Stress-Corrosion Cracking, Materials Performance and Evaluation, Second Edition Copyright © 2017 ASM International® R. Jones, editor All rights reserved. www.asminternational.org CHAPTER 1 Mechanisms of Stress- Corrosion Cracking* Revised by R.H. Jones, Pacific Northwest National Laboratory (Retired) and GT Engineering STRESS- CORROSION CRACKING (SCC) multaneous interaction of mechanical and is a term used to describe service failures in chemical forces results in crack propagation, engineering materials that occur by slow, envi- whereas neither factor acting independently or ronmentally induced crack propagation. The ob- alternately would result in the same effect. The served crack propagation is the result of the exact nature of this interaction is the subject of combined and synergistic interaction of me- numerous scientific investigations and is cov- chanical stress and corrosion reactions. ered in the section “Crack Propagation Mecha- Before SCC can be discussed in detail, several nisms” in this chapter. things must be clearly defined: the type of load- The stresses required to cause SCC are small, ing involved, the types of materials involved, usually below the macroscopic yield stress, and the types of environments that cause SCC, and are tensile in nature. The stresses can be exter- the nature of the interactions that result in this nally applied, but residual stresses often cause phenomenon. The term stress- corrosion crack- SCC failures. However, compressive residual ing is frequently used to describe any type of stresses can be used to prevent this phenome- environmentally induced or assisted crack prop- non. Static loading is usually considered to be agation. However, this discussion focuses on responsible for SCC, while environmentally in- the normal usage of the term as defined duced crack propagation due to cyclic loading is subsequently. defined as corrosion fatigue. The boundary be- One common misconception is that SCC is tween these two classes of phenomena is vague, the result of stress concentration at corrosion- and corrosion fatigue is often considered to be a generated surface flaws (as quantified by the subset of SCC. However, because the environ- stress- intensity factor, K); when a critical value ments that cause corrosion fatigue and SCC are of stress concentration, Kcrit, is reached, mechan- not always the same, these two should be con- ical fracture results. Although stress concentra- sidered separate phenomena. tion does occur at such flaws, it does not exceed The term stress- corrosion cracking is usually the critical value required to cause mechanical used to describe failures in metallic alloys. fracture of the material in an inert environment However, other classes of materials also exhibit (Kscc < Kcrit). Precorrosion followed by loading delayed failure by environmentally induced in an inert environment will not result in any sig- crack propagation. Ceramics exhibit environ- nificant crack propagation, while simultaneous mentally induced crack propagation (Ref 1.1), environmental exposure and application of and polymeric materials frequently exhibit craze stress will cause time-dependent subcritical cracking as a result of the interaction of applied crack propagation. The term synergy is used to stress and environmental reactions (Ref 1.2– describe this process because the combined si- 1.5). The mechanical properties of composites will degrade if exposure to the environment at- tacks the matrix, the reinforcing phase, or the *Originally by R.E. Ricker, National Institute of Standards and Technology, as Chapter 1, Mechanisms of Stress- matrix-to- reinforcement interface; if crack prop- Corrosion Cracking, in Stress- Corrosion Cracking, Materi- agation results during static loading, this degra- als Performance and Evaluation, ASM International, 1992. dation is SCC. Until recently, it was thought that 2 / Stress-Corrosion Cracking, Materials Performance and Evaluation, Second Edition pure metals were immune to SCC, but it is now either immune or susceptible. As a result, the list known that this is not true (Ref 1.6, 1.7). Be- of possible alloy/environment combinations that cause considerably more research has been con- cause SCC is continually expanding, and the ducted on the SCC behavior of metallic alloys, possibilities are virtually infinite. Some of the the discussion in this chapter focuses on SCC of more commonly observed alloy/environment metals and their alloys. combinations that result in SCC are listed in Environments that cause SCC are usually Table 1.1. aqueous and can be either condensed layers of In general, SCC is observed in alloy/environ- moisture or bulk solutions. Stress- corrosion ment combinations that result in the formation cracking is alloy/environment specific; that is, it of a film on the metal surface. These films may is frequently the result of a specific chemical be passivating layers, tarnish films, or dealloyed species in the environment. For example, the layers. In many cases, these films reduce the rate SCC of copper alloys, traditionally referred to as of general or uniform corrosion, making the season cracking, is usually due to the presence alloy desirable for resistance to uniform corro- of ammonia in the environment, and chloride sion in the environment. As a result, SCC is of ions cause or exacerbate cracking in stainless greatest concern in corrosion-resistant alloys ex- steels and aluminum alloys. Also, an environ- posed to aggressive aqueous environments. The ment that causes SCC in one alloy may not cause exact role of films in the SCC process is the sub- it in another. Changing the temperature, the de- ject of current research and is covered in the sec- gree of aeration, and/or the concentration of tion “Crack Propagation Mechanisms” in this ionic species may change an innocuous environ- chapter. Table 1.2 lists several alloy/environ- ment into one that causes SCC failure. Also, dif- ment combinations and the films that may form ferent heat treatments may make the same alloy at the crack tip. Table 1.1 Alloy/environment systems exhibiting 1.1 The Phenomenon of Stress- stress- corrosion cracking Corrosion Cracking Alloy Environment Carbon steel Hot nitrate, hydroxide, and carbonate/ bicarbonate solutions Stress- corrosion cracking is a delayed failure High- strength steels Aqueous electrolytes, particularly process. That is, cracks initiate and propagate at –9 –6 when containing H2S a slow rate (e.g., 10 to 10 m/s) until the Austenitic stainless steels Hot, concentrated chloride solutions; chloride-contaminated steam stresses in the remaining ligament of metal ex- High- nickel alloys High- purity steam ceed the fracture strength. The sequence of a brass Ammoniacal solutions events involved in the SCC process is usually Aluminum alloys Aqueous Cl–, Br–, and I– solutions Titanium alloys Aqueous Cl–, Br–, and I– solutions; divided into three stages: organic liquids; N O 2 4 • Magnesium alloys Aqueous Cl– solutions Crack initiation and stage 1 propagation Zirconium alloys Aqueous Cl– solutions; organic liquids; • Stage 2 or steady-state crack propagation I2 at 350 °C (660 °F) • Stage 3 crack propagation or final failure Table 1.2 Alloy/environment combinations and the resulting films that form at the crack tip Metal or alloy Environment Initiation layer a brass, copper- aluminum Ammonia Dealloyed layer (Cu) Gold- copper FeCl3 Dealloyed layer (Au) Acid sulfate Dealloyed layer (Au) Iron- chromium nickel Chloride Dealloyed layer (Au) Hydroxide Dealloyed layer or oxide High- temperature water Dealloyed layer or oxide a brass Nitrite Oxide Copper Nitrite Oxide Ammonia (cupric) Porous dissolution zone Ferritic steel High- temperature water Oxide Phosphate Oxide (?) Anhydrated ammonia Nitride CO/CO2/H2O Carbide CS2/H2O Carbide Titanium alloys Chloride Hydride Aluminum alloys, steels Various media Near- surface hydrogen Mechanisms of Stress-Corrosion Cracking / 3 The characteristics of each of these stages are pendent on a wide variety of parameters, such as subsequently discussed in greater detail. First, surface finish. The presence of flaws that con- however, the techniques used to measure SCC centrate stress or crevices that alter the environ- are reviewed briefly. Stress- corrosion cracking ment may dramatically change the threshold experiments can be categorized as: stress or the crack initiation time (Ref 1.9, 1.10). The entire crack initiation process is presently • Tests on statically loaded smooth samples • not well understood. Tests on statically loaded precracked samples Tests on statically loaded precracked sam- • Tests using slowly straining samples ples are usually conducted with either a constant More detailed information on these tests can applied load or with a fixed crack opening dis- be found in Chapter 17, “Evaluation of Stress- placement, and the actual rate or velocity of Corrosion Cracking,” in this book, and in Ref crack propagation, da/dt, is measured (Ref 1.11). 1.8. The magnitude of the stress distribution at the Tests on statically loaded smooth samples crack tip (the mechanical driving force for crack are usually conducted at various fixed stress lev- propagation) is quantified by the stress-intensity els, and the time to failure of the sample in the factor, K, for the specific crack and loading ge- environment is measured. Figure 1.1 illustrates ometry. As a result, the crack propagation rate, the typical results obtained from this type of test. da/dt, is plotted versus K, as illustrated in Fig. In Fig. 1.1, the logarithm of