Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Nuclear Engineering and Design 129 (1991)331-339

331

North-Holland

Behavior of pressure-vessel steels exposed to hydrogen from corrosion and environment

H. Pi r che r Thyssen StahlAktiengesellschaft, Duisbur~Germany

Received7 August 1989, revisedversion5 March

1990

Examples reflecting materials performance under service conditions are used to illustrate the potential deterioration of pressure-vessel steels exposed to hydrogen under varying corrosion and stress conditions. The fundamental reactions of hydrogen attack at elevated temperatures and pressures, of hydrogen-inducedcracking, and of various types of hydrogen-in- duced stress corrosion cracking are discussed. Steel selection possibilities, constructional and processing parameters, corrosive-mediuminfluences, and protectivemeasures are presented.

1. InWoduction

cracking (HIC) or hydrogen-induced stress corrosion cracking (HSCC).

Under service conditions, some metallic materials tend to pick up hydrogen as a result of reactions with environmental media. For steels with a bcc structure, i.e. with a ferritic, ferritic-pearlitic, bainitic or marten- sitic structure, this may occasionally lead unexpectedly rapidly to a deteriorat ion of the service propert ies and occasionally to a catastrophical failure. Well-known ex- amples are reflected in such problems as are caused by hydrogen at tack under pressure at temperatures above 200 °C in high-pressure reactors for ammonia produc- tion or coal hydrogenation. But, spectacular failures have also occurred as a result of hydrogen-induced corrosion during the exploitation or refining of wet, sour hydrocarbons. Hydrogen-induced corrosion of steel always takes place in two steps [1]. At first, atomic hydrogen is picked up by the steel from the surrounding media (aqueous solutions or gases). Fol lowing the concentra- tion gradient, the absorbed hydrogen then diffuses into the interior of the steel where, as the second step, hydrogen-induced cracking occurs at critical spots. Wi th respect to cracking by hydrogen from corrosion, a dif- ferentiation can be made between chemical and physical reactions. An example reflecting the first reaction is the reduct ion of cementite in C -Mn steels causing methane to originate from hydrogen at tack at elevated pressure and service temperatures [2,3]. The result of physical reactions are fissures stemming from hydrogen-induced

2. Hydrogen attack at e l evated pressure and tempera- tures above 200 ° C

An outstanding example showing the effect of hy- drogen at elevated pressure and temperature on steel is known from the Nobel-prize paper presented by C. Bosch in 1932 [2,4]. In the process of the first catalyt ic high-pressure hydrogenat ion of coal, the format ion of methane caused a plain-carbon steel fitting to undergo decarburizat ion followed by a slackening of the micro- structure and finally by brittle fracture. The causes of this type of hydrogen corrosion which is also called hydrogen at tack (HA) have been studied thoroughly, and extensive experience has been gained from analy- sing the damage sustained, from testing the materials used and from laboratory trials performed. They have been published by the American Petroleum Inst i tute in the form of the so-called Nelson diagrams; excerpts of the 1983 edition are shown in fig. 1 [3]. The higher the service temperatures and the hydrogen part ial pressures, the greater the percentage of such allowing elements as chromium, mo l ybdenum and occas iona l ly also vanadium and tungsten in steels resistant to hydrogen at tack at elevated service temperatures and pressures. Unl ike cementite, carbides containing these alloying elements present the resistance required to prevent

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