Hydrogen effect on local fracture emanating from notches in pipeline from steel API X52

Abstract

The assessment of local strength at notches in pipeline steel API X52 has been done for conditions of cathodic hydrogen charging. The relationship between hydrogen concentration and critical (failure) loading has been found. The existence of some critical hydrogen concentration which causes the significant loss of local fracture resistance of material was also shown.Keywords: hydrogen charging of metal, hydrogen concentration, cathodic polarization, current density, acoustic emission, static load, critical stress, work of local fracture.Introduction. Nowadays, pipelines for gas and oil transportation are integral components of national and global economic infrastructures. There are huge plans for installation of new transcontinental pipelines that require increased attention to their reliable and safe operation. In this frame, the hydrogen degradation of pipeline steels is the important problem among other structural integrity problems.Specific long-term operation of pipelines promotes of steel hydrogenating process. First of all, pipeline steels encounter hydrogen during transport of sour crude oil and other petroleum products [1]. Moreover, external environmental conditions cause free corroding processes, where hydrogen can appear on metal surface as result of cathodic counterpart of the anodic dissolution reaction. This fact has been proved by several studies [2][3][4]. Moreover, under service conditions when a cathodic protection system is in place, hydrogen charging of pipeline steels is possible too [4,5]. As result, there is the problem of structural integrity of aging buried pipelines having cathodic protection [5].Second reason, which turns one's attention to the problem of hydrogen degradation of pipeline steels, is the fact that hydrogen is supposed to play a decisive role in a future energy system, when fossil fuels become scarce and thus expensive and/or unsuitable from ecological reasons. The number of aspects related to the technical feasibility and economics of developing a hydrogen energy infrastructure are presented and discussed in literature during last decades [6,7]. The possible use of existing pipeline networks for mixtures of natural gas and hydrogen offers a unique and cost-effective option to initiate the progressive introduction of hydrogen as part of the development of a full hydrogen energy system [6,8].In the present study, the assessment local strength at notches in pipeline steel API X52 has been done for conditions of cathodic hydrogen charging. The relationship between hydrogen concentration and critical (failure) loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.Experimental Procedure. The objects of study were specimens from steel API X52 machined from real pipes of diameter D = 610 mm and wall thickness t =11 mm. The specimens were notched for modeling of the longitudinal external defects under operating internal pressure (Fig. 1). The chemical composition of...

Influence of Hydrogen Embrittlement on the Wierbiecki- Xu Failure Strain Criterion - Application to A Local Strain-Based Design

“…Figure 12 shows the evolution of the local strain resistance 𝜀 𝑓,𝑙 (𝛽, 𝜃 𝐿 ) versus the displacement . For small displacements in the range [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] mm, the relative difference between local strain resistance without HE and with HE is in the range [20%-9%], the local strain resistance without HE is higher. One notes that the failure elongation in pure tension after HE is decreasing of 25%.…”

“…Among then, the Volumetric Method (VM) [20] is based on the inflexion point on the stress distribution. This inflexion point is determined at the minimum of the relative stress gradient 𝜒 ∶ r (r)…”

“…The true stress-strain curve used for described the material behavior is given in Figure 4 . The effective local strain is obtained from the Volumetric Method (VM) procedure [20].…”

“…Hydrogen embrittlement has been obtained by the electrolytic method. Application of the (W-X) model has been done to get the local critical resistance used to predict fracture by the Volumetric Method (VM) [20]. An example is given for the case of an embedded pipe submitted to service pressure and lateral displacement.…”