Failure analysis is a vast concept that includes different fields such as electronics, mechanics, metallurgy, corrosion and so forth. However, for a corrosion engineer, failure analysis is defined as the study of a specific case of component or equipment failure—due to corrosion or the mechanical aspects of corrosion—which is conducted to determine the root causes for this failure and how it may be prevented in the future. There are a few important reasons to conduct a corrosion failure analysis according to industrial needs. They are:
- To determine the mechanism of corrosion in an industrial process. Failure analysis is well known as a preliminary process to understanding a corrosion process.
- Suggesting feasible guidelines and remedies to avoid the repetition of similar corrosion failure.
- Forensic analysis. In some cases, failure analysis is done to investigate the damage to metallic components after a chemical accident, fire, leak or environment pollution.
The failure analysis process is usually performed in three stages:
- Obtaining and gathering information related to a component or piece of equipment's history.
- Conducting a detailed examination of the failed part.
- Addressing the reasons that caused the failure.
These stages are discussed in more detail below.
1. Creating a Record of the Component and Equipment Background
Detailed information about both the failed component and the environment to which the component had been exposed is necessary for failure analysis. In fact, without having enough evidential data, analyzing the failure would not be possible.
For the failed components, it is important to know the metallurgical data about them, including chemical composition, microstructure, continuity, mechanical deformation and heat treatment history. Environmental conditions is the other issue that has to be considered in this stage. Although basic information such as chemical composition, temperature, fluid velocity and intensity must be determined, more detailed information should not be neglected. For example, pitting corrosion that occurs in a pump that had pumped seawater continuously seems impossible at the first survey because pitting corrosion will not happen in a moving solution. However, the records could show a brief overhaul or another scenario that would be enough to initiate pits. Therefore, it is very important to know any change in the environment, such as changes in composition, temperature, etc.
In addition, referring to the data sheets is not enough to gather the data. In most cases, the corrosion happens when conditions deviate from first design conditions. For example, in a buried pipeline that is broken due to corrosion, the electrochemical potential of the structure versus the soil had shown effective cathodic protection. However, engineers did not consider a new high-voltage transport line, which has the potential to cause AC corrosion. Therefore, any change in the surrounding environment should be considered carefully.
The inspection reports and related dates are other important factors. It is important to know the last time that the component was investigated and was found to have no significant corrosion. This will help determine the approximate time frame in which the corrosion occurred.
2. Examination of the Failed Part
In the second step, the failed components, and in some cases the adjacent components, are examined to obtain more information about the failure. It is necessary to document all the observations throughout this step by taking notes and photographs. This documentation provides information for the engineers to review and discuss.
Generally, the corroded components in industrial equipment are sent to the lab for more detailed investigation. In order to preserve the original form of samples and minimize sample damage, very careful sampling and sectioning practices are required. All the samples should be identified and placed in suitable packaging before being shipped to the laboratory.
In the lab, investigators can use various types of instrumental tools to examine the corroded samples. The simplest way to start is with a visual inspection. This should be focused on the type of corrosion present and its intensity. The issues that are often important in inspection depend on the type of corrosion being examined. For example, in pitting corrosion, evaluating the shape and density of pits is conducted at this step. However, for erosion corrosion, it is very important to know the pattern and depth of wear.
Optical or scanning electron microscopes are used to achieve more detailed information. Sometimes, the examination of a cross-section of a corroded sample can provide helpful clues to understanding the reasons behind corrosion. Cross-sectioning of samples for microscopic investigation is a good method for determining corrosion depth, the features of pits, the types of cracks (intergranular or transgranular) and so forth.
Determination of the morphology, volume and chemical composition of corrosion products is also very helpful in discovering the root cause of corrosion. For example, there are several feasible corrosion products that can be deposited on the surface of buried steel pipelines. Each of them forms in specific conditions. For example, siderite (FeCO3) will be formed in a high-pH, anaerobic solution containing carbonates. However, when a small amount of sulfur is present in the solution under the same conditions, the green rust (a complex composition of iron hydroxide and carbonate) will be more stable. In another example, one of the ways to recognize microbiologically induced corrosion (MIC) of steel structures by sulfate-reducing bacteria is the detection of mackinwalite (FeS) as a corrosion product.
There are many ways to recognize the composition and structure of corrosion products, including X-ray diffraction, X-ray fluorescence, energy dispersive X-ray spectrography, Fourier transform infrared spectrography and Raman spectroscopy.
Conducting mechanical tests is necessary when mechanical aspects such as wear or fracture contributed to a failure. In most cases, a hardness test along with metallographic structures provide enough information about the mechanical properties of the metallic samples in corrosion failure analysis.
3. Reporting Data, Deriving Conclusions, Making Suggestions
When all the information is collected and reviewed, corrosion engineers should analyze the data in a rational manner. It is very important that all the data be considered to avoid misinterpretation. This step needs sufficient knowledge and experience in order to prevent generating new failures or extra expenses due to the incompetence of the analyst.