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  • 1. Chai, Guocai
    et al.
    Ronneteg, Sabina
    Kivisäkk, Ulf
    Peng, Lin
    Johansson, Sten
    Mechanisms of Hydrogen Induced Stress Crack Initiation and Propagation in Super Duplex Stainless Steels2009In: STEEL RES INT, ISSN 1611-3683, Vol. 80, no 7, p. 482-487Article in journal (Refereed)
    Abstract [en]

    Austenitic and ferritic duplex stainless steels, DSS, have recently suffered from hydrogen stress induced cracking, HISC, in subsea components with a cathodic protection. This paper provides discussions on possible HISC mechanisms. HISC initiation can occur at the ferritic grain boundaries and phase boundaries at a stress lower than the yield strength, but dominantly at phase boundaries at a stress higher than the yield strength. EBSD analysis shows that HISC in DSS results from the interaction between the dynamic plasticity by creep and hydrogen diffusion. A model on the formation of microstresses in these two phases under creep conditions is proposed, which explains why HISC occurs mainly in the ferritic phase. Discontinuous two-dimensional HISC paths were observed. The austenitic phase acts as obstacles for crack propagation. The fracture covers "valleys" and "peaks" with the cleavage ferrite and the austenite with microfacets or striations due to the hydrogen-enhanced localized-plasticity.

  • 2.
    Kivisäkk, Ulf
    AB Sandvik Steel.
    A test method for dewpoint corrosion of stainless steels in dilute hydrochloric acid2003In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 45, no 3, p. 485-495Article in journal (Refereed)
    Abstract [en]

    When acid-containing gases condense on a steel surface a phenomenon called dewpoint corrosion can occur. In this case severe attack can appear and the attack cannot be predicted from traditional immersion test data. Dewpoint corrosion can for instance be found in refinery overhead condensers and in waste incineration plants. An experimental set-up consisting of a closed glass loop has been constructed in which stainless steel samples are exposed to condensing conditions simulating a formed condensate of about 1% HCl. In the loop three stainless steels have been tested. For all three materials higher corrosion rates were found in the loop than obtained in immersion tests but in the same range as can be predicted from service experience.

  • 3.
    Kivisäkk, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
    Influence of hydrogen on corrosion and stress induced cracking of stainless steel2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Hydrogen is the smallest element in the periodical table. It has been shown in several studies that hydrogen has a large influence on the corrosion and cracking behaviour of stainless steels. Hydrogen is involved in several of the most common cathode reactions during corrosion and can also cause embrittlement in many stainless steels. Some aspects of the effect of hydrogen on corrosion and hydrogen-induced stress cracking, HISC, of stainless steels were studied in this work. These aspects relate to activation of test specimens for uniform corrosion testing, modification of a test cell for dewpoint corrosion testing and the mechanism of hydrogen-induced stress cracking.

    The results from uniform corrosion testing of superduplex stainless steels indicated that there is a large difference between passive and activated surfaces in hydrochloric acid and in lower concentrations of sulphuric acid. Hence, initial activation of the test specimen until hydrogen evolution can have a large influence on the results. This may provide another explanation for the differences in iso-corrosion curves for superduplex stainless steels that have previously been attributed to alloying with copper and/or tungsten. In concentrated sulphuric acid, potential oscillations were observed; these oscillations activated the specimen spontaneously. Due to these potential oscillations the influence of activation was negligible in this acid.

    An experimental set-up was developed for testing dewpoint corrosion of stainless steels in a condensate containing 1 % hydrochloric acid. There was an existing experimental set-up that had to be modified in order to avoid azeotroping of the water and hydrogen chloride system. A separate flask with hydro chloric acid was included in the experimental set-up. The final set-up provided reasonably good agreement with field exposures in contrary to much higher corrosion rates in the original set-up.

    Relaxation and low temperature creep experiments have been performed with several stainless steels in this work. The aim was to understand how creep and relaxation relates to material properties and the relative ranking between the tested materials. For low temperature creep with a load generating stresses below the yield strength, as well relaxation at stress levels above and below the yield strength, the same ranking with respect to changes in mechanical properties of the steel grades was found. For low temperature creep with a load level above the yield strength, the same ranking was not obtained. This effect can most probably be explained by annihilation and generation of dislocations. During low temperature creep above the yield strength, dislocations were generated. In addition, low temperature creep experiments were performed forone superduplex stainless steel in two different product forms with differentaustenite spacing in the microstructure. The superduplex material experienced low temperature creep at a lower load level for the material with large austenite spacing compared to the one with smaller austenite spacing. Also this differenceis influenced by dislocations. In a material with small austenite spacing the dislocations have more obstacles that they can be locked up against.

    Studies of the fracture surfaces of hydrogen induced stress cracking, HISC, tested duplex stainless steels showed that HISC is a hydrogen-enhanced localised plasticity, HELP, mechanism. Here a mechanism that takes into account the inhomogeneous deformation of duplex stainless steels was proposed. This mechanism involves an interaction between hydrogen diffusion and plastic straining. Due to the different mechanical properties of the phases in a superduplex stainless steel, plastic straining due to low temperature creep can occur in the softer ferrite phase. A comparison between low temperature creep data showed that for the coarser grained material, HISC occurs at the load levelwhen creep starts. However, in the sample with small austenite spacing, HISC did not occur at this load level. Microhardness measurements indicated that the hydrogen level in the ferrite was not high enough to initiate cracking in the coarser material. The proposed mechanism shows that occurrence of HISC is an interaction between local plasticity and hydrogen diffusion.

  • 4.
    Kivisäkk, Ulf
    Sandvik Materials Technology.
    Investigation of low-temperature creep and microhardness of asuperduplex stainless steel and the relationship of these properties to themicrostructure and the mechanism for HISCIn: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936Article in journal (Other academic)
  • 5.
    Kivisäkk, Ulf
    Sankvik Materials Technology.
    Significance of activation of test specimens for the determination of iso-corrosion curves of stainless steels2005In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 61, no 6, p. 602-608Article in journal (Refereed)
    Abstract [en]

    The general corrosion resistance of a corrosion-resistant material, for instance, in an acid L's often illustrated in iso-corrosion curves; these curves can be used as a basis for material selection. Therefore, it is important that the information given in one diagram can be compared with diagrams from other sources. Information about the activation of the test specimens is not always given, which makes comparison difficult. Activation is often used to investigate whether the material has the ability to repassivate. In this paper two different forms of activation were used. either with a zinc rod or in boiling hydrochloric acid (HCl). For three superduplex stainless steels, UNS S32750, UNS S32760, and UNS S32520, with similar chemistry except for their copper and tungsten content, the influence of the activation method was studied using general corrosion rate tests in HCL The results were also compared with earlier results in sulfuric acid (H2SO4). No major influence of the alloying with copper and tungsten was found in the range where the materials are resistant, i.e., when they have low corrosion rates. However, in the reducing environments-HCl and H2SO4 at low concentrations-the activation of the test specimens narrowed this range. In contrast, in H2SO4 at high concentrations, which is a more oxidizing acid, it was indicated that activation did not appear to influence the corrosion rates. An activation procedure with boiling HCl seems to give higher corrosion rates than for specimens activated with zinc. However, the influence on the range seems to be minor and not as profound as without activation. Despite the fact that activation may significantly influence the corrosion rate, not all iso-corrosion diagrams indicate whether the experimental procedure includes activation. As a result, a material can be selected on misleading results.

  • 6.
    Kivisäkk, Ulf
    et al.
    Sandvik Materials Technology.
    Chai, G.
    Sandvik Materials Technology.
    Influence of test method and relaxation on the result from stress corrosion cracking tests of stainless steels in dilute neutral chlorides2003In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 59, no 9, p. 828-835Article in journal (Refereed)
    Abstract [en]

    Recently, as a part in the European Carbon and Steel Community (ECSC) project "SCC of newer SS grades" (NEWSCC), a u-bend test program in neutral dilute chlorides was performed. Earlier presented data of stress corrosion resistance on stainless steel (SS) in the same environment have been based mostly on constant load testing. The results from these two test programs differed, even though they were performed at the same environmental conditions and in the same equipment. The ranking of alloys inside the two material groups of austenitic and duplex SS was not changed. But, the ranking between the groups was changed. In the constant strain test program with u-bends, the austenitic SS received better results compared to the duplex SS, compared to the previously performed constant load test program. Another result was that AISI 304L (UNS S30403) steel experienced cracking first at a temperature as high as 125degreesC in 1% chloride solution. Experiences from service have given a maximum service temperature between 40degreesC and 60degreesC. Mechanical testing including relaxation tests at actual testing temperature was done to evaluate the differences. The austenitic SS experienced up to 30% relaxation but the higher alloyed duplex steels only similar to10%. The differences in relaxation behavior must be considered during evaluation of u-bend test programs. The degree of high relaxation of AISI 304L also might explain the improvement for u-bend specimens compared to constant load test data and service experience. The difference in relaxation behavior is also important when a test method shall be chosen for simulating practical experience. For example, constant strain samples such as u-bends are the most representative for conditions where a strain causes the load (for instance, in nonstress-relieved bent heat exchanger tubing). However, the huge difference in relaxation for different SS in a u-bend will not give representative results for a constant load application (for instance, stress-relieved heat exchanger tubing or pressure vessels).

  • 7.
    Kivisäkk, Ulf
    et al.
    Sandvik Materials Technology.
    Wallin, E.
    Sandvik Materials Technology.
    Results from low temperature creep and relaxation experiments of four different stainless steels2008In: NACE Corrosion 2008: Paper No. 08487, 2008Conference paper (Refereed)
    Abstract [en]

    In testing for environmental induced cracking several types of specimens can be used. For instance, tensile specimens can be used with constant load technique or u-bends and c-rings with constant strain can be used. Naturally the decrease of load with time during testing is of great importance. For a constant strain type of corrosion specimen relaxation takes place and for a constant load specimen, low temperature creep occurs. In several documents the test load level for specimens is given. The load level in one document can be different whether constant load technique or constant strain specimens are used. Low temperature creep and relaxation experiments at room temperature have been performed for stainless steels at 90% of the yield strength and at a load level corresponding to 2% strain. Four materials were included: the austenitic AISI 304L, 13Cr supermartensitic stainless steel and two duplex stainless grades UNS S32750 and UNS S32205. The creep experiments were performed with dead weight creep test equipment normally used for testing at high temperatures. For the same materials relaxation was also investigated with a tensile test machine. The experiments were performed at room temperature because this is the relevant temperature for HSIC-testing. The results show that there is a difference in ranking of the alloys between the influence of low temperature creep and relaxation. Further the results indicate that the ranking at 2% strain and at 90% of the yield strength is not the same. The results illustrates that it is relevant to consider load level and corrosion test specimen type when designing an environmental induced cracking experiment.

  • 8.
    Sathirachinda, Namurata
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
    Gubner, Rolf
    Pan, Jinshan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
    Kivisakk, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
    Characterization of phases in duplex stainless steel by magnetic force microscopy/scanning kelvin probe force microscopy2008In: Electrochemical and solid-state letters, ISSN 1099-0062, E-ISSN 1944-8775, Vol. 11, no 7, p. C41-C45Article in journal (Refereed)
    Abstract [en]

    A 2205 duplex stainless steel, which had undergone a slow cooling process in order to precipitate intermetallic phases, was characterized by means of magnetic force microscopy (MFM) and scanning Kelvin probe force microscopy (SKPFM), in addition to conventional scanning electron microscope and energy-dispersive spectroscopy analysis. MFM measurements yield information about the magnetic domain distribution, while SKPFM provides the variation in Volta potential between austenite (gamma), ferrite (alpha), and sigma phases (sigma). In general, paramagnetic austenite exhibits the highest Volta potential, followed by nonmagnetic sigma phase and ferromagnetic ferrite, respectively. Results show the applicability of MFM/SKPFM techniques for characterization of the individual phase properties of duplex stainless steel. Because a cross talk between magnetic and Volta potential signals has been observed, it is recommended to perform SKPFM measurements with nonmagnetic tips.

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