NACE TM0284 Hydrogen-Induced Cracking (HIC) Testing: Standard Evaluation of Steels for Sour Service

NACE TM0284 is the internationally recognized standard test method for evaluating the resistance of pipeline and pressure vessel steels to hydrogen-induced cracking (HIC) in wet hydrogen sulfide (sour) environments. The test exposes machined steel coupons to H2S-saturated solutions for 96 hours and quantifies cracking susceptibility through the crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR). This article explains the specimen requirements, test procedures, evaluation criteria, and industrial applications of NACE TM0284 HIC testing.

Technical Overview

NACE TM0284, "Standard Test Method: Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking," is the globally accepted laboratory procedure for assessing the susceptibility of carbon and low-alloy steels to hydrogen-induced cracking. The standard, originally published by NACE International and now maintained by AMPP (the association formed by the merger of NACE and SSPC), is widely specified in combination with NACE MR0175/ISO 15156 for materials intended for sour oil and gas service.

Hydrogen-induced cracking, also known as stepwise cracking, is a degradation mechanism that occurs without any applied external stress. When steel is exposed to an aqueous environment containing hydrogen sulfide (H2S), the corrosion reaction generates atomic hydrogen at the metal surface. H2S inhibits the recombination of atomic hydrogen into harmless molecular gas, causing a significant fraction of hydrogen atoms to diffuse into the steel. These atoms become trapped at metallurgical discontinuities such as elongated manganese sulfide inclusions, centerline segregation zones, and other planar defects, where they recombine into molecular hydrogen and build internal pressure. This pressure initiates small cracks that can link together in a characteristic stepwise pattern, potentially resulting in through-wall failure of pipelines and pressure equipment.

• The test evaluates the base metal resistance of plates and pipes used in wet H2S service.
• It is a laboratory screening method, not a simulation of full-scale service conditions.
• Results are expressed as three quantifiable cracking ratios used for material qualification and acceptance.
• The standard is typically applied together with NACE TM0177, which covers sulfide stress cracking (SSC) of steels under tensile stress.

Testing Samples

Standard Coupon Dimensions

The standard HIC test coupon is a rectangular specimen measuring 100 mm in length and 20 mm in width. The thickness is normally the full product wall thickness; however, when the wall thickness exceeds 30 mm, the specimen is machined down to a maximum of 30 mm by removing material from one surface. A complete test normally requires three coupons cut from the same material sample, and the results are averaged across all evaluated sections.

Sampling Locations and Orientation

Coupons must be representative of the product being qualified. For line pipe, specimens are taken from the pipe body at locations away from the longitudinal weld. For plate intended for pressure vessels, sampling is performed at the prescribed positions across the plate width to capture regions of potential centerline segregation. When required by purchase specifications, additional coupons can be taken to include weld metal and heat-affected zones for evaluation of welded joints.

Surface Preparation and Identification

Prior to testing, the coupon surfaces are machined or ground to remove scale, oxides, and any decarburized or work-hardened layer introduced during cutting. Specimens are degreased and cleaned with suitable solvents, rinsed, and dried. Identification markings are applied only in areas that will not interfere with subsequent sectioning and metallographic examination. All coupon dimensions are measured and recorded before exposure.

Testing Items

The primary output of a NACE TM0284 test is a set of three cracking ratios determined by metallographic measurement of the exposed coupons. Each coupon is sectioned after testing, and the cracks observed on each polished cross section are measured.

• Crack Length Ratio (CLR): The sum of the individual crack lengths divided by the section width, multiplied by 100. CLR reflects the extent of cracking propagating along the specimen width and is the most widely cited acceptance parameter.
• Crack Thickness Ratio (CTR): The sum of the individual crack widths (through-thickness dimensions) divided by the section thickness, multiplied by 100. CTR indicates how deeply cracks extend through the wall.
• Crack Sensitivity Ratio (CSR): The sum of the products of each crack length and width divided by the product of section width and thickness, multiplied by 100. CSR represents the effective cracked area fraction of the cross section.

Supplementary recorded items include the test solution type, initial and final solution pH, test temperature, exposure duration, visual observations of blistering, and photomicrographs of representative crack profiles. All ratios are reported as the average of the sections examined across the full set of coupons, typically nine sections from three coupons.

Testing Methods

Test Solutions

NACE TM0284 defines two standardized test solutions, and the choice between them depends on the intended service environment and the material purchase agreement.

• Solution A: An acidic brine containing 5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid in distilled or deionized water, saturated with H2S. This solution is identical to Solution A of NACE TM0177 and represents an aggressively sour, low-pH environment.
• Solution B: Synthetic seawater prepared in accordance with ASTM D1141 and saturated with H2S. This solution represents near-neutral sour service conditions, such as those encountered in offshore applications where steel contacts H2S-containing seawater or produced water.

Exposure Procedure

The cleaned coupons are placed in a test vessel made of glass or other inert material, with the solution volume maintained at an adequate ratio to the total specimen surface area as required by the standard. The test is conducted at 25°C plus or minus 3°C. The solution is first deoxygenated by purging with an inert gas such as nitrogen, after which H2S gas is bubbled through the solution for the prescribed initial period to achieve saturation. H2S saturation is maintained throughout the 96-hour exposure. The solution pH is measured and recorded at the beginning and end of the test, and the values must remain within the limits specified for the chosen solution.

Post-Exposure Evaluation

After the 96-hour exposure, the coupons are removed, cleaned, and visually examined for surface blistering. Each coupon is then sectioned transversely at the prescribed locations, typically producing three evaluation sections per specimen. The sections are mounted, ground, and polished using standard metallographic techniques. The polished surfaces are examined microscopically at 100x magnification, and the length and thickness of every crack exceeding the minimum measurable size are recorded. From these measurements, the CLR, CTR, and CSR values are calculated for each section and averaged for reporting.

Safety Requirements

Hydrogen sulfide is a highly toxic and flammable gas. Testing must be performed in ventilated enclosures or fume hoods equipped with continuous H2S detection, and personnel must be trained in gas handling, emergency response, and the use of appropriate personal protective equipment. All testing should comply with local occupational health and safety regulations.

Applications

NACE TM0284 HIC testing is a core qualification requirement for metallic materials used in wet H2S environments across the petroleum, petrochemical, and gas processing industries. It provides objective, quantitative evidence of a steel's resistance to stepwise cracking and supports compliance with NACE MR0175/ISO 15156 material selection requirements.

• Qualification of line pipe for sour service pipelines carrying wet sour gas and crude oil.
• Acceptance testing of carbon steel plates for pressure vessels, separators, and heat exchangers in refineries and gas plants.
• Material qualification for offshore platforms, flowlines, and subsea equipment exposed to H2S-containing produced fluids.
• Quality control for steel manufacturers producing HIC-resistant grades through sulfur control, calcium treatment, and clean steel practices.
• Research and development programs evaluating the effects of inclusions, microstructure, and centerline segregation on cracking susceptibility.
• Failure analysis and residual life assessment of equipment operating in sour, wet H2S service.

FAQ

What is the difference between HIC and SSC testing?

Hydrogen-induced cracking (HIC), evaluated by NACE TM0284, occurs in the absence of external stress and is driven by internal hydrogen pressure at inclusions and defects. Sulfide stress cracking (SSC), evaluated by NACE TM0177, is a form of hydrogen embrittlement that requires a tensile stress acting on a susceptible material. HIC tests use unstressed coupons, while SSC tests use stressed specimens such as tensile bars, bent beams, or C-rings.

How long does a NACE TM0284 test take?

The standard exposure duration is 96 hours of continuous H2S saturation. Including machining, preconditioning, exposure, sectioning, metallographic preparation, and evaluation, the complete laboratory turnaround is typically one to two weeks, depending on the number of specimens and reporting requirements.

What acceptance criteria are commonly applied?

The standard itself does not impose acceptance limits; these are defined by the purchaser or by referenced specifications. The most widely applied industry criteria, derived from ISO 15156-2 and related sour service guidelines, are CLR no greater than 15%, CTR no greater than 5%, and CSR no greater than 0.5%, calculated as averages over all evaluated sections.

Which test solution should be selected, Solution A or Solution B?

Solution A represents an aggressive acidic sour environment and is the default choice for most pipeline and refinery applications. Solution B, based on H2S-saturated synthetic seawater, is selected when the service environment is closer to neutral pH, such as offshore seawater exposure. The selection should be agreed between the purchaser and supplier based on the anticipated service conditions.

Can HIC and SSC testing be performed together?

Yes. It is common practice to qualify materials with both NACE TM0284 HIC coupons and NACE TM0177 SSC specimens in a single test program, often in the same H2S-saturated Solution A environment, to fully demonstrate sour service resistance as required by NACE MR0175/ISO 15156.

Can welded joints be evaluated by this method?

The standard primarily evaluates base metal, but welded test pieces can be included when specified. Coupons containing weld metal and heat-affected zones are exposed and sectioned using the same procedure, allowing the cracking susceptibility of the welded region to be assessed as a supplementary qualification requirement.