Crevice corrosion is a result of changes in the chemical balance and elements within a crevice. This could be caused by a change in conditions, buildup of chemicals, or a depletion of oxygen in the crevice. Common conditions occur when sand, mud, moisture, and salt accumulate in the surface of the crevice. This causes a depletion of oxygen and creates an electrochemical gradient between the fluids in the crevice and the surrounding metal. The result is an initiation site for crevice corrosion. These sites are apparent in stagnant environments and formed under gaskets, insufficient welds, insulation material, fastener heads, threads, clamps, jaw marks, or anywhere on the material where changes in local chemistry have occurred.
Chrome alloys, duplex stainless steels, austenitic stainless steels, and nickel-based alloys used for production in the oil and gas industry undergo many manufacturing and processing steps. These manufacturing processes occur prior to pipe joints and accessory equipment being mated or made-up via threaded ends and run downhole through a drilled petroleum or gas well for production. Certain manufacturing processes can create initiation sites for crevice corrosion.
Certain nondestructive testing (NDT) methods should be performed on material entering service to evaluate potential crevice corrosion initiation sites. This article focuses on crevice corrosion sites in the form of jaw marks produced during make-up of threaded pipe, inspection and evaluation of these initiation sites, and how to prevent them from acting as corrosion sites.
During make-up of a pipe string in which joints are mated from field end to coupling box end, mechanical jaws are used to grip the two joints of the material being mated. Usually bladders, “non-marking” jaws, or slip-resistant paper is used when mating joints to prevent the mechanical jaws from digging into the pipe and creating gouge marks. However, in some cases when torque requirements are too high or the pipe is not able to be gripped without slipping, these protective methods are not able to be utilized. This can cause the mechanical jaws to dig into the pipe and break the surface (Figure 1). Surface-breaking gouges can act as reservoirs for crevice corrosion if deeper than the allowable criteria of the manufacturer’s specification if not remediated.
The American Petroleum Institute’s specifications 5CRA and 5CT specify the technical delivery conditions for corrosion-resistant alloy seamless tubulars for casing, tubing, and coupling stock, as well as technical specifications for steel casing and tubing pipes used for oil wells in petroleum and natural gas industries (API 5CRA and API 5CT). These specifications do not allow any surface-breaking imperfection to be beyond 5% in depth of the nominal body wall of the pipe. However, if these gouges are remediated by light buffing, the remaining body wall must be verified to be equal to or greater than 90% of nominal wall thickness for cold-worked alloys or 87.5% for quench and tempered alloys.
Visual, dimensional, and ultrasonic (UT) inspection are the most efficient NDT methods for identifying these initiation sites created by jaw marks. The minimum lighting requirement to detect these imperfections should be 500 lux (50 footcandles) on the inspection surface as required by API 5CRA and 5CT. The inspector should visually evaluate the gouges to ensure that there is no lapped over metal around the edges of the gouge where the jaws penetrated the surface that could cause the pit or thread height gauge (Figure 2) to give an inaccurate depth reading. If there is metal protruding from the flat surface of the material around the edges of the gouge, these should be filed down as best as possible by the inspector to ensure the pit or thread height gauge can measure the depth on a flat surface. A pit or thread height gauge used to measure the depth of the gouges should be zeroed out against a flat calibration block prior to obtaining any depth readings to ensure accuracy.
The ultrasonic thickness device for this type of inspection could be a digital A-scan device with a dual element transducer. The device will be used to verify the remaining body wall of the material after remediation by buffing of the gouges. The frequency and diameter of the transducer will depend on the surface area of the material being inspected and the attenuation of the sound path within the material. In the case of the material inspected in this article, a dual element transducer with a probe diameter of 10 mm and a frequency of 5 MHz is preferred. The ultrasonic thickness device should be standardized to the actual material being tested so that the sound speed and attenuation on the ultrasonic thickness device are correct for the type of material being inspected in order to receive the most accurate thickness readings from the device. In order to do this, a mechanical caliper or device should be used to obtain the actual wall thickness of the part so that the UT device can be standardized to that known thickness. The visual, dimensional, and UT inspections shall be completed on the product prior to any coating, blasting, or painting to the surface so that the inspection results are correctly evaluated.
If jaw marks are visually and dimensionally inspected using a pit or thread height gauge and found to be deeper than the allowable criteria, then these jaw marks can be polished out with a light buffing wheel. The buffed area on pipe, couplings, and parts should be blended smoothly to avoid sharp edges. If buffing is done, the remaining body wall on the pipe or part shall conform to the minimum allowable tolerance in the API or customer’s product specification. The remaining body wall should be inspected via a UT thickness gauge to determine that the minimum wall thickness meets the product specification. Post remediation visual inspection should be conducted to view any possible tearing that may have occurred from jaw marks. If surface tearing is seen in the buffed area, surface liquid penetrant or magnetic particle testing may need to be conducted if required by internal or customer specifications.
In summary, during make-up of a pipe string in which joints are mated from field end to coupling box end, the mechanical jaws used to grip the two joints of material being mated can create gouge marks. These surface-breaking gouges can act as reservoirs for crevice corrosion if deeper than the allowable criteria and if not evaluated or remediated using the correct NDT methods discussed in this article.
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Justin Bouis, Global Operational Excellence Manager at Corrosion Resistant Alloys, (https://www.cralloys.com/), Houston, TX, JBouis@CRALLOYS.COM.
References
API, 2010, API SPEC 5CRA: Specification for Corrosion-resistant Alloy Seamless Tubes for Use as Casing, Tubing, and Coupling Stock, first edition, American Petroleum Institute, Washington, DC.
API, 2018, API SPEC 5CT: Specification for Casing and Tubing, tenth edition, American Petroleum Institute, Washington, DC.
ASNT, 2020, Recommended Practice No. SNT-TC-1A: Personnel Qualification and Certification in Nondestructive Testing, 2016 edition, The American Society for Nondestructive Testing, Columbus, Ohio.
ASNT, 2015, Level II Study Guide: Visual Testing Method, American Society for Nondestructive Testing, Columbus, Ohio.
ASNT, 2018, Relevant Discontinuities: Visual Testing, American Society for Nondestructive Testing, Columbus, Ohio.
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Photos courtesy of Justin Bouis.
Edited to replace Figure 1 photos, 31 January 2022.
lesson learned : Any and all precautions are helpful in avoiding mechanical damage and potential corrosion and improve safety and reduce risk