Stress Corrosion Cracking (SCC) (Source: http://www.slideshare.net/SHIVAJICHOUDHURY/damage-mechanism-in-boiler-thermal-power-plant)
The Rot From Within
On June 1, 1974, Britain experienced her worst peacetime explosion. This was the infamous Flixborough Chemical Reactor Explosion that claimed 28 lives and grievously wounded 36 while damaging 1,821 houses and 167 shops and factories.
In the words of a contemporary process safety campaigner, ‘the shock wave rattled the confidence of every chemical engineer in the country’. Fortunately, the reactor exploded outside office hours.
Investigations blamed the disaster on the failure of the bypass assembly introduced after one of the six reactors was withdrawn for repairs. And the reactor was removed because it had developed stress corrosion cracks, a result of an ill-informed decision to spray nitride-rich river-water on the reactor for cooling.
Although SCC is rare and its consequences are seldom so horrific, the victims of SCC include several power stations, boilers, oil refineries, and high-pressure pipelines. We can ignore this hazard only to our peril.
Basics of SCC
Composites and polymers have replaced metals in numerous but not all applications. Metals remain the material of choice in umpteen applications because of their toughness, strength, high-temperature tolerance, and stiffness.
Stress Corrosion Cracking (SCC) is the progressive cracking of metals and alloys caused by the combined effect of:
- SCC-Prone Microstructure
- Tensile Stresses High Enough to Induce SCC: sources of stress:
- service conditions
- residual i.e. introduced in the material during the manufacturing stage
- SCC-Facilitating Environs: chloride, hydroxide, carbonate, bicarbonate, ammonia, aerated water, acetate, thiosulphate, phosphate, polythionate, and methanol
Temperature and Time exacerbate the effect of all the above causes. Acting alone, stresses or corrosive ambiences cannot induce SCC. SCC-infested metals demonstrate trans-granular or inter-granular cracks.
What makes SCC such a dreaded phenomenon is its unpredictability. The affected material may fail without displaying any symptoms of an impending failure. Cracks penetrate deep into the material while the surface remains unaffected and the material fails with meager loss of material.
Furthermore, experimental data on SCC is exceptionally scattered preventing the derivation of useful conclusions. SCC-infested material can fail in hours after exposure or it can operate successfully for years. Many metal structures and nearly all alloys are SCC-prone under certain conditions.
Sources of residual stresses:
- Welding activates high levels of residual stresses that are nearly equal to the material’s yield strength
Heat Affected Zones (HAZs) of welding are most likely to hold residual stresses because their microstructure differs from that of the parent material
- Corrosion Products
- Cold Deformation
- Heat Treatment
Cracks form only after stress exceeds the threshold value. Deformation, welding defects, and minor design features such as acute section changes, notches, and corrosion pits push the magnitude of local stresses above the threshold value. Environmental and material factors influence the value of threshold stress intensity. Use its values with caution.
At stress values below the threshold stress intensity, cracks do not propagate. The crack propagation velocity jumps suddenly when stresses exceed the threshold stress intensity. Thereafter this velocity increases at snail’s pace but picks up again as the stress nears the critical stress intensity for fast fracture.
Mechanisms & Vulnerable Systems
SCC-causing mechanisms include:
- Hydrogen Embrittlement
- Active Path Dissolution
- Film-Induced Cleavage
Hydrogen Embrittlement starts when hydrogen dissolves in metals and alloys and makes them brittle by causing loss of ductility. And hydrogen dissolves easily in metals because it is a small atom that fits-in conveniently between the atoms of numerous metals-alloys and diffuses rapidly.
Points of triaxial stress concentration viz. cracks and notches attract hydrogen and hydrogen then expands these cracks and notches. Austenitic stainless steels are better at resisting hydrogen embrittlement than ferritic iron because their face centric cubic (FCC) structure slows down hydrogen diffusion.
In active path dissolution, the material cracks along the most corrosion-prone path inside the material. Normally, this path is the grain boundary where impurities are highly concentrated. The grain boundary erodes while the surface remains intact. This prevents early detection of SCC.
Tensile stresses do not initiate active path dissolution, but propagate it. Cracks spread at the rate at which materials corrode at the crack tip. This rate peaks at 10µm/s (micrometer-per-second). Average corrosion rates are below 10-5 µm/s (1mm in 3 years).
Film-Induced Cleavage produces trans-granular cracks in ductile materials coated with a brittle film. Ductile Blunting stops the spread of such cracks beyond 1µm inside the surface. If however corrosion has formed this brittle film, cracks form again at the blunted crack tip and the cycle continues.
Hydrogen embrittlement requires only a hydrogen source and a vulnerable material to set in. And, it is least dependent on the environment. Other SCC mechanisms start only with specific reactions at the surface and crack walls and another set of reactions at the crack-tip. This makes hydrogen embrittlement more common. It also makes the other forms of SCC more unpredictable because only a minor change in surroundings triggers them.
Specific SCC-prone systems:
- Chloride Cracking of Austenitic Stainless-Steel: in hot (over 700C), chloride-containing solutions. Residual stresses from fabrication and welding induce such cracking that is normally trans-granular. Sensitized austenitic stainless-steel exhibits inter-granular cracks
The nuclear industry uses austenitic steel at the said conditions. Researchers believe chloride SCC begins in the chromium carbide deposits along grain boundaries. Using low carbon steels and limiting the levels of oxygen and chloride ions in the environment prevents chloride SCC
Low chloride concentration is sufficient to induce trans-granular cracks at elevated temperatures while highly acidic solutions cause such cracking even at low temperatures
Sulphides add to the SCC-causing ability of chlorides. Free of residual stresses, austenitic steels of type AISI 316L, 317L, and high alloy 904L are SCC-resistant in chloride-rich desalination plants
- Hydrogen Embrittlement of High Strength Steels (HSS) at Static Loads: all steels are prone to hydrogen embrittlement but only HSS with yield strength above 600MPa are affected at static load
Welding, electroplating, corrosion, pickling, and exposure to gases containing hydrogen introduce hydrogen in steels. Baking HSS at 2000C lowers this susceptibility as it releases some hydrogen. Baking also relocates some to points where it is not so corrosive
- Low-Alloy and Carbon Steels in Passivating Environs: i.e. ambiences that tend to form passive, protective films over these steels. Such environs include caustic solutions, hot water, nitrates, phosphates, and carbonates. This phenomenon endangers gas pipelines
- Hydrogen Embrittlement in Aluminum Alloys: causes inter-granular cracks. Salt solutions and humid air introduce hydrogen. Aluminum’s FCC-microstructure slows down crack propagation. Heat treatment improves aluminum’s crack resistance
- Brass in Ammonia Environs develops inter-granular cracks
Electrochemical potential influences SCC of alloys. HSS steels with more negative potential are more prone to hydrogen embrittlement. However, these steels are also more vulnerable when their potential exceeds the normal, free positive corrosion potential.
The other two SCC mechanisms occur in a restricted electrode potential range. Practically, oxygen in the environment determines the electrode potential and the SCC-vulnerability of the environs.
- High strength alloys are more prone to hydrogen embrittlement
The necessity to make ships stronger, lighter, and more fuel-efficient has led to the increasing use of high strength alloys in shipbuilding. SCC-induced failures in the marine and offshore sector can cause toxic oil-gas leakages that are destructive for the environment, safety, and economy
Duplex Stainless Steels include all steels with a two-phase structure viz. delta ferrite and austenitic. They offer better SCC resistance and strength vis-à-vis austenitic steels and hence are widely used in the offshore oil-gas sector
- Low strength carbon steels are more liable to caustic cracking
- Changes in the composition of alloying elements affect the sensitivity of the alloy to different forms of SCC
While the addition of molybdenum hikes the resistance of low-alloy steel to carbonate-bicarbonate cracking, it lowers its defiance against caustic cracking
- Heat treatments similarly alter the inclination of alloys to various SCC mechanisms. Materials resistant to SCC in one particular ambience may become SCC-prone when heat treated to a different microstructure
Tests & Preventive Measures
Standard Tests check the SCC-vulnerability of materials in conditions known to promote SCC in that particular material. The Constant Extension Rate Test / Slow Strain Rate Test is a rigorous test. If a specimen does not fail here, it will most probably not fail in practice. The test slowly extends the specimen and measures:
- extension at failure
- time taken for failure
- appearance of fracture surface
Often, the vulnerability to SCC becomes obvious only after the equipment has gone operational. Risk-Based Assessment studies SCC in components and ranks them in decreasing order of the probability of failure and the consequences of failure. Those with high probability and high consequence are immediately replaced.
You can control SCC at the design stage through appropriate:
- Material Selection: choose a material least liable to SCC under the expected service conditions
However, you cannot choose low-strength material because they are more SCC-resistant in applications requiring high-strength materials. Certain environments such as hot water will cause SCC no matter what. And SCC-resistant material will be inevitably expensive
- Stress Restriction: limit stresses to below threshold levels. You can only restrict operational stresses when they are way below the threshold stress
Annealing relieves stress in carbon steels that possess high threshold stresses for most environments. The method cannot be used for austenitic steels that have low threshold stress for chloride environments
You can use annealing to partially relieve stress around welds and other critical areas if stress-relief annealing of the entire structure is not possible. Do this under expert guidance so as not to create new stresses
Mechanical procedures such as hydrostatic testing beyond yield strength distribute stresses uniformly and trim down maximum residual stresses. Other practices include grit-blasting and shot-peening. These create a useful surface compressive layer. Apply these uniformly lest they be counterproductive
- Environment Control: includes removing or replacing the SCC-promoting component. This approach is difficult because we have very little control over specific elements in the environment
And replacing / removing the causative element is unviable if that element is a necessary part of the operation. In this case we can:
- Modify the Metal’s Electrode Potential: carbon steel has lower electrode potential than stainless steel
- Use Inhibitors: specific to an alloy system to slash the corrosion rate. Choose them correctly, for inhibitors that restrict general corrosion may accelerate SCC
- Apply Coatings: of cadmium, zinc, or polymers. Damaged coatings promote SCC and you must factor-in this possibility before applying them. Electrochemical techniques that control other forms of corrosion can promote SCC
Zinc is more anodic than steel and therefore protects the underlying steel from corrosion even if the protective layer is damaged. But this low electrode potential facilitates hydrogen generation and thereby hydrogen embrittlement. Cadmium comes with a low hydrogen embrittlement risk but is toxic
Polymers offer greater electrical resistance to the underlying material i.e. they restrict the flow of SCC-promoting current. Pairing paints with inhibitors gives best results because oxygen and water penetrate inside most coatings
- Design to Minimize Stress: prevent crevices and other slots where chlorides, hydroxides, and oxygen can accumulate. SCC cracks often form at corrosion pits
- Laser Peening has improved SCC-resistance of materials
Despite research, SCC remains a little understood phenomenon. As yet, we are miles away from the stage when we can initiate concrete countermeasures with impunity.
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