Friday, September 17, 2010

RE: [MW:7048] Difference between SCC and SSC

Hi Limesh,

 

Stress Corrosion Cracking (SCC) in an environment  assisted cracking, produced under a combination of Tensile residual stress, corrosive media and temperature. SCC can happen for any material, under an incompatible operating media and operating conditions. In refining environment its’ very common, common examples are amine SCC for C.S/LAS, Chloride SCC for austenitic S.S, I have attached some descriptions of this problem (chloride SCC of austenitic S.S) and SSC:-

 

Chloride Stress Corrosion Cracking

Description of Damage

Surface initiated cracks caused by environmental cracking of 300 Series SS and some nickel base alloys under the combined action of tensile stress, temperature and an aqueous chloride environment. The presence of dissolved oxygen increases propensity for cracking.

Affected Materials

·        All 300 Series SS are highly susceptible.

·        Duplex stainless steels are more resistant.

·        Nickel base alloys are highly resistant.

 

Critical Factors

·        Chloride content, pH, temperature, stress, presence of oxygen and alloy composition are critical factors.

·        Increasing temperatures increase the susceptibility to cracking.

·        Increasing levels of chloride increase the likelihood of cracking.

·        SCC usually occurs at pH values above 2. At lower pH values, uniform corrosion generally predominates. SCC tendency decreases toward the alkaline pH region.

·        Cracking usually occurs at metal temperatures above about 140oF (60oC), although exceptions can be found at lower temperatures.

·        Stress may be applied or residual. Highly stressed or cold worked components, such as expansion bellows, are highly susceptible to cracking.

·        Oxygen dissolved in the water normally accelerates SCC but it is not clear whether there is an oxygen concentration threshold below which chloride SCC is impossible.

·        Nickel content of the alloy has a major affect on resistance. The greatest susceptibility is at a nickel content of 8% to 12%. Alloys with nickel contents above 35% are highly resistant and alloys above 45% are nearly immune.

·        Low-nickel stainless steels, such as the duplex (ferrite-austenite) stainless steels, have improved resistance over the 300 Series SS but are not immune.

·        Carbon steels, low alloy steels and 400 Series SS are not susceptible to Cl-SCC .

 

 

4.5.1.5 Appearance or Morphology of Damage SCC.

 

·        Characteristic stress corrosion cracks have many branches and may be visually detectable by a craze cracked appearance of the surface. Cracks could intergranular or transgranular, depending on the intensity.

·        Fracture surfaces often have a brittle appearance.

Prevention / Mitigation

·        Use resistant materials of construction.

·        When hydrotesting, use low chloride content water and dry out thoroughly and quickly.

·        Avoid designs that allow stagnant regions where chlorides can concentrate or deposit.

·        A high temperature stress relief of 300 Series SS after fabrication may reduce residual stresses.

·        However, consideration should be given to the possible effects of sensitization that may occur,

·        increasing susceptibility to polythionic SCC, possible distortion problems and potential reheat cracking.

 

Inspection and Monitoring

·        Cracking is surface connected and may be detected visually in some cases.

·        PT or phase analysis EC techniques are the preferred methods.

·        Eddy current inspection methods have also been used on condenser tubes as well as piping and pressure vessels.

·        Extremely fine cracks may be difficult to find with PT. Special surface preparation methods, including polishing or high-pressure water blast, may be required in some cases, especially in high pressure services.

·        UT.

·        Often, RT is not sufficiently sensitive to detect cracks except in advanced stages where a significant network of cracks has developed.

 

 

Sulfide Stress Cracking (SSC): is a form of  hydrogen stress cracking .As per API -571 it is classified as Wet H2S Damage ( such as Blistering /HIC /SOHIC /SSC). The description of the problem is as below:-

 

Sulfide Stress  Cracking (SSC)

 

·        Sulfide Stress Cracking (SSC) is defined as cracking of metal under the combined action of tensile stress and corrosion in the presence of water and H2S. SSC is a form of hydrogen stress cracking resulting from absorption of atomic hydrogen that is produced by the sulfide corrosion process on the metal surface.

SSC can initiate on the surface of steels in highly localized zones of high hardness in the weld metal and heat affected zones. Zones of high hardness can sometimes be found in weld cover passes and attachment welds which are not tempered (softened) by subsequent passes. PWHT is beneficial in reducing the hardness and residual stresses that render a steel susceptible to SSC. High strength steels are also susceptible to SSC but these are only used in limited applications in the refining industry. Some carbon steels contain residual elements that form hard areas in the heat affected zones that will not temper at normal stress relieving temperatures. Using preheat helps minimize these hardness problems

 

Affected Materials:-Carbon steel and low alloy steels.

 

 

I hope the above descriptions will clear your doubt.If you have access to API-RP-571Damage Mechanisms Affecting Fixed Equipment in the Refining Industry all the above and thounsands  more problems, causes, mitigation techniques are nicely explained by the industry leaders.

 

You may refer the atatched article on various forms of hydrogen damage in refinning environments, whioch includes SSC.

 

Thanks.

 

Pradip Goswami,

Welding & Metallurgical Engineer/Specialist

Email-pgoswami@sympatico.ca,

pgoswami@quickclic.net


From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of vinod kumar
Sent: Thursday, September 16, 2010 11:12 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:7044] Difference between SCC and SSC

 

Mr Limesh,
SCC - Stress Corrosion Cracking and it is produced by combined action of corrosion an Tensile  stress

SSC - Sulphide Stress Cracking and is produced by the combined action of Tensile Stress an corrosion in the presence of H2S


On Thu, Sep 16, 2010 at 3:04 PM, limesh M <limesh78@gmail.com> wrote:

Dear All,

 

 

What is the difference between SCC and SSC  as per NACE MR 0175? Both are same or different?

 

SCC and SSC test are same or different?

 

 

Regards,

 

Limesh

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Thanks and Best regards

Vinod

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