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Prevention of Hydrogen Embrittlement in Steels

Prevention of Hydrogen Embrittlement in SteelsH. K. D. H. BhadeshiaaaMaterials Science and Metallurgy, University of Cambridge, essential facts about the nature of the Hydrogen Embrittlement of steelshave now been known for 140 years. It is diffusible Hydrogen that is harmfulto the toughness of iron. It follows, therefore, that the harmful influence ofdiffusible Hydrogen can be mitigated by preventing its entry into steelorbyrendering it immobile once it penetrates the material. This review dealswiththe methods that might be implemented to design Steels and componentsthat resist Hydrogen Embrittlement by reducing the intake of Hydrogen orrendering it innocuous when it does penetrate the : Hydrogen Embrittlement , Hydrogen trapping, hydrogenpermeation, steel design, diffusion barriers, coatings1.

tendency for micropitting is reduced by the presence of black oxide during rolling-sliding wear tests, when compared to untreated steel [25]. Insum-4. The diffusion coefficient is a familiar mass transport parameter defined using Fick’s law and with units of m. 2. s. −1. Permeability is related to flow through a film and has units of ...

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Transcription of Prevention of Hydrogen Embrittlement in Steels

1 Prevention of Hydrogen Embrittlement in SteelsH. K. D. H. BhadeshiaaaMaterials Science and Metallurgy, University of Cambridge, essential facts about the nature of the Hydrogen Embrittlement of steelshave now been known for 140 years. It is diffusible Hydrogen that is harmfulto the toughness of iron. It follows, therefore, that the harmful influence ofdiffusible Hydrogen can be mitigated by preventing its entry into steelorbyrendering it immobile once it penetrates the material. This review dealswiththe methods that might be implemented to design Steels and componentsthat resist Hydrogen Embrittlement by reducing the intake of Hydrogen orrendering it innocuous when it does penetrate the : Hydrogen Embrittlement , Hydrogen trapping, hydrogenpermeation, steel design, diffusion barriers, coatings1.

2 IntroductionIn the year 1875, Johnson [1] revealed extraordinary changes in the tough-ness and breaking-strain of iron that was immersed temporarily in acid forjust a few minutes. He further observed that the change is not permanentsince with the lapse of time, the metal slowly regains it original toughnessand strength . Indeed, he went on to observe that the moistenedfracturesur-face of an embrittled steel liberated gas bubbles ( frothing , withthe bubbleseven seen under oil). The same paper found that a strong steel hasagreaterloss in toughness due to immersion in acid than one which is soft. A numberof acids were studied and only those that produce Hydrogen by their actionon iron were found to lead to a deterioration of properties.

3 Furthermore, thehydrogen had to be nascent, not molecular, since leaving it in Hydrogen gasdid nothing to the properties of the steel. To separate out the effect of acidand Hydrogen , he designed an electrochemical charging method using onlyPreprint submitted toJanuary 18, 2016 Manchester town s water, and proved that the iron electrode where hydrogenis liberated embrittled whereas the other one did paper by Johnson is a wonderful example of proper scientific method,which with elegantly simple experiments captured the essence of theembrit-tlement phenomenon, many aspects of which are rediscovered withmuchfanfare in modern literature. The following conclusions can justifiably bereached from this 1875 paper:1.

4 It is Hydrogen that embrittles steel, not the acid;2. that the Hydrogen is nascent, not molecular;3. it isdiffusiblehydrogen that embrittles1, so the phenomenon is re-versible;4. the effusion of diffusible Hydrogen from the steel leads to frothing(bub-bles);5. that stronger steel is more susceptible to Embrittlement than role of nascent Hydrogen became well-established in the fifty yearsthat followed, and unique experiments were published to relate the embrit-tlement to microstructure. For example, Pfeil [6] showed that large-grainedsamples are more sensitive to Hydrogen than those with fine structures. Hepostulated that Hydrogen decreases the cohesion across cubic cleavage planes,but does not affect slip.

5 Single crystals of iron were shown to be embrittledby Hydrogen , an effect attributed to machining strains. The detailsof Pfeil sand other contemporary work regarding cohesion or slip might be challengedin the light of modern understanding, but the 38,000 papers publishedsub-sequently on Hydrogen Embrittlement do not change the conclusionssum-marised above. One important phenomenon that emerged from diffusion1 The fact that diffusible Hydrogen embrittles is now widely recognised and forms thebasis of many designs where the transport of Hydrogen through the steel is impeded byintroducing traps [2 5, ]. Hydrogen is present in minute quantitiesin steel, usually lessthan 1 part per million, but is attracted towards stress fields of thetype associated witha crack tip.

6 It therefore diffuses there, concentrates and thereby alters the fracture mech-anism to the detriment of steel. Hence the need for diffusible Hydrogen for , is that diffusible Hydrogen can be trapped at sites such asboundaries [7].2 That which is likely to be sufficiently strongly trapped maynot harm the steel. It follows that to produce Steels that are resistant tohydrogen, all that is necessary is to control diffusible Hydrogen . This canbe done by introducing benign traps in the steel or preventing the ingressof Hydrogen . What follows below is based on this simple logic. We beginby considering methods that hinder the penetration of Hydrogen into of the coatings involved have multiple purposes, for example, aestheticappearance, retention of lubricant, etc.

7 But at the same time aresignificantbarriers to Hydrogen Black OxideThe so-called black oxide conversion-coating is generated on steelby im-mersion in an aqueous solution of 60-80% sodium hydroxide containinganoxidising agent such as 15-40% sodium or potassium nitrite or nitrateat atemperature of about 130-150 C, for 30 mins [9 11]. The final oxide ismagnetite [12], resulting from the following reaction sequence [9]:Fe2++2H2O!Fe(OH)2+2H+Fe(OH)2+OH !Fe(OH)3Fe(OH)3+OH !FeO 2+2H2 OFe2++2 FeO 2 Fe3O4(1)The oxide can also be a mixture of Fe3O4and Fe2O3. The alkaline nature ofthe solution is important because there is no Hydrogen evolved in theprocess(equation 1) which might otherwise embrittle the steel [13].

8 The originalgoal of the black oxide coating was to provide some resistance to atmosphericcorrosion and this function can be enhanced by immersion of the componentin hot oil because the thin oxide film, typically 1-3 m, can otherwise bepermeable3. The coating does not compromise the friction coefficient andhence can be used for components such as bearings, although thebenefits2 Pressouyre has suggested that even those features that repel Hydrogen should hinderits progress through the lattice [8].3It is argued in turn, that the presence of black oxide can improve the adhesion of thelubricant to the surface [11].3are not sustained under severe operating conditions [14, 15]. In many cases,black oxide coatings are introduced to enhance the aesthetic appearance ofthe product [16].

9 Black oxide has been applied to wind turbine bearings in an attempt toreduce the occurrence of axial cracks [17]. One interpretation is that theoxide retards the diffusion Hydrogen into the steel. Permeation4experimentson pure iron on which a passive oxide film was produced using an equivolumemixture of N Na2B4O7 10H2O and N NH3BO3on the anodic side ofa Devnathan and Stachurski cell, indicated a much lower influx of hydrogeninto iron that is coated [18]. The film studied was only 2-3 nm thick, somethree orders of magnitude thinner than the black oxide coatings discussedhere. Such a thin film is unlikely to be representative in the context ofthe porosity that is known to exist in black oxide coatings.

10 The detailedcomposition of the film was not stated in the original study. Nevertheless,the indications are that the diffusivity of Hydrogen (DH) in the thin oxidefilm is some twelve orders of magnitude slower than in the pure, annealediron [18]. There are no similar data for the thick black oxide films. However,when steel samples are stressed using a C-ring [19], it is claimed that thosethat are black-oxide coated and immersed in a corrosive solution faillaterthan uncoated controls; the evidence presented in support of this claim [11]is at best regarded as schematic5. Nevertheless, there is a general impressionin the industry that oxides of metals including steel, reduce the permeabilityof Hydrogen and its isotopes by at least an order of magnitude [20 22].