Author: Lucas Baldesberger, Engineer II
A metallurgical failure analysis is a type of failure assessment that investigates the damage mechanism(s) responsible for causing component failure or higher-than-expected degradation rates. Degradation commonly refers to corrosion but may also include crack growth, blistering, fouling/deposition, or microstructural anomalies, depending on the active damage mechanism(s). “Root cause” analysis terminology is commonly used interchangeably with “failure analysis,” but it is important to distinguish between the two unique investigation techniques. A root cause failure analysis is usually a more formal incident investigation that may examine procedural or cultural causes, commonly using a Why Tree, to determine the root cause that led to the event. A metallurgical failure analysis is more technical in nature, utilizing laboratory investigation results to identify the physical cause of failure.
Why Perform a Failure Analysis?
A successful metallurgical failure analysis provides factual insight into the key mechanism(s) at play, as the metallurgy holds truths to be discovered. Utilizing the correct laboratory techniques will produce results and provide data that will put biases and preconceived hypotheses to the side and let the facts direct the analysis. Laboratory techniques commonly used by the E2G Materials & Corrosion Engineering Team include positive material identification (PMI) via X-ray fluorescence (XRF) or optical emission spectroscopy (OES), mechanical testing to determine material-specific properties (tensile, creep, Charpy impact, hardness, etc.), glass bead blasting specimen cleaning, deposit chemical analysis via energy dispersive spectroscopy (EDS), and sample extraction and mounting for metallographic examination including optical imaging and scanning electron microscope (SEM) imaging in both the as-polished and etched conditions.
These laboratory results can then be used as key inputs to perform detailed damage mechanism reviews (DMRs), fitness-for-service (FFS) assessments, and/or remaining life assessments. The first step in damage mechanism mitigation, FFS assessments, and determining component remaining life is to correctly identify the mechanism type and cause of damage. A successful failure analysis can provide answers to both questions, leading to increased accuracy of follow-up assessments. For example, knowing the applicable damage mechanism when performing an FFS assessment is critical to accurately estimating damage progression rates (corrosion, crack growth, etc.), using the correct material properties, and identifying the most practical damage remediation strategies. Remember, findings from one failure analysis can be applied to additional locations of similar damage that are still in service to aid in lifecycle management!
Metallurgical failure analysis investigations are also not limited to pressure-boundary equipment. E2G has completed metallurgical failure analyses of injection quills, vessel water jacketing, and high-strength bolts, to name a few non-pressure boundary examples.
Utilizing Conclusions & Recommendations from an Analysis
Once the laboratory analysis is completed and conclusions are drawn, now what? Determining the cause of failure is valuable, but utilizing these findings for future insight can set a failure analysis apart from being a one-time value gain to instead being a movement towards continuous operational improvement. While in many cases failed components can be easily replaced or repaired, it is essential to learn from the incident and establish practical operational considerations to prevent future failures.
After the primary damage mechanism(s) is identified, integrity operating windows (IOWs) for equipment will be recommended by an experienced Corrosion Engineer to manage the mechanism. These “windows” typically establish bounding limits to operating process variables with the intent of maintaining predictable and reasonably low rates of equipment degradation, such that the equipment can be safely managed and operated without loss of containment while maintaining process efficiency. For example, a failure analysis may identify ammonium chloride corrosion due to salt deposition in the crude overhead system as the primary mechanism of failure. But what should be done to avoid this scenario in the future? IOWs may be generated for the crude column overhead temperature, maintaining them above the salt point to minimize deposition.
In addition to operational considerations, it is imperative to develop inspection strategies to aid in identifying and assessing severity of damage if similar components are still in service. The ability to identify and quantify present damage (flaw sizing, wall thinning, etc.) is key to performing accurate FFS assessments and determining component remaining life. By knowing what mechanism to inspect for, the correct nondestructive examination (NDE) techniques can be selected and possibly prevent similar future failures. All of these “action items” derived from the conclusions and recommendations are a part of the deliverable of every E2G failure analysis report and should be an expectation of the failure analysis investigation team. In the information age, a conclusion of “failed by internal corrosion” is not acceptable.
The E2G Difference:
Determining the correct laboratory approach is a vital step in completing a successful failure analysis and producing meaningful and accurate data. With the currently saturated market of laboratories providing testing and advertising failure analysis investigations, trusting the correct laboratory and engineering oversight is a critical decision when searching for a reputable source. The value in a failure analysis goes much deeper than the physical laboratory findings, as the true value is in the discussion on causes leading to the observed failure, future mitigation strategies, and deciphering the physical findings into qualitative and/or quantitative results.
Oftentimes, the most difficult (and valuable) part of any failure analysis is interpretation of the metallurgical lab results and identification of sensible engineering solutions. At E2G, Materials & Corrosion Engineers overseeing failure analyses have experience spanning a wide range of industries, from refining and petrochemicals to nuclear energy and fertilizer production. Such diversified knowledge promotes awareness of industry-specific mechanisms as well as fundamental issues affecting plants worldwide, such as steam generation or cooling water systems. Our thorough understanding of damage mechanisms, combined with our staff of subject matter experts (SMEs) who have 40+ years of experience within industry, allows E2G to perform technical, in-depth analyses with a focus on efficiency and optimization. Overall, damage mechanism familiarity and engineering expertise are both needed to extract the full value from a failure analysis.
Case Study #1: MIC of 316L SS Heat Exchanger Tube
A metallurgical failure analysis was recently completed for a petrochemical facility located along the Gulf Coast, USA. Several tubes were provided from a shell and tube heat exchanger which had required multiple retubes over the past few years due to tube leaks. The tubes were seam-welded 316L stainless steel with cooling water present on the shellside of the exchanger and an organic acid/water mixture on the tubeside. Macroscopic examination of the tubes identified OD-initiated pitting with some pits progressing through-wall. XRF/OES confirmed the tube composition fell within 316L (UNS S31603) composition limits, and no visible scale or deposits were observed. Upon sectioning the tube samples, longitudinal weld seams were identified along the tube ID. Cross-sectional mount samples were prepared through pit locations to allow for microscopic examination. Optical microscopy identified that spherical pits were generally cavernous, with significant sub-surface tunneling present, a morphology highly characteristic of microbiologically induced corrosion (MIC). Pits were then examined via SEM, which identified “pits within pits,” another telltale MIC morphology.
With the laboratory scope complete and damage mechanism identified, detailed conclusions and recommendations were then provided to the operator. MIC susceptibility increases in areas prone to low flow or stagnant conditions, such as when cooling water is on the shellside of an exchanger. These conditions may promote fouling/deposition, limit biocide treatment surface access, and prevent biofilms from being swept away – all factors that increase MIC susceptibility. In addition to the configuration inadequacies, review of operating conditions identified higher-than-optimal cooling water temperatures (may lead to scale formation) and some gaps in the plant-wide cooling water treatment program. These findings led to recommendations including reviewing the cooling water system with involvement of any chemical vendors, considering modifying the piping routing to the exchanger to allow for a bypass (temperature control), and assessing changing the tubeside and shellside services to minimize cooling water minimal flow locations. Finally, both eddy current (ECT) inspection and air-under-water testing were recommended for use of welded tubes to minimize the likelihood of defects being placed in service (previously, only a hydrotest was completed).
Case Study #2: Thermal Fatigue of Boiler Feedwater Piping & Weldolet
An NGL facility located in the Midwest, USA region recently experienced a failure in their boiler feedwater piping system. A several-foot section of a 16-inch boiler feedwater pipe (SA-106 CS) and adjacent weldolet (SA-105 CS) was provided, as the junction had experienced repeated historic issues of leaks and flaw identification via NDE. The macroscopic examination identified surface-breaking flaws throughout both samples, a previous circumferential weld repair along the bottom of the piping section, and a previous flaw grinding/weld repair of the weldolet sample. Optical imaging was then performed through flaws at two unique locations within each sample. Images identified straight, non-branched, carrot-shaped cracks with some oxide present in both samples, characteristic of thermal fatigue cracking.
Upon review of boiler operating conditions, it was discovered the failed piping segment originated from a spare pump and is not usually in service, meaning this piping segment is considered a deadleg. Deadlegs are more likely to maintain stagnant flow conditions and are prone to thermal fatigue as temperatures are usually much lower than adjacent in-use piping. Thermal fatigue was identified as the primary mechanism due to the temperature gradient between the piping segment and header weldolet, accelerated by the stresses present at the pipe-to-weldolet connection. Caustic stress corrosion cracking was identified as a secondary mechanism due to the significant amount of cracking that was present preferentially along the bottom of the piping and weldolet samples, indicative of stagnant conditions. Recommendations included maintaining deadleg insulation and heat tracing to reduce temperature gradients and ensure the line is properly drained when not in use to prevent stagnant liquid accumulation. Additionally, it was recommended to slowly introduce hot water to reduce expansion stresses when placing the line back into service.
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