechanical and corrosive components if each
were acting separately
• Not environment specific as SCC

21. S-N curve
There is a stress limit below which failure does not occurs

22. Dry fatigue vs Corrosion fatigue
• Endurance limit or fatigue limit for dry fatigue; for corrosion fatigue
there is no such endurance limit
• Dry fatigue gives single cracks, usually transgranular, whereas,
corrosion fatigue cracks occur in “families” and grow in width as
the process extends. They are also transgranular.
• Corrosion fatigue cracks in carbon steels often propagate generally
from the base of corrosion pits.

24. Mechanism: Dry and Corrosion Fatigue

25. Observations
1. There is no relation between corrosion fatigue strength and
tensile strength.
2. Medium-alloy steels have only slightly higher corrosion fatigue
strength than ordinary carbon steels.
3. Heat treatment does not improve corrosion fatigue strength of
either carbon or medium alloy steels
4. Corrosion-resistant steels, particularly steels containing
chromium, have higher corrosion fatigue strength than other
steels.
5. Corrosion fatigue strength of all steels is lower in salt water
than in fresh water.

26. Remedial Measures
• De-aeration: For mild steel, deaeration of a saline solution restores the normal fatigue limit in air
• Cathodic Protection
• Use of inhibitors: Addition of 200 ppm Na2Cr2O4 to tap water was found to reduce corrosion fatigue
of normalized carbon steel.
• Sacrificial coatings: Zinc or cadmium deposited on steel are effective
• Electro-deposits of Tin, lead, copper or silver on steel are effective
• Organic coatings is useful if they contain inhibiting pigments (e.g. ZnCrO4)
• Shot peening or introducing compressive stresses are beneficial

27. Hydrogen Cracking
• With a tensile stress or stress-intensity factor exceeding a specific threshold, the
atomic hydrogen interacts with the metal to induce subcritical crack growth leading
to fracture.
• In the absence of a corrosion reaction (polarized cathodically), the usual term used is
hydrogen-assisted cracking (HAC) or hydrogen stress cracking (HSC).

28. How Hydrogen Gets In
• Hydrogen, in atomic form, will enter and diffuse
through a metal surface whether at elevated
temperatures or ambient temperature.
• Once absorbed, dissolved hydrogen may be present
either as atomic or molecular hydrogen or in
combined molecular form (e.g., methane).
• Since these molecules are too large to diffuse through
the metal, pressure builds at crystallographic defects
(dislocations and vacancies) or discontinuities (voids
inclusion/matrix interfaces) causing minute cracks to
form.
• A metallurgical interaction occurs between atomic hydrogen and the crystallographic structure,
the ability of the material to deform or stretch under load is inhibited. Therefore, it becomes
“brittle” under stress or load.
• As a result, the metal will break or fracture at a much lower load or stress than anticipated.

29. Nature & Effect of Hydrogen Attack
Problem: Internal cracking or blistering.
Solutions:
• Use of steel with low levels of impurities (i.e., sulfur and phosphorus);
• modifying the environment to reduce hydrogen charging; use of surface coatings
and effective inhibitors.
Problem: Loss of ductility.
Solutions:
• Use of lower strength (hardness) or high resistance alloys;
• careful selection of materials of construction and plating systems;
• heat treatment (bakeout) to remove absorbed hydrogen.
Problem: High temperature hydrogen attack.
Solutions:
• Selection of material (for steels, use of low and high alloy Cr-Mo steels, selected
Cu alloys, nonferrous alloys);
• limit temperature and partial pressure H2.

30. Remedial measures
• Use clean steel with no void in the lattice
• Coatings or liners impervious to hydrogen penetrations
• Use of appropriate Inhibitors
• Alloy substitution: steel with nickel content or nickel alloys are often used to prevent
hydrogen cracking
• Removal of hydrogen evolution poisons; i.e. sulfide, cyanides which favors hydrogen
evolution.

31. Fretting Corrosion
• Occurs if:
– the interface is under load
– there is vibration or repeated relative motion between the
surfaces
– the load and the relative motion should be sufficient to cause
slip or deformation on the surfaces.

32. Wear oxidation and Oxidation wear

33. Remedial measures
• Lubrication at the interface
• Increase hardness of one or both metals
• Use gaskets
• Increase load to reduce slip between the surfaces