Engineering Failure Analysis 18 (2011) 2316–2323

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Engineering Failure Analysis

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Short communication

Failure analysis of a carbon steel screw under the service in the presenceof hydrogen sulphide

F. Nasirpouri ⇑, H. Alizadeh, M. HosseingholizadehDepartment of Materials Engineering, Sahand University of Technology, Tabriz 51335-1996, Iran

1. Introduction

The petroleum industry generally has different processes which bring a wide variety of corrosive environments in contactwith structural materials in service. Each year, tens of millions of dollars are spent to replace or repair pipes and vessels thatsuffer excessive localized metal loss, stress corrosion cracking (SCC) or hydrogen embrittlement (HE). One of the most com-mon failures in the industry would happen due to the brittle fracture known as sulphide stress cracking (SSC) when the sur-rounding environment contains sulphurous compounds such as hydrogen sulphide (H2S) [1].

Under a wet H2S atmosphere, steels become susceptible to SSC. Under such circumstances atomic hydrogen, resultedfrom an electrochemical reaction between the metal and the H2S-containing medium enters the steel at the corroding sur-face. Atomic hydrogen, in turn, can partly be absorbed into the steel. In the absence of applied stress, the diffused hydrogencould cause blistering or hydrogen induced cracking (HIC) initiated at hard phase constituents and non-metallic inclusions.In the presence of applied and residual stress, the failure process can occur either by sulphide stress corrosion cracking(SSCC) through hydrogen embrittlement or stress oriented hydrogen induced cracking (SOHIC). The latter occurs mainlyin lower strength steels, such as API pipeline grades [2]. SSC is a form of hydrogen stress cracking resulting from absorptionof atomic hydrogen produced by the sulphide corrosion process on the metal surface [3].

All variables known to influence SSC of iron-based alloys are classified into two major categories: (i) environmental ef-fects that include hydrogen sulphide concentration (or pressure), temperature and applied potential, and (ii) mechanicaland metallurgical variables such as alloy composition, microstructure, heat treatment, cold work, hardness or strength level,and surface condition [4–10].

One particular study has shown that tempered martensite is a less sensitive phase to SSC, meaning that the higher themechanical properties of steel, the lower the SSC resistance. The higher volume fraction of martensite with a minimum valueof 95% is suggested to make the steel SSC resistant [11]. Even in the presence of sodium chloride beside hydrogen sulphide,steels with a martensitic microstructure has the highest corrosion rate up to two orders of magnitude higher than that forferritic–bainitic or ferritic microstructures [12].

Environmental variables that control SSC and accelerate its effect include low pH, high partial pressure of hydrogen sul-phide, increasing chloride concentration, temperature and implied potential increasing [11]. SSC is most probably found inhard weld and heat affected zones and in high strength components including bolts, relief valve springs, valve trim, compres-sor shafts, sleeves, springs and can occur throughout the refinery wherever there is a wet H2S environment present [3].

Screw steels are often subject to environments containing hydrogen sulphide (H2S) in the presence of tensile stress. Underspecified conditions, these steels are known to be highly resistive against a severe sulphide stress cracking (SSC) failure. Forconventional industrial applications the materials are assumed to have high SSC susceptibility if the hardness of screw steelsexceeds 22 HRC (equivalent to 550 MPa yield strength) [13]. Also about pipeline, EN ISO 15156-1 standard implies that thehardness of welds, heat-affected zones and base metal of the pipeline play an important role in determining its sulphidestress cracking susceptibility, which is high in regions presenting hardness greater than 22 HRC [14–16].

1350-6307/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2011.07.001

⇑ Corresponding author. Tel.: +98 412 3459450; fax: +98 412 3444333.E-mail address: nasirpouri@sut.ac.ir (F. Nasirpouri).

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On the effect of the alloying upon the SSC resistance, hydrogen permeability tests have shown that the addition of nio-bium as an alloying element decreases the hydrogen entrapment from sour working media. Niobium micro-alloying is,therefore, a suitable way to improve the SSC resistance of high-strength steels. Also, both vanadium and molybdenum havean improving effect on the SSC resistance of steels. The useful effect of chromium addition is evidenced when it is alloyedbetween 0.07 and 1.3 %wt, while a ‘‘threshold effect’’ seems also to be observed because the SSC resistance does not changeanymore above 1.3% of Cr. In H2S saturated synthetic sea water at pH 5, copper alloying has been reported to have a bene-ficial effect on the general corrosion of steels [11].

Zhao et al. realized that steels with acicular ferrite exhibit a better SSC resistance than those with a martensitic micro-structure [14]. It is currently recognized that the best suited grades for manufacture of SSC-resistant components are steelsof the AISI 4100 series with a tempered martensite microstructure. However, because of the development of deep and sourwells in the recent years, there is an increasing demand for higher strength H2S-resistant steels so that it is now difficult tomeet the requirements of customers by using these grades [8,14].

In this paper, we report a failure analysis on a carbon steel screw broken after a short time service in H2S containingenvironment.

2. Material and methods

A ruptures screw was achieved from a petroleum industry. The processing unit in which the screw was failed contained astream flow containing H2S gas. The screw was made of steel whose composition was analyzed using quantumetry and isshown in Table 1. According to the manufacturer’s data sheet, the screw was subjected to rolling procedures.

The failed screw was first investigated by visual examinations to record any obvious failure remarks. Different samplesfrom different parts of the specimen were selected for metallographic, electron microscopy and hardness examinations.Metallography samples were prepared from cross sections of cracked and sound regions. Samples were etched in Nital 2%solution. Scanning electron microscopy (SEM- MV2300 Cam Scan) and energy dispersive X-ray spectroscopy (EDS-Oxfordinstruments) were used to study the surface morphology and chemical composition of fractured surface and also corrosionproducts.

Microhardness in Rockwell was performed using 100 g load with a dwell time of 15 s to obtain the hardness profilearound the cracked area.

3. Results

3.1. Visual examinations

Fig. 1 shows an image of the failed screw taken in the lab after the failure. Visual test was primarily performed and re-vealed that a thin yellowish film exists on the specimen. Several cracks were also detected on the outer surface of the screw,as shown in Fig. 2. It was observed that the cracks have propagated from outer surface into the center of the sample. Cracksobserved have been developed without any deformation in the material.

3.2. Metallographical examinations

Fig. 3 shows optical micrographs taken from different parts of the sample. Fig. 4 illustrates an optical micrograph takenfrom the cross section of mainly cracked region. The cracks detected on the surface of the screw are shown in Fig. 5. Thesecracks like large crakes produced on the surface and grew into the center of the screw.

Metallographical examinations after polishing and etching procedures under appropriate conditions imply that themicrostructure of steel is fine grain bainite near to the outer surface, in the center and around the cracks developed inthe screw.

Table 1Elemental chemical composition of the screw steel taken from the core of the specimen.

C Si Mn P S Cr Mo Ni Al Co

0.354% 0.211% 0.730% 0.028% 0.031% 1.160% 0.210% 0.076% 0.016% 0.014%

Cu Nb Ti V W Pb Sn Mg As Zr

0.204% <0.002% 0.004% 0.005% 0.020% <0.001% 0.019% <0.001% 0.016% 0.002%

Ca Ce Sb Ta Zn N Fe

0.001% <0.001% <0.001% 0.054% <0.001% 0.011% 96.834%

Fig. 1. An image of the failed screw taken in the lab after the failure.

Fig. 2. Images of the fracture surface and cracks and corrosion products formed on the screw from different angles.

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3.3. Hardness profilometry

Microhardness test was carried out on both sound and cracked regions of the specimen. Profiles of hardness of the metalacross the crack and inside the sound sample are shown in Fig. 6. We observed that hardness is the highest on surface whileit gradually decreases along the propagation line of the crack towards the center of the sample and the crack tip where thehardness is the lowest.

Fig. 3. Optical micrographs after etching the screw steel in Nital 2% taken from (a) outer surface, (b) the center of the sample, and (c) the region surroundinga typical crack.

Fig. 4. Optical micrographs of the region around one the main cracks occurred inside the steel screw after etching in Nital 2% in (a) 200� and (b) 1000�magnification.

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3.4. Fractography

Randomly selected fracture surfaces of the cracked screw were studied using SEM. Fig. 7 shows SEM micrographs of thefracture surface and also corrosion product. Fractograms show that band-like morphology is formed with an oriented direc-tion. A close look at the morphology of the fracture surface reveals that the band-like structure has been developed along thecracks propagation direction and therefore bank-like routes have been formed on the fractured areas. However, the corrosionproduct has a porous structure formed on the surface.

Fig. 5. Optical micrographs of the region around a typical surface crack occurred on the outer surface towards the core of steel screw after etching in Nital2% in (a) 200� and (b) 1000� magnification.

Fig. 6. The hardness profile of steel (a) across the whole screw and (b) along the fractured surface. Upper panels show the corresponding image and linescan on which the hardness was measured.

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3.5. Chemical analysis

EDS analysis suggests that sulfur exists in the corrosion products. Most probably a simple combination of iron substratewith sulfur produces iron sulphide (FeS) under the condition of service. EDS elemental analysis is given in Table 2.

Fig. 7. SEM images of (a) a typical fracture surface of the screw steel, (b and c) band-like routes for crack growth and (d) of the morphology of the corrosionproduct formed on the surface containing a compound of sulfur.

Table 2EDS elemental analysis of the corrosion product formed on the fracture surface.

Element App Intensity Weight% Weight% Atomic%Conc. Corrn. Sigma

O K 4.32 0.6952 11.02 0.53 26.15S K 11.85 0.7923 26.52 0.30 31.39Fe K 33.13 0.9402 62.46 0.45 42.46

Totals 100.00

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4. Discussion

In general, four conditions are necessary for SCC to occur: (a) susceptible material, (b) sustained tensile stress, eitherresidual or applied, (c) aggressive environment containing specific ions, i.e., H2S and (d) presence of an electrolyte (water/moisture) [16]. NACE MR0175 standard procedure [17] for materials for use in H2S-containing environments in oil andgas production recommends the standard chemical composition and mechanical properties of the materials for such appli-cation in screws. Under the given conditions predicted by the standard, SSC can significantly be prohibited during serviceunder H2S condition. The ASTM A-320 standard procedure [18] recommends a maximum hardness of 235 HB in steels whichshould be quench-hardened and tempered at a minimum temperature of 620 �C. Once this heat treatment process is con-ducted, ferrite phase forms and becomes dominant, rendering the steel to have a hardness of below 22 HRC (or 235 HB)[17,18].

Metallographic images taken from the material at the core of the screw shows that bainite is detected as the stable phase.This phase generally preserves a high hardness in steels. Microhardness measurements carried out on the central part of the

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screw farther to the crack exhibit a hardness of value of 34 HRC which exceeds the recommended hardness by the standards,i.e. 22 HRC. Therefore, the material is basically susceptible to various types of brittle fracture under the service.

During service, the steel is exposed to acidic environments containing H2S, which may in turn form atomic hydrogenbeing adsorbed on surface according to the following reactions:

H2Sþ e� ! HS� þHads; ð1Þ

Hads þH2Sþ e� ! HS� þH2; ð2Þ

Hads () Hads; ð3Þ

where Hads and Habs are the adsorbed hydrogen atoms on surface and adsorbed hydrogen atoms in steel, respectively. In factadsorbed hydrogen on surface is a unique source for hydrogen diffusion and accumulation in steel. Increasing H2S in envi-ronment intensifies this effect and more hydrogen atom will diffuse in steel with increasing time during service [3].

On the other hand, another possible reaction takes place as a consequence of direct contact of steel and hydrogensulphide:

H2Sþ Fe! FeSþ 2H: ð4Þ

It is generally expected that when an adhesive FeS crust is formed on the steel surface, further diffusion of atomic hydro-gen is inhibited. However, the FeS crust in humid H2S environment cannot avoid diffusing the hydrogen atoms into the plas-tic zone of crack tip due to a weak strength between crust and background. Therefore, FeS crust dissociates [3].

On the origin of the initiation of crack and its propagation in the screw, crack growth patterns and the fractured surfacesshow important features. Based on our examinations, a major and considerable issue that exists in the material is the dif-ference between the hardness of the region surrounding the crack tip and the surface of steel material. Hardness graduallydecreases from the surface of the steel screw and therefore brittleness increases from crack tip to the surface.

We interpret this evidence with the diffusion of hydrogen atoms from surface into the steel which is accompanied withhydrogen embrittlement. However, an improper pre-heat treatment can probably be another reason for the higher hardnessof the material. EDS analysis shows that the corrosion product on surface is most likely FeS. This brittle phase also is one ofthe destruction reasons.

Fig. 4 shows that how the main crack propagates inside the screw steel. The main crack nucleates from defects and holeson the outer surface as a result of the hydrogen embrittlement and because of stress deformation bands connecting crackedges to each other. Presence of a brittle and porous FeS phase on surface intensifies this process. Small cracks then nucleatein deformation bands (Fig. 7) and join each other. If this process repeats crack will grow towards the center and finally sam-ple is ruptured by stress corrosion.

5. Conclusions and recommendations

Based on the failure analysis given in this investigation it can be concluded that higher hardness of the specimen than theallowed value recommended by NACE MR0175 standard procedure is the main factor causing the failure in the steel screw.An improper pre-heat treatment before service is probably the most reasonable description for the material deficiency. How-ever, the formation of a porous and brittle FeS layer observed by SEM and EDS has caused the hydrogen embrittlement form-ing the cracks to initiate on the surface. It can be suggested that addition of a corrosion inhibitor, gas dehumidification,improving of the steel properties by chemical component, steel making, rolling treatments control, homogenous microstruc-ture, improving weld ability and harden ability may be done to prevent SSC corrosion [3,11].

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