Corrosion of stainless steel by hot caustic research using solutions of chemically pure caustic sodium hydroxide naoh led to the development of a diagram that attempts to delineate the param eters of concentration and tem perature governing stress corrosion cracking scc of type. One needs to ensure that there is good distribution of caustic by using an injection quill in the hot process water line. Case histories are given and alternative alloys are suggested. Stresses can be either residual weld,cold working or applied stresses. Stress corrosion cracking international institute of. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel. Caustic stress corrosion may concentrate in such regions as the water evaporates rapidly, but sufficient concentration of caustic by such a mechanism to induce stress. The problem of stress corrosion cracking scc, which causes sudden failure of metals and other materials subjected to stress in corrosive environments, has a significant impact on a number of sectors including the oil and gas industries and nuclear power production. Or a structure under static tensile stress, much below the yield stress, in contact with corrosive environment may fail due to scc. Specimens were obtained from samples of ruptured apollo fuel tanks and from virgin solutiontreatedandaged titanium alloy sheet.
Corrosion doehdbk1015193 specialized corrosion rev. Stress corrosion cracking scc is a progressive fracture mechanism in metals that is a result of the simultaneous interaction of a corrodent and a sustained tensile stress. Structural failure due to scc is often sudden and unpredictable, occurring after as little as a few hours of exposure, or after months or even years of satisfactory service. Incorporates findings of a 1999 survey as they apply to refinery applications.
Prior to the discovery of this new form of cracking, scc was thought to be a relatively wellunderstood, intergranular. Stress corrosion cracking of duplex stainless steels in. Effects of chemical compounds on the stress corrosion. A case study on caustic corrosion in refinery piping. Stress corrosion cracking is the cracking of a susceptible metal under the mixed influence of a tensile stress either residual or applied in a corrosive environment 46. Caustic corrosion is a type of corrosion that occurs in metal equipment e. Caustic injection may occur in the slurry preparation area of anore preparation plant opp as caustic is often injected into the hot process water stream prior to the primary mix box. The useful safe limit of carbon steel is approximately 150 o f65 o c, both with regard to caustic stress corrosion cracking cscc and corrosion. However, many dss grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfidecontaining caustic. Stresscorrosion cracking scc is a term used to describe service failures in engineering materials that occur by slow, environmentally induced crack propagation. Analysis, interestingly, revealed that the observed caustic stress corrosion cracking cscc was not directly related to process parameters.
Stress corrosion cracking scc is the cracking induced from the combined influence of tensile stress and a corrosive environment. Caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 482f 250c. The cracks reduced the vessel material toughness, which eventually led to large flaws resulting in the catastrophic failure. Caustic embrittlement is mainly caused by a caustic environment, stress levels and temperature. Mild steels steels with low carbon and low alloy content and stainless steels will crack if they are exposed to concentrated caustic high ph environments with the metal under a tensile stress. All samples were extracted from known operating conditions from the field failures. Stress corrosion cracking behavior of nickel base alloys. This due to high concentration of the caustic soda added to the boiler water to control ph ph 9. Temperature is a significant environmental factor affecting cracking.
This type of failure is known as stress corrosion cracking, often abbreviated to scc. Their geometry is such that if they grow to appropriate lengths they may reach a critical size that results in a transition from the relatively slow crack growth rates associated with stress corrosion to. The observed crack propaga tion is the result of the combined and synergistic interaction of mechanical stress and corrosion re actions. Sp04032015, avoiding caustic stress corrosion cracking. Stress corrosion cracking of stainless steel components in. Environments that may cause stress corrosion cracking in nickel alloys include hot caustic solutions, high temperature water and hot and wet hydrofluoric acid. The required tensile stresses may be in the form of directly applied stresses or in the form of residual. Avoiding caustic stress corrosion cracking of refinery equipment and piping. Stress corrosion cracking of carbon steel in caustic. Designing, fabricating, andor maintaining carbon steel equipment and piping that is exposed to caustic environments. Stress corrosion cracking scc has been reported in a number of steam generator tubes of nuclear power plants in a caustic environment with or without lead species in the sludge accumulated on the top of the tube sheets. For caustic stress corrosion cracking, what is the effect of temperature and concentration mechanisms.
Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. From the chemical composition point of view, corrosion resistant nickel based alloys are categorized as pure nickel, nickelcopper alloys, nickelmolybdenum alloys, nicrmo alloys and nicrfemo alloys. The impact of scc on a material usually falls between dry cracking and the fatigue threshold of that material. The stress corrosion cracking scc behavior of cold worked mild steel in hot, aqueous, 33 pct naoh solutions was studied with prefatigue cracked double cantilever beam specimens. Corrosion under insulation cui is one of the most wellknown phenomena in the process industries, and yet it still makes up an inordinately large percentage of global maintenance expenditures. With rising temperature and increasing caustic concentrations can increase the likelihood and severity of cracking. Stress corrosion cracking caustic embrittlement of super. Duplex stainless steels dss with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical and pulp and paper mills. Caustic cracking is a form of stress corrosion cracking most frequently encountered in carbon steels or ironchromiumnickel alloys that are exposed to concentrated hydroxide solutions at. The interaction of corrosion fatigue and stresscorrosion. This result is not true in the case of stress corrosion tests where phosphorusdoped steel is more susceptible to stress corrosion cracking.
Avoiding stress corrosion cracking of carbon low alloy and. Stress corrosion cracks propagate over a range of velocities from about 103 to 10 mmh, depending upon the combination of alloy and environment involved. The stress corrosion cracking behavior in caustic solutions 200 gl sodium hydroxide, 10 gl sodium chloride of three austenitic 18cr10ni2. Potentiostatically controlled tests were conducted near the activepassive transition, using fracture mechanics testing techniques and fatigue precracked double cantilever beam specimens. Crack propagation can increase dramatically with temperature and can grow through the wall in a matter of hours during temperature excursions especially in conditions that promote caustic concentration. The paper discusses stress corrosion cracking scc of stainless steel components due to the ingress of caustic into steam systems.
Stress corrosion cracking was the likely failure mechanism that caused the cracks. Cracking can occur even at low caustic levels if a concentrating mechanism is present. The stress corrosion cracking scc behaviour of medium carbon steel mcs in caustic soda, potash and a mixture of these reagents was studied by fracturing this material on a tensometer after every seven days of immersion in these media. Sp04032015, avoiding caustic stress corrosion cracking of carbon steel refinery equipment and piping sorry this product is no longer available. Stress corrosion cracking of steel and aluminum in sodium. Although hydrogeninduced cracking and stress corrosion cracking are similar phenomena, both types of damage are usually described and treated separately. The downcomer of an incinerator suffered sever cracking and loss of quenching water. In such cases, caustic concentrations of 50 to 100 ppm are sufficient to cause cracking 4. Nace sp04032015 avoiding caustic stress corrosion cracking of carbon steel refinery equipment and piping.
Fracture mechanicsbased testing was used to quantify the stresscorrosion cracking and corrosion fatigue behavior of a precipitationhardened martensitic stainless steel custom 465h950 in. This report outlines the likely mechanisms for scc of 316l304l in white liquor oxidizer systems, and suggests ways in which scc can be prevented. Stress corrosion cracking of 316 stainless steel in. These results therefore strongly suggest that the hydrogen embrittlement mechanism does not play a major role in stress corrosion cracking of. Caustic stress corrosion cracking, also referred to as caustic embrittlement, is a form of degradation that is caused when a component operates in a caustic environment. Had ndk conducted regular inspections, it would have discovered that the acmite coating was not protecting the vessel walls. Caustic stress corrosion cracking inspectioneering. Caustic corrosion an overview sciencedirect topics. Stress corrosion cracking scc or called as environmentally assisted cracking eac may take place at the electrochemical potential range. Scc kinetics were studied under freely corroding potentials e corr. Stresscorrosion cracking, materials performance and. Through an investigation of the field failure analysis and laboratory experiment, a study on stress corrosion cracking scc behavior of steel and aluminum was performed.
Analysis and research indicate that the cracking is due to a caustic stress corrosion cracking mechanism although its occurrence at the relatively low caustic concentrations of the digester process was unexpected. Anodic polarization curves were also determined for the steel, together with those for ni, cr and fe. Stress corrosion cracking scc is the simultaneous action of a tensile stress and an aggressive environment on an alloy system to produce a brittle fracture in a normally ductile material at stress levels far below engineering design limits. Stainless steels are more resistant to general corrosion compared with carbon steel. Consequently, caustic corrosion called caustic embrittlement more accurately, a specific kind of stress corrosion cracking was engendered on the carbon steel tubes under both concentrated naoh and tensile stress because the operational pressure of the tube side was larger than the shell side, and corrosion products including na 3 feo 3, na. Caustic stress corrosion cracking caustic embrittlement. On the mechanism of caustic stress corrosion cracking in. Cui is a subject that is wellresearched and understood. Avoiding stress corrosion cracking of carbon low alloy and austenitic stainless steels in chloride and caustic environments 1. As with other cracking mechanisms, there is a threshold stress level where cracking will not occur. The mechanism is similar to that of chloride stress corrosion.
Caustic stress corrosion cracking of mild steel springerlink. Pdf stress corrosion cracking scc of nickelbased alloys. It is a form of stress corrosion cracking characterized by surfaceinitiated cracks that propagate dramatically. Damage due to hydrogen embrittlement and stress corrosion. In stress cracking that is induced by a caustic environment, the presence of dissolved. Alloy 400 n04400 and alloy 600 n06600 also have excellent stress corrosion resistance. Caustic stress corrosion cracking of 304 ss incinerator. S by means of potentiostatically controlled slow strain rate testing techniques ssrt. Similar but accelerated laboratory test was subsequently conducted in such a way as to mimic the field failures. The role of hydrogen in the mechanism of stress corrosion cracking has been known since the 1970s so that preventative measures have since been introduced.
1509 753 376 1475 616 337 1577 455 1667 1633 559 243 388 1388 1583 1023 326 467 351 989 1212 850 226 767 1339 943 89 441 837 1412 894 722 1073 1259 759 1077 532 1334 1112