 Table of Contents ECNDT '98 Session: Chemical, Petrochemical | Controlling Ultrasonic-Inspection Intervals by Monitoring Hydrogen PermeationPeter Bo Mortensen, Curt Christensen, Carsten Peyk
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Hydrogen induced damages in the form of laminar cracking, stepwise cracking and stress oriented cracking are briefly discussed. Hydrogen permeation data are presented and compared with hydrogen concentration profiles and hydrogen induced cracking behaviour. A concept for monitoring the hydrogen permeation flux/concentration on refinery vessels in service in order to evaluate the need for ultrasonic-testing will be described.
Hydrogen in steel may have a devastating effect on strength and integrity and may result in internal fissuring, loss of ductility and cracking. Hydrogen may enter the steel at high temperature during casting, forging and welding, and it is common industrial practice to minimize possible hydrogen pick-up from the environment during these processes.
Hydrogen may be introduced into steel also at ambient temperatures as a result of electrochemical reactions such as acid pickling, plating and corrosion. In oxygen free wet H2S environments as frequently encountered in the oil and gas industry, the corrosion process involves discharging of hydrogen ions at the corroding surface and part of the liberated hydrogen atoms are absorbed into the steel substrate as interstitial atoms. Within the steel it may also segregate as molecular hydrogen at non-coherent phase boundaries (inclusions) or it may concentrate in areas of higher solubility (chemical segregation's) or high lattice dilatation (triaxial stresses).
Hydrogen may exert its damaging effects through a number of partly competitive, partly complimentary mechanisms generally known as:
In practice, the corrosion induced hydrogen damage is referred to as sulphide stress cracking, SSC, and hydrogen induced cracking, HIC. The latter encompasses stepwise cracking, SWC, and stress oriented hydrogen induced cracking, SOHIC. The types of damage are schematically shown in Fig 1.
 Fig1: Three typical hydrogen induced damages |
Laboratory work and in-service experience suggest the existence of a threshold hydrogen permeation flux/concentration, below which no damage is observed. This threshold value is specific to the steel, its microstructure and applied stress/strain. Quantification of the hydrogen flux/concentration can be of great value, not only as a tool to determine specific threshold values in laboratory work prior to service, but also as in-service monitoring to assess the applicability of a given steel to a given environment.
Interstitially absorbed hydrogen atoms are highly mobile in steel. Unless trapped the atom is free to diffuse through the wall and to exit at the outer steel surface. The driving force is concentration gradients as for instance set-up between the internal surfaces with high hydrogen activity and the external surface where the hydrogen activity can be virtually zero. In a homogeneous steel microstructure the local concentration falls linearly with distance below the hydrogen entry surface.
The hydrogen concentration in steel is not easy to measure directly. It is possible, however, to measure it indirectly by hydrogen permeation flux measurements using the dry contact non-intrusive hydrogen probes developed at the FORCE Institute. The hydrogen concentration, CHo at the hydrogen entry surface relates to the permeation flux as illustrated in Fig 2. The diffusion coefficient necessary to calculate the CHo may be determined by either time lag considerations, or initial risk characteristics, but depending on the method used, some scatter in coefficient may be expected. In work aiming at determining the threshold hydrogen concentration for a given steel or the applicability of a steel to a given environment the diffusion coefficient may be considered as a specific material constant thus using hydrogen permeation current density and wall thickness (diffusion length) as discriminators without the need to determine the diffusion rate.
 Fig 2: Relationship between hydrogen concentration and hydrogen permeation |
On its way through the steel the hydrogen may segregate as molecular hydrogen at non-coherent phase boundaries as previously mentioned. Gas filled voids are formed and the pressure in these voids equilibrates the concentration of hydrogen in the surrounding matrix [5] [6]. Based on Sieverts Law the hydrogen gas pressure, pH, can be expressed as
As CH beneath the hydrogen entry surface may be 10-100 times higher than LH it follows that the hydrogen pressure may reach 102-104 bar. Such high pressures are certainly capable of separating the phase boundaries and set-up shear stresses in the surrounding material to such an extent that local yielding and cracking may take place along sensitive planes [7] [8]. Cracking is further enhanced by the supersaturated hydrogen in the matrix and also by the fact that the non-coherent phase boundaries (inclusions) tends to lie in bands of low temperature transformation microstructures (bainite and martensite islands), associated with segregations. Such microstructures are known to be more susceptible to hydrogen stress cracking than the ferritic matrix. The applied and residual stresses interact with the stress fields around hydrogen pressure induced cracks, thus contributing to cause the resolved shear stress to exceed the yield strength. Yielding is characterized by dislocation movements (slip) along activated slip planes, hence, in addition to the lattice diffusion, hydrogen can also be transported to the crack tip area by dislocation sweeping mechanisms. This enhances link-up cracking in regions intersected by the activated slip planes between otherwise small and individually non-significant initiation sites for hydrogen induced blister cracks.
As mentioned in the introduction all steel seems to have a threshold value of hydrogen concentration, CHth, below which HIC is unlikely to occur [9] [10] [11]. Due to local variations in inclusion morphology and density, microsegregation and centreline segregations etc., the threshold hydrogen concentration need not to be homogeneous across the pipe wall [9] [12] [13] [14] [15]. A possible variance in threshold value is depicted by the dashed lines shown in Fig 3. By superimposing the possible throughwall hydrogen concentration profile, as it could develop with time in e.g. immersed metallic clean samples and single side exposure descaled steel panels in NACE TM O1 77 solution, it is evident that the risk of cracking is higher in immersion exposure (i.e. laboratory tests) than in single side (in-service situation) exposure. As the hydrogen concentration varies across the pipe wall, and the possible hydrogen pressure in voids decreases exponentially with the concentration (Sieverts Law), the propensity for pressure induced cracking is highest near the corroding surface and so is the propensity of forming blisters [16]. It is also apparent that both the peak height of the hydrogen permeation current and the decay time can be very decisive in relation to cracking [17]. Both of these factors are influenced by the steel chemistry, microstructure and surface film forming kinetics. In case of mill scale covered or prerusted surfaces the hydrogen uptake in the steel may be so suppressed that HIC is not possible.
 Fig 3: Hydrogen concentration profile in steel wall |
If during a high hydrogen concentration surge, small hydrogen induced pressure cracks are formed, these may progress also after the surge as the decayed hydrogen concentration may still be sufficient to propagate HIC in the resulting local stress-strain field around the cracks [7] [8] [18].
In service the condensed water phase of a wet sour gas will contain C02 and H2S in proportion to their partial pressures in the gas and their solubility in the aqueous phase at the prevailing temperature. Typically, the initial pH can be low, i.e. in the range of 3.0 to 3.5 for a gas containing 30 bar partial pressure of C02 and 5 bar of H2S at 20°C. In absence of hydrocarbon condensate or other organic compounds the freshly formed condensate is corrosive towards steel [9] [15].
Corrosion of steel in oxygen free environments results in liberation of atomic hydrogen at the corroding surface and subsequent recombination to molecular hydrogen gas. In the presence of sulphide (and other "promoters" like cyanides and arsenic compounds) a substantial part of the atomic hydrogen becomes absorbed into the steel substrate, where it saturates available interstitial vacancies and build up high concentrations of diffusible hydrogen. The ability to maintain a high hydrogen activity and concurrently high hydrogen concentrations in the steel matrix depends on the local electrochemical potential which again depends on the solution chemistry, presence of soluble corrosion products and film forming characteristics of the actual steel. As a result of the ongoing corrosion the aqueous phase close to pipe wall will be enriched in soluble corrosion products (ferrous carbonates, sulphides). The near wall pH is increased due to the electrochemical reactions [19], and eventually the solubility of the ferrous corrosion products is exceeded resulting in the precipitation of the corrosion products. Hydrogen permeation studies in sour environments have shown that the H-uptake rate diminishes not only with increase in pH but also with the formation of sulphide films at the surface as this results in lower corrosion rate [5] [15] [20].
The basis of this concept mentioned in the title of this paper is the continuous use of hydrogen permeation probes and periodically ultrasonic scanning. Until now it has been standard practice only to do ultrasonic examinations with fixed intervals but with this concept it is the intention to evaluate the need for ultrasonic examinations based on the hydrogen permeation measurements and on a longer term to reduce the number of ultrasonic examinations if the hydrogen permeation in periods is low as indicated in Fig 4. The first approach has been to apply the concept to an old vessel that is known to have hydrogen induced damages.
From numerous experiments using the hydrogen permeation probes on pressure vessels and pipelines exposed to simulated sour service it has been found that a connection between environment severity and hydrogen induced damages exist for a given steel. The quantitative relation between the two has not been studied in these experiments but will be included in the first phase of this concept.
 Fig 4: The risk of hydrogen induced damage as function of H-flux |
A minimum of 1 temperature probe and 5 hydrogen permeation probes are mounted on the vessel in selected positions. These positions should be chosen in an area where damages are known to exist or expected to occur, but in a way so that existing laminar hydrogen caused defects are not shielding the hydrogen permeation probes. The use of these hydrogen permeation probes in laboratory work and in-service has shown that large variations in hydrogen permeation flux may occur in neighbouring locations e.g. and it is therefore crucial that the correct location for the hydrogen permeation probes is selected. The easy mounting procedure for the probes gives the advantage that the probes can be moved to new locations in order to select the most critical position.
The measured current on the individual hydrogen permeation probes is collected and stored in a multi purpose battery powered data logger with capacity for storing approximately 30,000 data points. With the expected relatively slow variations in hydrogen permeation the storing capacity will be sufficient for at least one month of unattended service. For the final data processing and presentation of the data a conventional PC with a spreadsheet can be used.
The hydrogen permeation measurements may be launched at any convenient time but preferably not later that 1 to 2 months before a planned shut down. By doing so the hydrogen permeation is monitored during shut-down and during re-start of the equipment which previously have shown to be two periods that may have significant influence on the hydrogen damages.
During normal operation an effort can be done to keep the H-flux at a sufficient low level. As the solubility of hydrogen decreases with temperature the risk of hydrogen induced damages increases at shut-down. By comparing the results from ultrasonic examinations done shortly before and after a shut-down and re-start it has been indicated that such an action can cause a development in the number and sizes of hydrogen induced damages. Mounting the probes before a planned shut-down is therefore found to be convenient in order to collect results that may help explaining this situation.
In the first phase on old vessels where the concept has been implemented and the results have been accepted as valuable information. In this period the ultrasonic examinations are continued as normal. The results of the ultrasonic examinations are then to be compared with the hydrogen permeation measurements in order to empirically establish the relation between the hydrogen permeation and the development in hydrogen induced damages. The use of ultrasonic examination for mapping the hydrogen induced damages will therefore be continued as previously until a connection between hydrogen permeation and damages has been established.
As the hydrogen permeation probes are mounted on the outer surface of the vessel the wall thickness will evidently introduce a time lag between the hydrogen activity on the inner surface and the measured hydrogen at the outer surface as it was illustrated on Fig 2. To compensate for the possible errors this time lag may introduce in the interpretation of the results, a different type of hydrogen probe can be inserted flush with the inner surface. This type of probes are also sealed Devenathan cells, but they are in sealed steel cases with a thin steel membrane of the experimental steel. Because of the thin steel membrane the time lag measured with this type of cell are neglectable and it is therefore suitable as reference cell in this set-up.
Having collected hydrogen permeation data for a period of time including a number of ultrasonic examinations it is the intention in a second phase to prolong (or if needed shorten) the ultrasonic examination interval according to the hydrogen permeation level in the past period. This can be done if a critical hydrogen permeation level has been established during the first phase. The number of needed shut-downs for the individual vessels are therefore expected to be reduced leading to a more safe and cost effective production in the long run.
At the present moment the concept has only been employed in practice for a short time and the on-site results are therefore sparse, but when sufficient results are collected it is the intention that they should be published, too. In the near future it is the intention that the concept shall be used on a new vessel which combined with laboratory examinations of the vessel steel gives an excellent possibility to avoid the most severe hydrogen damages.
The significance of hydrogen concentration in the steel and the effect of environment severity have been discussed.
A concept has been described that gives the possibility of determining the optimal inspection intervals for the ultrasonic examinations of refinery vessels. The concept is based on the use of on-line hydrogen permeation measurements with non-contact hydrogen probes on the outer surface of the vessel and if possible direct measurements of environment severity by intrusive hydrogen probes.
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