Hydrogen-induced cracking and blistering
Hydrogen damage to steel is a well-known consequence of corrosion in sour service. A corrosion inhibitor program can reduce hydrogen entry into the metal. This decreases or eliminates hydrogen damage, resulting in higher profitability and plant availability
Berthold Otzisk and Agnoula Gourzoulidou, Kurita Europe
Frank Dean, Ion Science
Klaus Bernemann, Evonik Oxeno
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Corrosion costs billions of dollars worldwide. Corrosion damage may lead to a drop in production, the expense of replacing equipment, maintenance and repairs costs, safety hazards or environmental pollution.
If corrosion is slow and well identified, costs can be controlled, as end of life is predictable. Equipment replacement or rehabilitation can be planned and orchestrated years in advance, to cause minimum disruption to the normal, efficient running of the plant. Corre-spondingly, unanticipated corrosion costs are often compounded by a lack of immediately available services and materials needed for equipment rehabilitation and unplanned upsets, both upstream and downstream of the severe corrosion event. Hydrogen induced cracking (HIC) and the blistering of carbon steel are both such unexpected corrosion phenomenon.
Hydrogen damage (Figure 1) is encountered in many steel processes, such as pickling, electroplating and welding. One essential contributing feature of hydrogen damage to welds is that mobile hydrogen, by virtue of its very increased solubility and diffusivity through steel at elevated temperatures, is generated at a relatively high concentration in certain regions of the weldment (the heat affected zone). As the weldment cools, the solubility decreases, and so the activity of this mobile hydrogen increases.
However, HIC during steel service does not normally involve the cooling of steel after exposure to hydrogen at high temperatures. Usually, active, aqueous acid corrosion by hydrogen promoters — the weak acid hydrides of P, As, Sb, S, Se, Te and F — causes atomic hydrogen to enter the steel at extremely high activities. For instance, the Grabke and Reicke1 report promoted mobile hydrogen concentration levels in steels at activities of over 1000 bar1/2. That is to say, the gaseous hydrogen pressure required to generate the hydrogen in steel concentrations generated by promoters would need to be over one million bar. No wonder steel can be susceptible to hydrogen cracking. The most important hydrogen promoter industrially, and probably the most vigorous,2 is H2S, sour gas.
With regards to sour corrosion, the rate of hydrogen entry into the sub-surface of a steel subject to sour corrosion is now well known to vary as [H2S]0.2 to [H2S]0.25 (ie, only weakly with sour gas concentration3-6). It appears the promoter acts catalytically in favouring the entry of atomic hydrogen into the steel. By contrast, in acid corrosion by non-promoters, such as HCl,7 cathodically formed atomic hydrogen associates to form molecular hydrogen, which desorbs to be carried off into the process stream as â€¨hydrogen gas.
Hydrogen entry is not known to occur through the sulphide scale that frequently results from sour corrosion. Indeed, the passivation of sour corrosion by a variety of sulphide scales that form on carbon steel are a major reason why sour corrosion is not more prolific. Also, sour corrosion can be successfully controlled by inhibitors.
In the absence of inhibitors, hydrogen damage is contingent on the scale being removed due to:
— Dissolution at a pH below about 3–5 (depending on steel surface chemistry8)
— Oxidation of sulphide scale; for example, during internal inspection, followed by removal of the more soluble iron oxide product
— Erosion corrosion
— At a high pH by complexation of sulphide to form soluble thiocyanide, Fe(SCN)42-. It is doubtful that cyanide is a promoter itself. It only acts to expose steel to sour gas at a high pH.
Steel’s susceptibility to HIC depends upon hydrogen entering the steel at a certain activity and migrating through it. Migrating hydrogen atoms may encounter non-metallic inclusions at the steel centre line and lattice defects, where they segregate and recombine to form molecular hydrogen. Eventually, sufficient pressure forms inside a micro-crack for it to propagate, forming a hydrogen-induced crack discernable by ultrasonic testing. The cracks normally run parallel to the steel surface, which is, therefore, the favoured direction of stress propagation. Apart from the presence of sulphides such as MnS, steel’s susceptibility to cracking is influenced by the banding of grains, also running parallel to the surface.
As cracks elongate and join up, the hydrogen activity required to propagate them further decreases, and eventually the metal will deform to produce a discernable blister on the steel’s exterior. Such large blisters should not be confused with fish eye-type blisters forming near the surface of relatively soft carbon steels subject to, say, sour gas corrosion.
HIC is observed in steels with high tensile strength, which may be high-carbon steels or non-stainless alloy steels. Another variant of hydrogen-related corrosive damage — sulphide stress corrosion cracking — affects high- strength steels, in which hydrogen migrates to a crack formed on the steel’s surface.
HIC may occasionally be delayed beyond the normal timescale of a few hours to a day, to allow for the diffusive migration of hydrogen into and, if not trapped, out of the steel. A high hydrogen entry activity is required, but, in addition, the affected steel is subject to high internal or external stress. Signs of a need for delayed HIC are notches on the work piece’s surface or its interior, where hydrogen has concentrated in elastically widened lattice areas.
Hydrogen-induced damage can reveal fracture surfaces that cannot be easily distinguished from cleavage fractures. Intergranular cleavage fractures are often difficult to distinguish from cracks that have been formed by intergranular stress corrosion cracking. In addition, there is the possibility of confusion with intergranular fatigue cracks or intergranular hot cracks.
Corrosion measurement and control
Low carbon and low alloy steels are the preferred materials for most pipes and vessels in the oil industry. Typically, corrosion rates are quoted in units of mm/year or mils/year wall loss, a severe corrosion rate being 1 mm/yr or 40 mil/yr. Corrosion measurement is crucial at all stages of oil production and refining. The most direct measurement of corrosion rate is wall thickness loss, determined by ultrasonic thickness (UT) testing. These small handheld instruments measure the back wall echoes to determine the thickness of the metal.
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