The Phenomenon

Sudden brittle fracture in high strength steels resulting from hydrogen embrittlement represents an extremely dangerous phenomenon to industry, particularly since it is usually the result of factors that occur during the manufacturing process.

Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, fatigue strength and fracture toughness are also dramatically reduced. Brittle fracture occurs without warning and can be immediate, within hours of manufacture, or after years in service. Hydrogen embrittlement failures have even been observed in unassembled parts in inventory, a phenomenon known as “shelf popping”.

Generally, the higher the strength of the steel, the more at risk it is to hydrogen embrittlement and the more vulnerable it is to lower levels of hydrogen. Embrittlement at levels of 10 parts per million and less are not uncommon. Some research suggests this relationship is exponential. In other words, doubling the strength, quadruples the susceptibility to hydrogen embrittlement.

Although hydrogen embrittlement occurs in many different metal alloys, high strength steel appears to be the most sensitive, is the most widely used and accounts for the largest number of hydrogen embrittlement failures. This article offers an overview on hydrogen embrittlement as it relates to high strength steels only, though details of the phenomenon generally apply to other susceptible metals.

First Appearance

In the late 1940’s a revolution was underway in aviation. Jet propulsion was rapidly replacing the old piston engine driven propeller technology and aircraft performance began to exceed levels that had been considered physically impossible just ten years earlier. Weight reduction and more power propelling airframes that could withstand higher loading were critical to these improvements. This resulted in demands for higher strength alloys from which smaller, lighter and stronger components could be made.

Low alloy steels such as 4130 had been used in aviation in the past. However, these materials were typically used in the normalized heat treated condition, at tensile strengths in the 90,000 to 120,000 psi range – well below levels susceptible to hydrogen embrittlement. In response to demands for more strength, “radical” heat treatments to tensile strengths approaching 200,000 psi were applied to 4130 and other “anemic” low alloy steels. Some of the first hydrogen embrittlement failures appeared, though they weren’t initially recognized as such.

Enhanced low alloy steels, such as 4140 and 4340 were used in response to these failures, and the cycle was repeated, with the demand for more performance from smaller components resulting in processing to ever higher strength levels.

One of the unfortunate consequences of increasing the strength of low alloy steels is a corresponding reduction in corrosion resistance. To combat increased corrosion in service, a variety of electroplated coatings, such as chromium, nickel and cadmium, were applied. With a new potent source of hydrogen now available from the plating baths used in these processes, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and in other industries to which the new materials technology had filtered down.

Part 2 in this series will discuss this phenomenon from a metallurgical perspective – what actually occurs, on a microscopic scale, that causes hydrogen embrittlement?

In Part 3, we will discuss the conditions that render steels susceptible to hydrogen embrittlement, and how to prevent its occurrence in both the manufacturing and service environments.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

The objective in analyzing contaminants is the identification and elimination or isolation of their source. Techniques used make this identification include Optical Stereomicroscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive  X-Ray Spectroscopy (EDS). The accompanying case studies describe a selection of contaminant investigation and analyses performed here at Metallurgical Associates. Part 2 of this discussion will describe examples of the analysis of contaminants for a variety of sources.

Read part 2 of Analyzing Contaminants, Stains and Debris.

The cost of corrosion to the US economy is estimated at 4.2% of the Gross National Product according to a recent study. That amounts to over $350 billion annually which, until this year, exceeds the cost of all oil imports into the US.

Corrosion is the most common and costly failure mode impacting engineered and structural materials, yet it tends to be accepted as inevitable precisely because it is so pervasive. The same study, however, indicates that 40% of these costs, or $140 billion, could be saved through the application of existing practices and technologies. Although numbers of this magnitude tend to be overwhelming, they translate into real costs and lost savings by industry, right down to individual manufacturers and product end users.

The term “corrosion” describes a number of processes driven by a wide range of electro-chemical factors. At the root of these is the inherent instability, at the atomic level, of most industrial metals which predisposes them to return to their naturally occurring form, oxides.

One of the more unusual forms of corrosion results from the interaction of bacteria with a wide range of metals and alloys. Microbiologically Induced Corrosion (MIC) “technically” functions as an accelerant to more conventional corrosion processes. The rate of acceleration, however, may be from 10 to 1000 times conventional corrosion rates, requiring that MIC be addressed as a distinct corrosion process, from a practical standpoint.

MIC initiates and propagates primarily by two processes. The first is the formation of corrosion cells on a metal surface. Colonies of micro-organisms generate sticky biofilms which adhere them to their host surface and create a micro-environment that is significantly different from the surrounding metal. Variations in dissolved oxygen, pH, and organic and inorganic compounds in these micro-environments result in electrical potential differences with the surrounding metal, producing highly active corrosion cells.

The second is by direct chemical attack.  The metabolic by-products of many micro-organisms are highly corrosive. Two related organisms, sulfur reducing bacteria (Disulfovibrio) and sulfur oxidizing bacteria (Thiobacillus thiooxidans), produce hydrogen sulfide and sulfuric acid respectively. Localized sulfuric acid concentrations as high as 10% have been observed from these by-products. Other bacteria species produce a wide range of organic acids such as acetic acid, as well as ammonia.

Both aerobic bacteria, which thrive in an oxygenated environment, and anaerobic bacteria, which thrive in a minimal or non-oxygen environment have been documented in MIC. In some cases, these two bacterial types share a symbiotic relationship as aerobic bacteria deposit biofilms under which an oxygen depleted zone is formed at the metal interface. This oxygen depleted zone then becomes an ideal environment for the growth of anaerobic bacteria colonies.

The formation of tubercles is also often associated with MIC.  Tubercles resemble blisters of corrosion product and are initiated from biofilm deposits and iron oxidizing bacteria, particularly at low flow velocity areas in fluid piping systems. The growth and decomposition cycle of the tubercle releases sulfates and provides a site for anaerobic sulfate reducing bacteria on the interior of the blister. Tubercles also form an efficient oxygen concentration cell, dissolving iron under the blister.  Unchecked tubercle growth in fluid transport systems will severely limit or even completely block fluid flow.

Click image to enlarge.

Classic hallmark of a common MIC failure type. The cross-section above shows a Type 304 stainless steel heat exchanger tube that failed by MIC at the longitudinal seam weld, perforating the 0.065” thick tube wall.  MIC began at the tube ID due to an anaerobic bacteria species introduced through incomplete drying following hydrostatic testing.

Contact of the metal’s surface with water is a pre-condition to MIC. Since the bacteria species responsible for MIC pose no human health risk, “safe” drinking water systems are just as much at risk as non-potable water systems. Cooling systems and heat exchangers, wells, fire and agricultural automatic sprinkler systems and liquid storage tanks are among the more obvious potential sites for MIC to develop. However, fluid products not normally associated with water such as gasoline, oil and machining and cutting lubricants all contain at least trace levels of water which are sufficient to support bacteria that initiate MIC. Virtually all processed fluid products including food and beverage, petrochemical and other commercial and industrial products also contain varying amounts of water and are susceptible to MIC.

See Part 2 of this series – Microbiologically Induced Corrosion

See Part 3 of this series – Microbiologically Induced Corrosion Prevention and Analysis

Fatigue is the most common type of fracture in engineered components. Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure. The component, whether it’s the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure.

A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory. In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates. But a practical understanding of the process is extremely beneficial and has direct application to its prevention, and the manufacturing environment, as discussed below.

To the non-technically inclined, the term “fatigue” suggests this type of failure is related to the age of a component, that the material is “tired”. In fact, fatigue fracture can occur within hours of a component going into service. Conversely, even large, highly stressed components can operate for decades with no fatigue cracking or failure.

Fatigue fractures result from repeated, or cyclic, stresses. These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation.  Regardless of the variation in direction, the stress on the component at the point of fatigue fracture is always tensile stress, in which the fracture initiation site is being “stretched”, or pulled in opposite directions. To illustrate this, visualize a tube which is being repeatedly bent in one direction. The side of the tube that is concave when it is bent is being compressed. The side of the tube which is convex is being “stretched”, or subjected to a tensile stress. This is the side on which a fatigue crack will initiate.

Fatigue cracks initiate at stresses below the tensile strength of the material. Tensile strength is the stress, or load, at which a material breaks when pulled in two opposing directions. This load is a specific value for each metal alloy, varying somewhat depending on heat treating and other processing operations. These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references. The fact that fatigue cracks can occur at stress levels below the tensile strength of a material is difficult to explain. Theories on this focus on physical and structural changes at the microscopic (0.0001” or less) area of crack initiation.

Fatigue is a progressive fracture mechanism. Once a fatigue crack initiates, it is driven further into the component with each stress cycle. This crack growth process continues as long as the component is subjected to cyclic stress. Depending on the magnitude and frequency of these stresses, the crack may grow over time ranging from hours to years.  Eventually, the crack advances to a point where the remaining intact cross section of the component can not sustain the next cyclic stress load – “the straw that breaks the camels back” – and complete fracture of the component occurs.

Read part 2 – Fatigue in the “Real World”