Corrosion Testing
| Alloy | 42% MgCl2 | 33% LiCl | 26% NaCl |
| Austenitic SST | |||
| Type 304L (S30403) | SCC | SCC | SCC |
| Type 316L (S31603) | SCC | SCC | SCC |
| 904L (N08904) | SCC | SCC | No Cracking |
Does chloride cause corrosion?
The presence of chlorides enhances the corrosion of materials. It aggravates corrosion conditions in stress corrosion and pitting corrosion of materials and concrete. Harmful chlorides are the cause of corrosion. Some are more corrosive than others.
How stress influences stress corrosion cracking?
- The concentration of hydrogen in the steel
- The applied tensile stress
- The stress concentration, composition and microstructure of the steel
What is the effect of stress on corrosion?
Stress Corrosion Cracking
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How to prevent stress cracking?
How to prevent stress corrosion cracking? Stress corrosion cracking can be prevented through: Avoid the chemical species that causes SCC. Control of hardness and stress level (residual or load). Introduce compressive stress by shot-peening for example. Use of materials known not to crack in the specified environment.
What do you mean by stress corrosion cracking?
Stress corrosion cracking (SCC) is defined as the growth of cracks due to the simultaneous action of a stress (nominally static and tensile) and a reactive environment [1]. SCC is the result of the combined and synergistic interactions of mechanical stress and corrosion/oxidation reactions [2].
How do chlorides affect corrosion?
The chloride ions present in the water promote local corrosion such as crevice corrosion and pitting, and the higher the chloride content, the more corrosive the water. The chloride concentration of the water that passes through sewage treatment plants is usually lower than the chloride concentration of seawater.
What leads to stress corrosion cracking?
What causes stress corrosion cracking? Stress corrosion cracking results from the conjoint action of three components: (1) a susceptible material; (2) a specific chemical species (environment) and (3) tensile stress.
What is transgranular stress corrosion cracking?
Transgranular corrosion is a type of localized corrosion which occurs along cracks or faults across the crystals in metals and alloys. It follows the pattern of grains in the individual lattices of the material. Stress corrosion of austenitic steels is usually transgranular.
Are chlorides corrosive?
Chlorides create devastating corrosive effects on many materials. Some stainless steels, when exposed to high amounts of chlorides, will begin corroding in the form of pits. The negatively charged chloride ions are able to penetrate the passive oxide layer of stainless steel.
What is meant by chlorides?
Definition of chloride 1 : a compound of chlorine with another element or group especially : a salt or ester of hydrochloric acid. 2 : a monovalent anion consisting of one atom of chlorine.
How does chloride affect stainless steel?
Wet and humid environments containing chloride ions can cause pitting corrosion and crevice corrosion of austenitic stainless steel components. Components under an applied or residual stress can deteriorate further by stress corrosion cracking in these conditions.
What is stress corrosion cracking and how can it be avoided?
Means of reducing or preventing stress corrosion cracking are : elimination of residual stresses by stress relieving heat treatments, purification of the medium, choice of the most appropriate material, improvement of the surface condition, avoid surface machining stresses, perform peening treatments on welds to induce ...
How do you fix stress corrosion cracking?
If SCC occurs, do not attempt to repair the tank by welding the damaged area. Welding will only make the cracking worse. Unfortunately there is no way to repair SCC once it has taken place. Your best option is to first identify the conditions that lead to the corrosion, then replace the damaged equipment.
What causes transgranular fracture?
Intergranular fracture, intergranular cracking or intergranular embrittlement occurs when a crack propagates along the grain boundaries of a material, usually when these grain boundaries are weakened. The more commonly seen transgranular fracture, occurs when the crack grows through the material grains.
What does stress corrosion look like?
The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks.
What causes hydrogen embrittlement?
Hydrogen Embrittlement occurs when metals become brittle as a result of the introduction and diffusion of hydrogen into the material. The degree of embrittlement is influenced both by the amount of hydrogen absorbed and the microstructure of the material.
What causes a stainless steel to crack?
The combination of tensile stress and a specific corrosive environment can crack stainless steels. This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides. Although no stainless steel grade is totally immune to chloride SCC, ...
Which family of stainless steels is most susceptible to chloride?
The relative resistance to chloride SCC is dependant on the stainless steel family. The austenitic family of stainless steels is the most susceptible. The resistance of austenitic stainless steels to SCC is related to the nickel content of the steel.
Is stainless steel resistant to chloride?
The ferritic family of stainless steels, which includes grades such as type 430 and 444 is very resistant to chloride SCC. The duplex stainless steel with their dual austenite/ferrite microstructures has a resistance that is in between that of the austenite and ferrite grades.
Why does stainless steel crack?
Chloride cracking of 300 series stainless steels continues to occur in a number of places, including: Cracking from corrosion under insulation (CUI) which contains small amounts of chloride or where chlorides are present in the atmosphere; When a process is inadvertently contaminated with chlorides by unsuspecting people;
Why are Cl-SCC failures rare?
Fortunately, catastrophic failures from Cl-SCC are rare because of the very high toughness of stainless steel - although they can occur. The consequences from most leaks tend to be economic in nature, although this can still be devastating to some plants due to the high costs associated with replacing equipment.
What is a cl-scc?
Chloride Stress Corrosion Cracking (Cl-SCC) is a type of Stress Corrosion Cracking (SCC) and is one of the most well known forms of SCC in the refining and chemical processing industries. It can be detrimental to austenitic stainless steels, one of the main reasons these steels are not considered a cure-all for corrosion problems. Damage due to Cl-SCC is easily identifiable by the telltale spiderwebbed and lightening-array type network of highly branched cracks.
What causes cracking on startup?
When a process is inadvertently contaminated with chlorides by unsuspecting people; Equipment that is is hydrotested with chloride contaminated water and left to dry out (concentrating the chlorides into small pools of highly aggressive salt solutions), which causes cracking on startup;
Why is stress corrosion cracking?
Stress corrosion cracking can be caused by the type of material being used. This is because different materials are more/less susceptible to stress corrosion cracking than others. Poor material selection can lead to stress corrosion cracking due to the material being susceptible to SCC in the corrosive environment that it is operating in.
Does stress corrosion prevent cracking?
This prevents stress corrosion cracking from occurring. However, this can be a very difficult factor to control if corrosive species are naturally present in the environment where the material is located, for example austenitic stainless steels in seawater. Other methods of prevention include controlling the temperature to ensure ...
What is CLSCC mechanism?
The mechanism of CLSCC is complex and the current understanding is discussed in Section 5 of this report. Essentially CLSCC involves a combination of the electrochemistry of metal dissolving over a highly localised area, i.e. at the base of a pit or crevice, and microstructural
What were the cracks in the reactor vessel?
Cracking in the reactor vessel was found to be extensive. Through-wall cracks were found around three nozzles in the vessel base; in two cases the cracks extended around 2/3rds of the circumference. There were also four positions of through-wall cracking on the top main circumferential seam. It was considered that the structural integrity of the vessel was severely compromised.
What is CLSCC propagation?
CLSCC propagation occurs when cracks grow more quickly from the pit or crevice than the rate of corrosion.
What causes a CLSCC to be intergranular?
An austenitic stainless steel is described as being sensitised if a heat-treatment or welding procedure allows holding or slow cooling between 9000C and 4800C. Holding or slow cooling through this temperature range leads to chromium depletion at grain boundaries because chromium carbides are formed. Sensitisation impairs corrosion resistance significantly and leads to intergranular attack in a wide variety of environments that would otherwise cause little deterioration. The intergranular attack is caused by the chemical and physical differences between the grain boundaries and the bulk material. These differences are the result of precipitation of carbides, depletion of alloying elements, and segregation of impurities at the grain boundaries. Sensitisation is therefore very detrimental to CLSCC susceptibility.
What is a CLSCC?
CLSCC requires the presence of a residual or applied tensile stress and the presence of chloride ions under specific environmental conditions. Cracks form and propagate with time at approximately right angles to the direction of the tensile stress. Cracks propagate at stress levels much lower than those required to cause normal tensile failure. Cracks that run across the grain boundaries are called “transgranular” and those that follow the grain boundary and are termed “intergranular”. CLSCC normally progresses until the remaining ligament of material separates by ductile overload fracture. Hence the fracture surface will contain both areas of brittle fracture that are characteristic of CLSCC as well as microvoid coalescence.
How to prevent CLSCC?
The main options for preventing or controlling CLSCC are management of chloride levels, temperature, and pH. Inhibitors can also be effective where the process allows it. As noted above, electrode potential has a major effect on CLSCC susceptibility and while positive potentials increase cracking, lowering the potential can prevent cracking by cathodic protection [29]. Wrapping with aluminium foil or coating with thermally sprayed aluminium are established techniques for preventing external CLSCC when the pipework or vessels are insulated [76]. Austenitic stainless steel heat exchanger tubes are also thought to be cathodically protected from pitting and CLSCC when used with carbon steel tube plates and shells [51].
Does hydrogen sulphide affect CLSCC?
In the absence of hydrogen sulphide, these chloride concentrations would not cause cracking at ambient temperature.
What is stress corrosion cracking?
While rare, Stress Corrosion Cracking (SCC) offers very little in terms of visual hallmarks, material deformation, or other common detection methods. This means that metal which otherwise looks shiny and in excellent condition can suddenly catastrophically fail with little to no warning. Fortunately, while detection often requires complex ...
Can corrosion be detected before failure?
Many forms of corrosion progress in an easy-to-detect manner, causing visual distortion, discoloration, cracking, and more. This means that detection often occurs well before the point of failure. While resolving the solution might be costly and inconvenient, it’s typically something for which a company can make plans and correct ...
Can stainless steel be contaminated with chlorine?
Austenitic stainless steels are particularly susceptible to hot, concentrated chloride solutions and chlorine-contaminated steam. However, duplex steels — with their blend of austenitic and ferritic metallurgic composition — can typically withstand higher temperatures before succumbing to SCC attack.
