Modern analytical techniques in failure analysis of aerospace, chemical, and oil and gas industries

Seifollah Nasrazadani*; Shokrollah Hassani†    * Engineering Technology Department, University of North Texas, Denton, TX, USA
† BP America Inc., Houston, TX, USA

Chapter Outline

1 Microscopy Techniques

1.1 Optical Microscopy

Human curiosity about the shapes and appearance of objects at both macro and micro scales has helped us develop microscopes with higher resolving powers. Human eyes with resolution power of 0.1 mm are not sufficient to observe features smaller than 0.1 mm. Therefore, microscopes with higher resolution are needed to examine materials' features at micro, nano, or angstrom size. By definition, optical resolution is defined as the shortest distance between two points that can be differentiated. Optical microscopes can resolve features approximately 0.2 μm apart. The resolving power of optical microscopes represents three orders of magnitude improvement over that of human eyes. Applications of optical microscopes in the oil and gas industry includes but not limited to geoscience studies, mineral identification, microfossils and biostratification, geochronology as well as porosity analysis. Among the mentioned applications, porosity analysis appears to be more popular due to the fact that the presence of interconnected pores affects permeability of oil and gas in sandstones and this facilitates oil and gas extraction from a reservoir.

1.2 Scanning Electron Microscopy

Scanning electron microscopy (SEM) is the most popular technique in the analysis of failed components in the oil and gas industry. The popularity of SEM is due to the fact that failure mode(s) can be easily identified through morphological features obtained from analysis of the fracture surfaces. Specifically one can look for formation of dimples in a fracture surface of a highly ductile alloy such as low carbon steels as well as all nonferrous alloys, such as aluminum and copper, indicating severe plastic deformation prior to failure. High carbon steels hardened through heat treatment processes can undergo brittle fracture when exposed to subzero temperatures or a sudden blow, exhibiting cleavage fracture morphology that is indicative of fast fracture without any demonstration of plastic deformation. Ferrous and nonferrous alloys experiencing fatigue typically exhibit striation marks indicating application of a cyclic load prior to failure. Oftentimes alloys experiencing fatigue failure demonstrate striation marks but absence of striation marks does not eliminate the possibility of fatigue failure. Figure 2.1a–c show examples of ductile failure (a), brittle failure (b), and fatigue failure (c).

One of the issues in the oil and gas industry is flow-accelerated corrosion (FAC). SEM has been effectively used in identification of the flow regime based on morphology of the damaged steel surfaces. Steel pipes used in the oil and gas industry where highly energetic fluids such as steam passing through elbows (Figure 2.2a) cause severe FAC in carbon steels and even alloy steels showing scalloped formation shown in a single-phase flow regime (Figure 2.2b) whereas two-phase flow regime results in the formation of tiger bands.

A SEM image is formed in an electron gun through a shower of electrons (~ 100 nA) produced by either thermionic or field emission mechanisms. Primary electrons bombard surface of the sample under investigation to produce secondary electrons through the inelastic collision of primary electrons and valence electrons orbiting the nucleus of the samples' atoms. A set of electromagnetic lenses guide the shower of electrons to raster sample surface where as a scanning coil controls the spot size of the electron shower. A secondary electron detector is used to collect them and be utilized in image formation. Backscattered electrons are also produced through elastic collision between primary electrons and the nucleus of the sample atoms that can be collected using a backscattered electron detector to produce an image with differentiable contrast in regions with concentration of high atomic number as compared to those of low atomic number elements. X-ray signals are also produced and utilized for chemical identification of materials on the surface of atoms. This aspect will be discussed in detail in the following sections.

Samples used in SEM analysis must be vacuum (10− 6 torr) compatible though recent developments in new designs of SEMs (called environmental SEM) capable of operating at pressure of 13 torr that is much closer to atmospheric pressure (760 torr). Exposure of nonconductive samples to an electron shower results in electrical charging of the sample producing a useless image with glaring spots. Deposition of ~ 100 Å gold or carbon coating eliminates the charging issue.

1.3 Focused Ion Beam

Focused ion beam (FIB) technique is analogous to SEM with the exception of using Ga ions in place of electrons (used in SEM) to form an image. FIB has a highly enhanced resolution that allows observation of much finer features such as porosity in core samples cut and drilled in the geological analysis for oil and gas industry used in oil exploration efforts. Application of FIB in core sample analysis reduces oil and gas explorations' risks. Pore analysis can provide quantitative chemical analysis of minerals that contain pores at nanometer scale. Organic matters also contain pores at the nanometer scale that require highly resolved images to be observed. Simultaneous observations on pore size and chemical quantification can be achieved through transmission electron microscopy (TEM). FIB can be used to prepare samples for TEM analysis saving sample preparation time and cost.

Sample preparation to the nanometer thickness has been a challenge in the past but the advent of FIB has reduced time and cost needed for sample preparations for TEM significantly. Figure 2.3 shows a FIB image of a mineral sample...