Understanding Corrosion And Stress Corrosion Cracking In Metals And Alloys

What is Corrosion?

Question:

Discuss about the Intergranular Stress Corrosion Cracking.

Corrosion can be defined as a natural process with the help of which refined metal is converted into more chemically- stable form such as its sulfide, hydroxide or oxide (Talbot & Talbot, 2018). It is the slow destruction of materials which are usually metals by electrochemical and / or chemical reaction within their environment. Materials other than metals such as polymers or ceramics also suffer from corrosion, however, the term degradation is more commonly used in this context.  The valuable properties of materials and structures are degraded as a result of corrosion including the appearance, strength and permeability to gases and liquids. Passivation is beneficial in the mitigation of corrosion damage. The hindrance of passivating film forming capability can even lead to the corrosion of high- quality alloy. Right grade of material is required to be selected for the particular environment in order to ensure the long- lasting performance of such group of materials. If the passive film suffers from a breakdown as a result of mechanical or chemical factors, the resultant modes of corrosion may include crevice corrosion, pitting corrosion and stress corrosion cracking. The focus of this report is on ammonia stress corrosion cracking.

Stress corrosion cracking (SCC) can be demarcated as the growth of crack formation in a corrosive environment (Cunningham, Bottleberghe & Greene, 2017). The protective oxide layer is sometimes destabilized by the corrosive environment under definite conditions without causing general corrosion. The reformation of oxide is prevented after a crack due to destabilization.  Ductile metals that are subject to a tensile stress have chances of facing an unexpected sudden failure specifically at elevated temperature. Stress corrosion cracking is caused by the chemical environment which is only a little corrosive to metal (Cheng, 2013).  Brightness is noticed in metals with faces severe stress corrosion cracking while they are filled with cracks that are microscopic. This is the most common factor as a result of which the stress corrosion cracking goes undetected before its failure. Stresses can also be the outcome of crevice loads because of stress concentration or can be instigated by residual stresses or type of assembly.

Ammonia stress corrosive cracking is a form of SCC that takes place in brass tubes in cooling water service which has been contaminated by ammonia as a result of biological growths or any other contamination. The occurrence of this cracking can also be the consequence of intentional addition of ammonia as a neutralizer to the process streams by someone who is not aware its impact on the brass tubes. Brittle fracture is experienced on bending by the brass condenser tubes on the presence of significant ammonia stress corrosion cracking (Raja & Shoji, 2011).   

The Effects of Corrosion

Carbon steel equipment are also affected by the ammonia stress corrosion cracking but unlike the mechanism of cracking on brass which takes place in an aqueous solution, steal equipment cracking takes place in an anhydrous ammonia. In other words, the presence of liquid ammonia in oxygen can result in stress corrosion cracking in carbon steels (Shi, et. al., 2015). With the increase in the yield strength of the plate material, local hardness of the welds and increased strength of welds, the probable problem of stress corrosion cracking is also increased. Vulnerability to this issue is also faced by the systems that are contaminated with oxygen/ air. However, cupro- nickel alloys are not found to be vulnerable to ammonia stress corrosion cracking (McDougal & Stevenson, 2005).

During the normal operations, the applied stress levels are not high as required to initiate cracking.The usage of eddy current such as eddy current array or pulsed eddy current testing is considered to be one of the best techniques for the purpose of inspection of ammonia stress corrosion cracking in brass tubes.    

With specific reference to 18Cr-8Ni steels, the characteristics of stress corrosion cracking provides that susceptibility is found in alloy to transgranular stress corrosion cracking at the time when there is possibility of a passive/ active transition or when a noble surface is produced by the responses in alloy surface which may be metal or oxide, co- planar arrays of dislocations are exhibited by the alloy or high work- hardening rate is exhibited by the alloy, specific electrochemical reactions are there (which are considered to be the chloride’s frequent penetration of passive films) (Kim, Kwon, Kim, Hwang & Kim, 2011).

The proposal of various mechanisms has been made for the purpose of explaining the synergistic stress corrosion interactions the occurrence of which is found at the crack tip. Stress corrosion cracking is expected to be caused as a result of more than on process. The mechanisms have been classified into two main categories namely cathodic mechanisms and anodic mechanisms. The occurrence of both cathodic and anodic reactions is a must during corrosion. This means that the phenomenon resulting in crack propagation may be linked with either form (Parkins, 2011).

 In alloys, the mechanism of anodic dissolution related to stress corrosion cracking is due to the reason of heterogeneity in precipitates distribution with diverse electrochemical anodicity associated with alloy matrix. Then there is a rapid dissolution of the anodic precipitates which further results in the extension of intergranular crack until there is the occurrence of re-passivation (Newman, 2002).  

Passivation

 Simple active dissolution and elimination of material from the crack tip is the most common anodic mechanism (Javidi & Horeh, 2014). On the other hand, the most common cathodic mechanism is absorption, hydrogen evolution, embrittlement and diffusion.  The Crack- Propagation Mechanisms assumes that the occurrence of the breakage in interatomic bonds of crack tip is the result of either mechanical fracture (brittle or ductile) or chemical solvation and dissolution. Normal fracture processes are included in the mechanical fracture that is expected to be induced or stimulated by any of the following interactions between environment and material.

• Surface reactions
• Surface films
• Absorption of ecological species
• Responses in metal ahead of crack tip

All the mechanical fractures that have been proposed are found to contain some of these processes as an important step in the process of stress corrosion cracking.

Surface absorption mechanism suggests that the absorption of active surface components on the hard surface can subsequently result in reducing the deformation resistance and strength of solids. In this mechanism, the interatomic bonds are weakened by the absorbed species at the crack tips thereby resulting in either the promotion of growth in cracks by decohesion or the facilitation of emission of dislocations and nucleation. The enhance dislocation emission further leads to the promotion of joining of the crack tip. Consequently, intergranular fracture or brittle cleavages are produced. This is similar to hydrogen embrittlement but limited to a surface one such that effect can be produced by elements other than hydrogen (Marcus, 2011).

It is believed that two diverse mechanisms cause the occurrence of stress corrosion cracking. These are the hydrogen embrittlement and active path corrosion. It is found that the cracking in active path corrosion form is caused by local corrosion of crack tip. It further proceeds on its electrochemically active way with respect to surrounding metal. Cracking in hydrogen embrittlement mechanism is found to be caused by entrance of hydrogen into metal which in turn results in reducing the deform ability (Chene, 2016). Due to the non- consideration of hydrogen embrittlement as a corrosion process, therefore, the cracking that results due to this mechanism is often excluded from the stress corrosion cracking (Eliaz, Shachar, Tal & Eliezer, 2002).

Stress corrosion cracking is suffered by certain aluminium alloys and austenitic steels in the existence of chlorides, copper alloys and nitrates in the presence of ammoniacal solutions and mid steel in the existence of alkali (Brandl, Malke, Beck, Wanner & Hack, 2009). Therefore, the usefulness of austenitic stainless steel is limited to containing water with more than little ppm content of chlorides at temperature that is greater than 50°C (Li, Chu, Wang & Qiao, 2003). Structural steels that are highly tensile are supposed to crack in an unexpectedly brittle way in the entire variety of aqueous environments, specifically in the presence of chlorides. The subcritical crack growth is demonstrated i.e. the propagation of small surface flaws under fracture mechanics predicts that there should be no occurrence of failure.

What is Stress Corrosion Cracking?

Environmental stress cracking which is a similar process is found to occur in polymers when exposure is faced by the products to aggressive chemicals or specific solvents such as alkalis and acids. Similar to the case of metals, the attack is limited to the particular chemicals and polymers. Sensitivity is faced by the polymers to environmental stress cracking where there is no compulsory chemical degradation of materials by the attacking agents. Sensitivity to degradation by acids is faced by Nylon which is considered to be a process termed as hydrolysis. When strong acids attack nylon mouldings, it results in severe cracking.

Elastomers also suffer from the formation of cracks by ozone attack which is a different form of stress corrosion cracking in polymers. Double bond are attacked in the rubber chain with styrene- butadiene rubber, natural rubber and nitrate butadiene rubber due to the small quantity of gas in air as they are considered to be most sensitive to degradation. Critical strain is found to be very little when ozone cracks are formed in products under stress. The orientation of cracks is at the correct angle to strain axis so that its formation will take place around the boundary of rubber tube.  The occurrence of such cracks in fuel pipes have proved to be very dangerous due to the growth in cracks from the surfaces that are exposed from outside into the bore of the pipe. This has the chances of resulting in fire and fuel leakage. The prevention of ozone cracking can take place by the addition of anti- ozonants to the rubber prior to vulcanization. Automobile tire sidewalls often suffered from ozone cracks but now such problem have been reduced as a result of increasing use of additives. However, the recurrence of problem is seen in the equipment such as unprotected products like seals and rubber tubing.

Ceramics do not suffer a lot from this effect as they are more resistant to chemical attacks. However, phase changes are commonly noticed in ceramics under stress which subsequently results in hardening instead of failure. The toughening mechanism has the capability of enhancing the oxidation of decreased cerium oxide which further results in slow growth of the crack and impulsive failure of dense ceramic bodies.      

The effect of Stress corrosion cracking on a material typically falls between fatigue and dry cracking threshold of that particular material. The requisite tensile stresses may be in directly applied stresses form or in residual stresses form. Carbon steel equipment are also affected by the ammonia stress corrosion cracking but unlike the mechanism of cracking on brass which takes place in an aqueous solution, steal equipment cracking takes place in an anhydrous ammonia.

Ammonia Stress Corrosive Cracking

The consequence of stress corrosion is the appearance of corrosion cracks on the metal surface. Moreover, intergranular cracking in stress corrosion cracking advances without apparent preference for boundaries (King, Johnson, Engelberg, Ludwig & Marrow, 2008). The occurrence of both types of cracking depends on metal structure or the environmental structure and is in the same alloy.  Stress corrosion cracks are initiated at surface flaws which then grow into small macroscopic evidence of mechanical deformation in alloys and metals that are usually considered to be ductile.

Pits formation are found in electrochemical terms when potentials exceeds the pitting capability. The same parameters are considered as the basis of evolution between pitting and cracking that control the stress corrosion cracking (Lu, Chen, Luo, Patchett & Xu, 2005). The change in the potential and corrosive environment within a pit may be essential for enabling the pit to perform the function of a crack initiator. There are a variety of examples in which the initiation of stress corrosion cracks is noticed at the base of a pit due to intergranular corrosion. The intergranular stress corrosion cracking also leads to crack propagation in these cases (Arioka, Yamada, Terachi & Staehle, 2006). It is expected that the development of same local electrochemistry is not common in the preexisting pit like in the case of one grown during service.

The initiation of stress corrosion cracks are also noticed in the absenteeism of pitting by slip- dissolution or intergranular processes (Arafin & Szpunar, 2009). For this, the requirement arises for differences in bulk chemistry and grain- boundary chemistry (Knight, Birbilis, Muddle, Trueman & Lynch, 2010). Such conditions are mostly experienced by sensitized austenitic stainless steels or with impurities segregation such as sulfur, phosphorus or silicon in various materials (Meng, Zhang, Zhuang & Zhang, 2016).

The propagation’s subcritical nature may be credited to the release of chemical energy as the crack propagates. This can be presented in the following equation form:

Release of elastic energy + chemical energy = deformation energy + surface energy

The damage is caused at a slow rate as SCC is a delayed failure process. The appearance of SCC is observed in environment/ alloy combinations resulting in the creation of a film on the surface of metal. These films may be in the form of tarnish films, passivating layers or dealloyed layers. The rate of uniform or general corrosion is subsequently reduced due to these films which in turn make the alloy necessary for battling against uniform corrosion in the environment. 

Carbon Steel and Stress Corrosion Cracking

The combination of three factors results in stress corrosion cracking namely exposure to a corrosive environment, a susceptible material and tensile stresses above edge. The initiation of stress corrosion cracking becomes impossible if any of such factors are eliminated. However, a variety of approaches can be utilized for the purpose of preventing or delaying the onset of SCC. Firstly, the control strategy needs to be selected for stress corrosion cracking that will initiate its operations at the design stage. It will further focus on the material selection, stress limitation and control of the environment. The development of new alloys that are more resistant to stress corrosion cracking can be chosen as a conventional approach for the purpose of controlling the problem. However, this proposition is costly and requires enormous time investment for achieving only marginal success (Nugent & Khan, 2014).

For the purpose of controlling stress corrosion cracking, awareness needs to be created regarding the possibility of its occurrence at the design and construction stages. The problems related to stress corrosion cracking can be avoided if due attention is paid at the time of making selection of materials. Those materials should be chosen that are not susceptible to stress corrosion cracking in the service environment and their correct processing and fabricating should be ensured (Je & Kimura, 2014). High temperature water is the example of environments that are aggressive and result in causing stress corrosion cracking of most materials.  High yield strength and other mechanical requirements can create difficulty in reconciling with SCC resistance.

The existence of stress in the components causes stress corrosion cracking. Therefore, in order to control SCC, the method adopted should consist of eliminating or reducing that stress below the starting point of SCC (Seong, Frankel & Sridhar, 2015).

Controlling the environment is considered as the straightest way of controlling stress corrosion cracking by removing or replacing that particular component of environment that is liable for the difficulty. When the cracking is the result of species that are the necessary components of the environments then the options related to environment control consists of addition of inhibitors, modification of the electrode capability of the metal or creating isolation for the metal from the environment with the help of coatings.

There are several ways with the help of which ammonia stress corrosion can be prevented in steel equipment. Greater susceptibility is faced by the systems that have not undergone postweld heat treatment (PWHT). Therefore, there is a requirement for proper PHWT in order to prevent ammonia stress corrosion cracking. The cracking of wheel can be easily obstructed with the help of adding a little quantity of water i.e. nearly 0.2% to the anhydrous ammonia.

Detection of Stress Corrosion Cracking

Conclusion

Therefore, it can be concluded that ammonia stress corrosion cracking is the form of corrosion that takes place in brass tubes in cooling water service which has been contaminated by ammonia as a result of biological growths or any other contamination.  The proposal of various mechanisms has been made for stress corrosion cracking such as cathodic and anodic mechanisms and crack propagation mechanism. Various materials and equipment suffer from stress corrosion cracking such as aluminium alloys, austenitic steels, polymers, elastomers, ceramics and carbon steel equipment, etc. The ammonia stress corrosion appears in the form of corrosion cracks on the surface of metal. Stress corrosion cracks are initiated at surface flaws which then grow into small macroscopic evidence of mechanical deformation in alloys and metals that are usually considered to be ductile. Furthermore, various steps can be undertaken for the purpose of prevention or mitigation of ammonia stress corrosion cracking. This includes the elimination of the factors such as corrosive environment, a susceptible material and tensile stresses above edge. Moreover, awareness needs to be created for the purpose of selection and controlling of materials along with controlling the stress and environment.

References

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