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Hydrogen Embrittlement

It is the process by which various metals, most importantly high-strength steel , become brittle andfracture following exposure to hydrogen . Hydrogen embrittlement is often the result of unintentional introduction of hydrogen into susceptible metals during forming or finishingoperations and increases cra All steels are affected by hydrogen, as is evidenced by theinfluence of hydrogen on corrosion fatigue crack growth, and the occurrence of hydrogen-inducedcracking 5 under the influence of very high hydrogen concentrations. However,hydrogen embrittlement under static load is only experienced in steels of relatively high strength.There is no hard-and-fast limit for the strength level above which problems will beexperienced, as this will be a function of the amount of hydrogen in the steel, the applied stress, theseverity of the stress concentration and the composition and microstructure of the steel. As a roughguide hydrogen embrittlement is unlikely for modern steels with yield strengths below 600 MPa,and is likely to become a major problem above 1000 MPa.cking in the material.of routes, including welding, pickling, electroplating, exposureto hydrogen-containinggases and corrosion in service. The effects of hydrogen introduced into components priortoservice may be reduced by baking for a few hours at around 200 C. this allows some of

the hydrogen to diffuse out of the steel while another fraction becomes bound torelativelyharmless sites in the microstructure

Hydrogen embrittlement is also used to describe the formation of zircaloy hydride. Use of the term in this context is common in the nuclear industry .

The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures,the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen candiffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atomsre-combine in minuscule voids of the metal matrix to form hydrogen molecules, they createpressure from inside the cavity they are in. This pressure can increase to levels where the metal hasreduced ductility and tensile strength up to the point where it cracks open ( hydrogen induced cracking , or HIC). High-strength and low-alloy steels, nickel and titanium alloys are most susceptible. Austempered iron is also susceptible. [citation needed ] Steel with an ultimate tensile strength of less than 1000 MPa or hardness of less than 30 HRC are not generally considered susceptible to

hydrogen embrittlement.

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Stress Corrosion Cracking

Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually fallsbetween dry cracking and the fatigue threshold of that material. (SCC) is the growth of

cracks in a corrosive environment. It can lead to unexpected sudden failure of normallyductile metals subjected to a tensile stress , especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC onlywhen exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metalotherwise. Hence, metal parts with severe SCC can appear bright and shiny, while beingfilled with microscopic cracks. This factor makes it common for SCC to go undetected priorto failure. SCC often progresses rapidly, and is more common among alloys than puremetals. The specific environment is of crucial importance, and only very smallconcentrations of certain highly active chemicals are needed to produce catastrophiccracking, often leading to devastating and unexpected failure .[1]

The stresses can be the result of the crevice loads due to stress concentration , or can becaused by the type of assembly or residual stresses from fabrication (e.g. cold working); theresidual stresses can be relieved by annealing .

Austenitic stainless steels suffer from stress corrosion cracking in hot solutions containingchloride. A high chloride concentrationis required, although relatively small amounts of chloride resufficient at heated surfaces, where chloride concentration canb occur, or wherechloride is concentrated by pitting or crevicebcorrosion, and problems can be experiencedin tap water

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Galvanic Corrosion

Dissimilar metals and alloys have different electrode potentials and when two or morecome into contact in an electrolyte a galvanic couple is set up, one metal acting as anodeand the other as cathode. The potential difference between the dissimilar metals is the

driving force for the accelerated attack on the anode member of the galvanic couple. Theanode metal dissolves into the electrolyte, and deposition is formed on the cathodic metal.

The electrolyte provides a means for ion migration whereby metallic ions can move fromthe anode to the cathode . This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of an electrolyte and a electronic conducting path between themetals is essential for galvanic corrosion to occur.

Cathodic protection (CP) is a technique used to control the corrosion of a metal surface bymaking it the cathode of an electrochemical cell .[1] The simplest method to apply CP is by

connecting the metal to be protected with another more easily corroded "sacrificial metal " to actas the anode of the electrochemical cell. For structures where passive galvanic CP is notadequate, including long pipelines , an external power source provides the current. Cathodicprotection systems are used to protect a wide range of metallic structures in variousenvironments. Common applications are; steel water or fuel pipelines and storage tanks ; steelpier piles ; ships and boats; offshore oil platforms and onshore oil well casings and metalreinforcement bars in concrete buildings and structures.

Cathodic protection can, in some cases, prevent stress corrosion cracking .

Pipelines

An air cooled cathodic protection rectifier connected to a pipeline.

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Pipelines are routinely protected by a coating supplemented with cathodic protection. An ICCPsystem for a pipeline would consist of a DC power source, which is often an AC poweredrectifier and an anode, or array of anodes buried in the ground (the anode groundbed ).

The DC power source would typically have a DC output of between 10 and 50 amperes and 50

volts , but this depends on several factors, such as the size of the pipeline. The positive DC outputterminal would be connected via cables to the anode array, while another cable would connectthe negative terminal of the rectifier to the pipeline, preferably through junction boxes to allowmeasurements to be taken .[14]

Anodes can be installed in a vertical hole and backfilled with conductive coke (a material thatimproves the performance and life of the anodes) or laid in a prepared trench, surrounded byconductive coke and backfilled. The choice of grounded type and size depends on theapplication, location and soil resistivity .[15]

The output of the DC source would then be adjusted to the optimum level after conducting

various tests including measurements of electrochemical potential .

It is sometimes more economically viable to protect a pipeline using galvanic anodes. This isoften the case on smaller diameter pipelines of limited length .[16]

Crevice corrosion

It refers to corrosion occurring in confined spaces to which the access of the working fluid fromthe environment is limited. These spaces are generally called crevices. Examples of crevices aregaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spacesfilled with deposits and under sludge piles.

Crevice corrosion of type 316 stainless steel

This photo shows that corrosion occurred in the crevice between the tube and tube sheet (bothmade of type 316 stainless steel) of a heat exchanger in a sea water desalination plant .[1]

The corrosion resistance of a stainless steel is dependent on the presence of an ultra-thinprotective oxide film (passive film) on its surface, but it is possible under certain conditionsfor this oxide film to break down, for example in halide solutions or reducing acids. Areas

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where the oxide film can break down can also sometimes be the result of the waycomponents are designed, for example under gaskets, in sharp re-entrant corners orassociated with incomplete weld penetration or overlapping surfaces. These can all formcrevices which can promote corrosion . To function as a corrosion site, a crevice has to be of sufficient width to permit entry of the corrodent, but narrow enough to ensure that the

corrodent remains stagnant. Accordingly crevice corrosion usually occurs in gaps a fewmicrometres wide, and is not found in grooves or slots in which circulation of the corrodent is possible. This problem can often be overcome by paying attention to the design of thecomponent, in particular to avoiding formation of crevices or at least keeping them as openas possible. Crevice corrosion is a very similar mechanism to pitting corrosion ; alloysresistant to one are generally resistant to both. Crevice corrosion can be viewed as a lesssevere form of localized corrosion when compared with pitting. The depth of penetrationand the rate of propagation in pitting corrosion are significanatly greater than in crevicecorrosion.

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