Liquid Metal Embrittlement

Liquid metal embrittlement is the decrease in ductility of a metal caused by contact with liquid metal. The decrease in ductility can result in catastrophic brittle failure of a normally ductile material. Very small amounts of liquid metal are sufficient to result in embrittlement.

Some events that may permit liquid metal embrittlement under the appropriate circumstances are listed below:

	Brazing
	Soldering
	Welding
	Heat treatment
	Hot working
	Elevated temperature service

In addition to an event that will allow liquid metal embrittlement to occur, it is also required to have the component in contact with a liquid metal that will embrittle the component.

Liquid Metal Embrittlement Failures

The liquid metal can not only reduce the ductility but significantly reduce tensile strength. Liquid metal embrittlement is an insidious type of failure as it can occur at loads below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component.

Intergranular or Tran granular cleavage fracture are the common fracture modes associated with liquid metal embrittlement. However reduction in mechanical properties due to decohesion can occur. This results in a ductile fracture mode occurring at reduced tensile strength. An appropriate analysis can determine the effect of liquid metal embrittlement on failure.

Wear Failures

Wear may be defined as damage to a solid surface caused by the removal or displacement of material by the mechanical action of a contacting solid, liquid, or gas. It may cause significant surface damage and the damage is usually thought of as gradual deterioration. While the terminology of wear is unresolved, the following categories are commonly used.

	Adhesive wear
	Abrasive wear
	Erosive wear

Adhesive wear has been commonly identified by the terms galling, or seizing. Abrasive wear, or abrasion, is caused by the displacement of material from a solid surface due to hard particles or protuberances sliding along the surface. Erosion, or erosive wear, is the loss of material from a solid surface due to relative motion in contact with a fluid that contains solid particles. More than one mechanism can be responsible for the wear observed on a particular part.

Galvanic Corrosion

Galvanic corrosion is frequently referred to as dissimilar metal corrosion. Galvanic corrosion can occur when two dissimilar materials are coupled in a corrosive electrolyte. An illustration of galvanic corrosion would be joining two dissimilar metals in electrical contact in seawater.

In a galvanic couple, one of the metals in the couple becomes the anode and the other metal becomes the cathode. The less noble material becomes the anode. The anodic metal corrodes faster than it would all by itself. The cathodic metal corrodes slower than it would all by itself.

Many boaters use this knowledge to their benefit. Sacrificial zinc anodes are commonly used to protect metal components on boats. The zinc anode corrodes preferentially there by protecting the boat component. The zinc anodes are maintained and replaced as required to insure continued protection. Other alloys are also used as sacrificial anodes. Aluminum or magnesium sacrificial anodes provide better protection in some cases.

Ductile and Brittle Metal Characteristics

Ductile metals experience observable plastic deformation prior to fracture. Brittle metals experience little or no plastic deformation prior to fracture. At times metals behave in a transitional manner - partially ductile/brittle.

Ductile fracture has dimpled, cup and cone fracture appearance. The dimples can become elongated by a lateral shearing force, or if the crack is in the opening (tearing) mode.

Brittle fracture displays either cleavage (transgranular) or intergranular fracture. This depends upon whether the grain boundaries are stronger or weaker than the grains.

The fracture modes (dimples, cleavage, or intergranular fracture) may be seen on the fracture surface and it is possible all three modes will be present of a given fracture face.

Brittle Fractures

Brittle fracture is characterized by rapid crack propagation with low energy release and without significant plastic deformation. The fracture may have a bright granular appearance. The fractures are generally of the flat type and chevron patterns may be present.

Ductile Fractures

Ductile fracture is characterized by tearing of metal and significant plastic deformation. The ductile fracture may have a gray, fibrous appearance. Ductile fractures are associated with overload of the structure or large discontinuities

Uniform Corrosion

Uniform or general corrosion is typified by the rusting of steel. Other examples of uniform corrosion are the tarnishing of silver or the green patina associated with the corrosion of copper.

General corrosion is rather predictable. The life of components can be estimated based on relatively simple immersion test results. Allowance for general corrosion is relatively simple and commonly employed when designing a component for a known environment.

Some common methods used to prevent or reduce general corrosion are listed below:

	Coatings
	Inhibitors
	Cathodic protection
	Proper materials selection

Stress Corrosion Cracking

Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Temperature is a significant environmental factor affecting cracking.

For stress corrosion cracking to occur all three conditions must be met simultaneously. The component needs to be in a particular crack promoting environment, the component must be made of a susceptible material, and there must be tensile stresses above some minimum threshold value. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses are commonly below the yield stress of the material.

Stress Corrosion Cracking Failures

Stress corrosion cracking is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Pitting is commonly associated with stress corrosion cracking phenomena.

Aluminum and stainless steel are well known for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion cracking in the right environment.

Controlling Stress Corrosion Cracking

There are several methods to prevent stress corrosion cracking. One common method is proper selection of the appropriate material. A second method is to remove the chemical species that promotes cracking. Another method is to change the manufacturing process or design to reduce the tensile stresses. AMC can provide engineering expertise to prevent or reduce the likelihood of stress corrosion cracking in your components.

Corrosion Failures

Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties. This may result in failure of the component. Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure. Examples are listed below:

	Type of corrosion
	Corrosion rate
	The extent of the corrosion
	Interaction between corrosion and other failure mechanisms

Corrosion is is a normal, natural process. Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by changing the environment. Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

Stress corrosion cracking necessitates a tensile stress, which may be caused by residual stresses, and a specific environment to cause progressive fracture of a metal. Aluminum and stainless steel are well known for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion cracking in the right environment.

Laboratory corrosion testing is frequently used in analysis but is difficult to correlate with actual service conditions. Variations in service conditions are sometimes difficult to duplicate in laboratory testing

Pitting Corrosion

Pitting is a localized form of corrosive attack. Pitting corrosion is typified by the formation of holes or pits on the metal surface. Pitting can cause failure due to perforation while the total corrosion, as measured by weight loss, might be rather minimal. The rate of penetration may be 10 to 100 times that by general corrosion.

Pits may be rather small and difficult to detect. In some cases pits may be masked due to general corrosion. Pitting may take some time to initiate and develop to an easily viewable size.

Pitting occurs more readily in a stagnant environment. The aggressiveness of the corrodent will affect the rate of pitting. Some methods for reducing the effects of pitting corrosion are listed below:

	Reduce the aggressiveness of the environment
	Use more pitting resistant materials
	Improve the design of the system

Crevice Corrosion

Crevice corrosion is a localized form of corrosive attack. Crevice corrosion occurs at narrow openings or spaces between two metal surfaces or between metals and nonmetal surfaces. A concentration cell forms with the crevice being depleted of oxygen. This differential aeration between the crevice (microenvironment) and the external surface (bulk environment) gives the the crevice an anodic character. This can contribute to a highly corrosive condition in the crevice. Some examples of crevices are listed below:

	Flanges
	Deposits
	Washers
	Rolled tube ends
	Threaded joints
	O-rings
	Gaskets
	Lap joints
	Sediment

Some methods for reducing the effects of crevice corrosion are listed below:

	Eliminate the crevice from the design
	Select materials more resistant to crevice corrosion
	Reduce the aggressiveness of the environment Crevice Corrosion

Description

Crevice corrosion is a particular form of localized corrosion (i.e., including phenomena somewhat related to pitting corrosion reactions) which occurs in a crevice formed between two surfaces, one at least of which is a metal.

Examples of such crevices are: flanged or threaded connections, shielded areas on metal surfaces (e.g. as a result of deposit formation), etc.

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation materials, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps. Crevice corrosion is initiated by changes in local chemistry within the crevice:

	Depletion of inhibitor in the crevice
	Depletion of oxygen in the crevice
	A shift to acid conditions in the crevice
	Build-up of aggressive ion species (e.g. chloride) in the crevice

Corrosion - Mechanism

The mechanism of crevice corrosion is generally accepted at this time and can be described as a four-stage process:

	The crevice solution becomes deoxygenated due to initial corrosion in the crevice and the diffusion rate of oxygen into the crevice is not sufficiently rapid to replace its rate of depletion due to the local cathodic process.
	The cathodic process moves outside the crevice, separating the anode and the cathode. Due to current flow and mass transport phenomena, chloride ions migrate to, and build up inside the crevice, causing a reduction of pH in the crevice solution.
	The build up of chlorides and the depressed pH create a crevice solution that causes a breakdown of the passive film on the alloy.
	Once the passive film is compromised, corrosion propagation commences.

Seawater as a Corrosive Medium

Composition

The composition of seawater is substantially the same worldwide, the principal variations being the ratio of water to total salt content. Total dissolved solids (TDS) vary from as low as approximately 8000 ppm (mg/L) in the Baltic Sea to as high as 60,000 ppm in bay areas of the Arabian Gulf, due to the evaporation by tropical heat in the desert air. The "nominal" dissolved solids, upon which formulae for artificial seawater are based, is about 34,500 ppm, of which about 25,000 ppm is taken to be sodium chloride. Salinity and Chlorinity

The concept of salinity is sometimes misunderstood, commonly taken as being the corrosivity of seawater. Historically, salinity has been defined as "the total weight in grams of inorganic salts in one kilogram of seawater, when all bromides and iodides are replaced by chloride equivalents and all carbonates by oxide equivalents."However, salinity is usually determined either by conductivity measurements or from the chlorinity. Chlorinity, in turn, is "the mass in grams of silver required to precipitate the halogens in 0.3285234 kilo grams of seawater." Fortunately, it is nearly equal to the mass of chloride in the same. Chlorinity is related to salinity as follows:

S % = 1.80655 Cl% [1.0]

Carbonates and Sulfates

While deep ocean waters are usually undersaturated with respect to carbonates, surface waters are usually saturated due to wave action and exposure to carbon dioxide in the atmosphere. This saturation will affect the deposition of calcium and magnesium salts in cathodic reactions during corrosion or in the application of cathodic protection. The presence of 2000 to 3000 ppm of sulfate ions in seawater of average composition facilitates corrosion under anaerobic conditions, due to the action of sulfate reducing bacteria (SRB's).

Dissolved Oxygen

The nominal saturation for dissolved oxygen (DO) in seawater is about 6-8 ppm at 25-30 oC (77-86 oF). However, the DO may go as high as 12 ppm, depending upon wave action and seawater temperature, and supersaturation may occur due to photosynthesis by phytoplankton bloom. On the other hand, the DO may decrease to practically zero due to bacterial action or biological or chemical oxygen demand. The presence or absence of DO largely overshadows everything else. Steel corrosion, for example, increases with DO despite decreasing temperature.

pH

The pH of seawater is usually in the range from 7.7 to 8.3 in surface waters, due to the buffering effects of carbonate salts. Much more acidic conditions can be produced in deep water, with pH's of 3-4 being produced by bacterial action. Simple hydrocarbons and oxygenated organic compounds produce carbon dioxide and water upon oxidation, also lowering the pH. In the latter instance, the pH would be roughly 5-6. On the other hand, living plants consume carbon dioxide by photosynthesis, where sunlight is available, raising the pH. Decomposing nitrogenous organic material (e.g., fish) will form amines and ammonia, also tending to increase the pH. It is the net result of all these possible activities, plus the effect of such specific ions as sulfides from SRB activity, which must be considered.

Localized Variations

Besides stratification effects in deep, quiet waters, temperature decreases with depth. From a surface temperature about 13oC (55oF), the temperature may diminish to about 5oC (41oF) at 3000 feet (1000 meters) depth. The DO may simultaneously drop from 6 ppm to about 1 ppm if biological or bacterial activity occurs. Otherwise, the DO will be greater because of increased oxygen solubility at the lower temperature.

Geographical Variations

Temperature extremes may vary from 2-4 oC (36-39 oF) in the polar regions to 32oC (90oF) or more in shallow waters in the tropics. The dissolved oxygen will diminish about 50% with depth in the Gulf of Mexico at 1500 feet (500 meters), but is unchanged at the same depth in the North Sea.

Characteristics

Besides compositional variations, seawater will contain varying amounts of silt, dissolved gases other than DO, and decaying animal and vegetable matter, which contribute to fouling and corrosion. Seawater, utilized for cooling purposes in chemical or other process plants, is usually taken from relatively shallow water sources or from seawells, which rarely exceed 60 feet (20 meters) in depth. Shallow estuaries or bay water are notoriously susceptible to municipal or industrial pollution. In southern climates particularly, there may be "Red Tides" (due to dinoflagellates) which tend to diminish dissolved oxygen and increase suspended solids in the form of dead marine life.

Under stagnant conditions, as when a plant cooling system is shut down during a turnaround, the seawater may become anaerobic, due to consumption of oxygen by corrosion, the effects of BOD or COD, or bacterial action (or by a combination of such factors). The action of SRB's on sulfates will produce "sour" conditions from the production of small amounts of hydrogen.

Fouling

Strictly speaking, fouling includes four separate types of phenomena. Fouling by crystalline deposits (e.g., calcareous deposits, calcium sulfate deposits) requires temperatures greater than about 90oC (190oF) and is not usually encountered in heat exchangers at the usual seawater pH. An exception would be where alkaline streams (caustic, ammonia or amines) leaked into the cooling water, raising the pH above calcium salt saturation.

Fouling from suspended solids and biological growths are normally the major concerns. The effect of sand in abrasion or erosion corrosion is well known and its capability for setting up oxygen concentration cells adequately proved. Silt from estuary waters contain not only rock particles, usually 0.05 inches (1.25 mm) or less in diameter, but also vegetable matter and possibly metallic oxides from contamination or pollution. More adherent than sand, silt contributes to fouling, plus adding to corrosion problems, not only by concentration cell effects but also contributing decomposition products. The deposition of sand and silt within the heat exchanger tubes is primarily controlled by maintaining recommended minimum velocities, usually about 5-6 fps (1.5-1.8 mps).

Marine Corrosion

Marine corrosion includes the immersion of components in a seawater, equipment and piping that use seawater or brackish water, and corrosion in marine atmospheres. Exposure of components can be continuous or intermittent. Ships, marinas, pipelines, offshore structures, desalination plants, and heat exchangers are some examples of systems that experience marine corrosion.

Corrosion of a component, such as a bolt, can vary markedly depending on if it is simply in a marine atmosphere, a splash zone, or submerged in seawater. Maintenance costs for ships, offshore structures and related equipment are dependent on how marine corrosion issues and failures are managed.

In addition to the salt (NaCl) in seawater there are other commonly occurring constituents, dissolved gases, living organisms, and various other materials found in seawater. Rivers, temperature, dissolved oxygen, and pollutants are some examples of issues that may affect the corrosion of a given component in seawater.

Marine atmospheres are generally considered to be one of the more aggressive atmospheric corrosion environments. Some factors that affect corrosion rates in marine atmospheres are listed below:

	Humidity
	Wind
	Temperature
	Airborne contaminants
	Location
	Biological organisms

Alloy selection, metallic coatings, organic coatings, and cathodic protection are commonly used methods for providing proper corrosion protection to various components. Thermal spray, galvanizing, and for specific circumstances electroplating are metallic coatings used in various marine corrosion applications.

Organic coatings have changed in recent times due to environmental requirements. Organic coatings have various constituents to enhance properties that provide corrosion protection, antifouling, adhesion, good mechanical strength, or other desirable characteristics.

Cathodic protection can be accomplished by either using an impressed current system or by using sacrificial anode system. Magnesium, aluminum and zinc alloys are the most frequently used sacrificial anode systems.

Fatigue Failures

Metal fatigue is caused by repeated cycling of of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe.

The process of fatigue consists of three stages:

	Initial crack initiation
	Progressive crack growth across the part
	Final sudden fracture of the remaining cross section

Schematic of S-N Curve, showing increase in fatigue life with decreasing stresses.

Stress Ratio

The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax).

	If the stresses are fully reversed, then R = -1.
	If the stresses are partially reversed, R = a negative number less than 1.
	If the stress is cycled between a maximum stress and no load, R = zero.
	If the stress is cycled between two tensile stresses, R = a positive number less than 1.

Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress than it does under a fully reversed stress.

Preventing Fatigue Failure

The most effective method of improving fatigue performance is improvements in design:

	Eliminate or reduce stress raisers by streamlining the part
	Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes
	Prevent the development of surface discontinuities during processing.
	Reduce or eliminate tensile residual stresses caused by manufacturing.
	Improve the details of fabrication and fastening procedures

High Temperature Failure Analysis

Creep occurs under load at high temperature. Boilers, gas turbine engines, and ovens are some of the systems that have components that experience creep. An understanding of high temperature materials behavior is beneficial in evaluating failures in these types of systems.

Failures involving creep are usually easy to identify due to the deformation that occurs. Failures may appear ductile or brittle. Cracking may be either transgranular or intergranular. While creep testing is done at constant temperature and constant load actual components may experience damage at various temperatures and loading conditions.

Creep of Metals

High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. A typical creep curve is shown below:

In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slope of the curve, identified in the above figure, is the strain rate of the test during stage II or the creep rate of the material.

Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.

Stress Rupture

Stress rupture testing is similar to creep testing except that the stresses used are higher than in a creep test. Stress rupture testing is always done until failure of the material. In creep testing the main goal is to determine the minimum creep rate in stage II. Once a designer knows the materials will creep and has accounted for this deformation a primary goal is to avoid failure of the component. Stress rupture tests are used to determine the time to cause failure. Data is plotted log-log as in the chart above. A straight line is usually obtained at each temperature. This information can then be used to extrapolate time to failure for longer times. Changes in slope of the stress rupture line are due to structural changes in the material. It is significant to be aware of these changes in material behavior, because they could result in large errors when extrapolating the data.

Fatigue Failure Analysis

Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength. This can result in an unexpected and catastrophic failure in use.

Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure.

Hydrogen Embrittlement

When tensile stresses are applied to a hydrogen embrittled component it may fail prematurely. Hydrogen embrittlement failures are frequently unexpected and sometimes catastrophic. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses to cause cracking are commonly below the yield stress of the material.

High strength steel, such as quenched and tempered steels or precipitation hardened steels are particularly susceptible to hydrogen embrittlement. Hydrogen can be introduced into the material in service or during materials processing.

Hydrogen Embrittlement Failures

Tensile stresses, susceptible material, and the presence of hydrogen are necessary to cause hydrogen embrittlement. Residual stresses or externally applied loads resulting in stresses significantly below yield stresses can cause cracking. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component.

Very small amounts of hydrogen can cause hydrogen embrittlement in high strength steels. Common causes of hydrogen embrittlement are pickling, electroplating and welding, however hydrogen embrittlement is not limited to these processes.

Hydrogen embrittlement is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. While high strength steels are the most common case of hydrogen embrittlement all materials are susceptible.