Engineering Failure Analysis 20 (2012) 147–155

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Engineering Failure Analysis

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Failure assessment of the hard chrome coated rotors in the downholedrilling motors

Khalil Ranjbar ⇑, Majid SababiDepartment of Materials Science and Engineering, Shahid Chamran University, Ahvaz 61355, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 August 2011Received in revised form 21 October 2011Accepted 7 November 2011Available online 20 November 2011

Keywords:Hard chrome coatingRotorStatorSpallingSolid content

1350-6307/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.engfailanal.2011.11.007

⇑ Corresponding author. Tel.: +98 6113330010 19E-mail address: k_ranjbar@scu.ac.ir (K. Ranjbar).

In this investigation, the premature failure of the hard chrome coated rotors of thedownhole drilling motors is studied. The main part in these motors is the power section,composed of a metallic rotor and an elastomeric stator. The rotor is manufactured from17-4 PH stainless steel and rotates within a stator and displaces the drilling fluid forward.Drilling fluid characteristics change upon circulation and influxes, and cause erosion andcorrosion of the rotor. Different types of damages such as scratches, deep and shallow cut-tings, spalling, pits, micro and macrocracks are observed on chrome plated rotor surface.Effects of working parameters such as bottom hole temperature and solid content on lifetime of rotors are investigated. Chemical analysis and microscopic examinations of rotorand its coating are also carried out to identify the causes of failure. It was found that, dril-ling fluids are corrosive in nature and their solid content are too high for chrome coatedrotors.

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1. Introduction

Hard chrome coating (plating) is widely used in different industries such as aircraft, vehicles, ships, and military sectors.Chrome has some distinct characteristics that are advantages in resisting wear: It is very hard, slippery and it is resistant tomost corrosive environments. A thick chrome deposit is of value when it provides additional wear surface and distinguishesit from a thinner decorative coating. But it may develop a pattern of tiny cracks when the stresses become greater than thestrength of the coating. Cracks and the porosities due to residual stresses, are the characteristics of chrome plating [1]. Thesecracks often form an interlacing pattern to the base metal allowing corrosive liquid or gas to penetrate.

In oil exploration drilling, mud motors utilize mud as working fluid to drive the rotor inside the stator, as well as tolubricate the drill bit and flush out the debris. In such a system, rotor and stator which are called power section, denotingits utilization to transmit power to the drill bit. Usually a mud motor, piping and other auxiliary equipments are insertedinto and operate in a hole (i.e. future potential oil well).

Rotors are generally manufactured from high strength steels such as 17-4 PH stainless steel [2] and hard chrome coated inorder to reduce corrosion, erosion, abrasive wear, and maintain smooth sealing surface. Rotor and stator are in contact withdrilling fluid and may subject to different types of damages depending on the characteristics of this fluid. The hard chromecoating of rotor may be attacked by drilling mud which is usually composed of various compounds such as calcium chloridesand alkali salts. Acid gases such as carbonic acid and hydrogen sulfide can influx into the mud system. These gases reduce thepH of mud and greatly accelerate the corrosion. Chlorides, oxygen, CO2, and H2S are the most well-known components whichcan cause pitting corrosion [3] and combination can even have synergetic effects. The use of chrome as a coating is not so

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Stabilizer P

Lobe

Fig. 1. Different parts of a downhole drilling motor and cross-sectional view of a 3:4 lobe power section.

Table 1Chemical composition of the rotor (wt.%).

C = 0.035 Cr = 17.24 Co = 0.039 Nb = 0.235 W = 0.010 P = 0.031Si = 0.366 Ni = 3.510 Cu = 4.050 Al = 0.003 Ti = 0.011 Fe = Bal.Mn = 0.484 Mo = 0.261 Sn = 0.030 V = 0.005 S = 0.030

100 µm

(a)

(b)

Fig. 2. The micrographs of the as received rotor (17-4 PH stainless steel), (a) optical image of rotor shows tempered martensite microstructure, and (b)scanning electron micrograph of the same clearly reveals the parallel arrays of white tempered martensite lath within the grains. In both cases, samples areetched by Villella’s reagent.

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effective in hostile environment containing chloride concentrations. When the mud which is containing calcium chloride re-mains in contact with the chromium surface and absorbs carbon dioxide, pH decreases due to following reactions:

(a)

(d)

(b)

(c)

Fig. 3. SEM–EDX mapping of rotor for Cr, Fe, and Ni, elements, (a) an image of rotor where chrome coating is partly lost, (b) mix mapping for Cr, Fe, and Ni,(c) mapping for distribution of Fe, and (d) mapping for distribution of Cr, and the area where it is lost.

Fig. 4. Spalling on chrome coated rotors surface. In both the rotors shown in figure (a) and (b) the maximum effects observed on lobes. The image of figure(b) is magnified and revealed better in figure (c).

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Fig. 5.the cut

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CaCl2 þH2Oþ CO2 ¼ CaCO3 þ 2HCl ð1Þ

2NaClþH2Oþ CO2 ¼ Na2CO3 þ 2HCl ð2Þ

This acidic environment combined with dissolved oxygen and other parameters, causes dissolution of the chromium layerwith the possible reaction

2Crþ 6HClþ 1:5O2 ¼ 2CrCl3 þ 3H2O ð3Þ

As far as the base metal is uniformly covered with chromium coating, the corrosion mechanism can be operated chem-ically. But once interface between coating and the base metal destroyed, salt solutions penetrate through the coating. In sucha case, not only peeling of the chromium coating can cause abrasive wear on rotor and stator, but also the base metal is goingunder attack.

The mud in the drilling process can be classified as oil base or water base mud. Water based drilling mud consists pri-marily of bentonite clay and water, additives such as organic polymers, dispersants, wetting agent, weighting materials, thin-ners, and lubricants. The mud properties considerably change because of fluid/solid and gas influxes or additions ofimpurities, during drilling process [4].

On a drilling rig, mud composition and properties such as density, viscosity, and pH are controlled, modified and period-ically tested in the system so called mud pits, to ensure properties and improve drilling efficiency. The mud is pumped downthe hole and further re-circulated through the drill string where it sprays out on drill bit, cleaning and cooling the bit. Then,the mud carries the cuttings up to the surface. In the circulation, mud is mixed and contaminated with cuttings, and must befiltered in order to be used again. Large cuttings can be removed from the drilling fluid in steps by means of solid removalequipment including shale shaker, mud gas separator, desander and desilter. Some of the cuttings are in the form of fine dis-persion, and cannot be removed effectively. These fines are called ‘‘drilled solids’’ and should be kept low for optimum dril-ling operations. In these operations, high pressure fluid is pumped into the top of the power section where it fills the first set

Failure of chrome coated rotors surface in the form of: (a) spalling and cutting of coating, (b) corrosion pits (shown inside the circles) formed withinting region of coating after being thinned, and (c) more corrosion pits and magnified spalling, shown by circles and arrow respectively.

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of open cavities. The pressure differential across two adjacent cavities forces the rotor to turn and thereby cavities gotopened alloying the fluid to flow progressively down the length of the power section. The cavities are separated from eachother by a series of seal lines, between rotor and stator. In fact, the pressure capability of a pump is a function of the numberof times the seal lines are repeated. If cavity pressure increased beyond the seal limits, the seal lines will open and fluid willslip at a very high velocity [5]. High speed particles (sand, and other hard particles) traveling through pump cavities abraderotor and stator. This causes seal lines to be less effective and higher slippage in the pump.

The failure analysis of rotors in downhole drilling has not been explored extensively, and only a single study has beenreported [2]. So, this work is aimed to investigate the failure of hard chrome coated rotors of the downhole drilling motors,and also to identify dominant failure mechanisms. Finally, possible mechanisms are illustrated for better understanding.

2. Experimental procedures

Premature and continuous failures of hard chrome plated rotor in downhole motors (commonly known as mud motors) insouthern part of Iran, Khozestan province, are reported. Rotor and stator as the main part of power section and their positionin drilling motor is shown in Fig. 1. The main function of these motors is to convert the hydrolic energy of drilling fluid (dril-ling mud) to mechanical energy to turn the drill bit. Rotor was made of 17-4 PH stainless steel with chrome as top coat. Thechemical composition of base metal is presented in Table 1.

Several failed rotors were visually examined. One of the rotors was cut to pieces for metallographic examination andchemical analysis of base metal and its coating. Microstructural features at higher magnification were revealed with theScanning Electron Microscope (SEM) model LEO 1455VP, equipped with Energy Dispersive X-ray spectroscopy (EDX). Forthe metallographic observations, samples after usual mounting and polishing were etched with Vilella’s reagent. The hard-ness of the chrome layer and the base metal were measured. The thickness of chrome layer was in the range of 80–130 lm,with no under coating applied on rotor. Usually, plating thickness is not uniform due to complex geometry of the rotor.

(a)

(b)

Fig. 6. An extensive surface failure of rotor: (a) detachment of chrome layer and macrocracking (shown in circle), and (b) SEM image of a interconnectednetwork of microcracking on chrome coated surface. Hard chrome coating by its nature is inherently porous and full of cracks.

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Various types of water based drilling mud is used in drilling, but in this study it was bentonite mud. Its qualitative phaseanalysis was done by X-ray Diffraction (XRD), using diffractometer Philips model 1840 with the Cu Ka1 radiation k = 1.54 A.

Effects of bottom hole temperature (BHT), and solid content of drilling mud upon the life time of the rotor were alsoinvestigated. The data for these two parameters are collected from the daily progress report log sheets of the oil well beingdrilled.

3. Results and discussion

Chemical composition of rotor (Table 1) and its metallographic examinations (Fig. 2), are identical with ASTM A705 grade630 corresponding to 17-4 PH (precipitation hardening) stainless steel, which is a martensite stainless steel containing about

Fig. 7. SEM images of chrome layer failure: (a) a part of layer is lost and wear grooves are clearly seen on chrome layer, and (b) a part of layer is removeddue to abrasive wear of a hard indenting particle, embedded in the stator.

Fig. 8. XDR pattern of the water based bentonite drilling mud.

(b)

Bottom Hole Temperature (°C)

Dri

lling

tim

e (h

)D

rilli

ng t

ime

(h)

(a)

Fig. 9. Variations of rotor life time in terms of bottom hole temperatures for two different size drilling motors, (a) 4 4.3 inch size motor and (b) 6 3.4 inchsize motor. It seems change in BHT (at least within the range shown) is not a controlling factor for the rotor life.

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3% Cu and strengthened by dispersion of copper rich particles in the martensite matrix after applying proper solution anneal-ing, quenching and aging heat treatment within the temperature range of 480–620 �C [6,7]. Fig. 2a and b shows the opticaland SEM images of the rotor respectively. The former image is a typical tempered martensite, and the later microstructurereveals the lath morphology of the martensite stacked in near parallel array within the packets.

Vickers hardness (HV) measurements were carried out on the coating as well as the base metal using 1 kgf load. The hard-ness values of chrome coated surface and the base metal were in the range of 1050–1100 HV and 340–350 HV respectively.As mentioned earlier, due to complex shape of the rotor, the coating thickness is not uniform, its low in the valleys and highin the lopes [2]. On the other hand, hard chrome coating has microcracks and pores which affects the hardness measure-ments. That’s why a range of hardness values less than 700 HV to more than 1100 HV are reported [2,8] for this coating.Microstructure, hardness and the working condition indicate that, the rotor has tempered at around 620 �C after annealingand quenching treatment. In other words, it is in over-aged condition (H1150) and found to meet the requirement. In fact,17-4 PH stainless steel is more common than any other type of precipitation hardening steels and due to its favorable com-bination of mechanical properties and corrosion resistance has been used in variety of applications including power plants,chemical processing industries, air craft fittings, gears, paper mills, drilling, etc. [8,9].

Microscopy and chemical mapping from the peeled area of rotors is shown in Fig. 3. Evidently, there is no primer coatingunder top chrome coat which could improve adhesion of top coat to substrate.

The power section of a drilling motor consists of a helical shaped rotor and a stator. As shown in Fig. 1, the rotor has onelobe less than stator. Single or multi lobe configurations may be used in drilling. Stator is made stationary and elastomeric innature for handling abrasives. In the present study, it was found that, the base polymer used for manufacturing of stator is abutadiene. The results of analysis indicated that, it should be a nitrile rubber (NBR), which is usually obtained by copolymer-ization of butadiene with acrylonitrile. NBR has excellent physical properties and oil resistance.

The morphology of failed area is shown in Figs. 4–7. Visual inspections are not the same in different sites. Along the wholerotor, the chrome coating was lost its uniformity from some areas and disbonding was occurred in which base metal is at-tacked. In addition, the maximum damages to coating were observed on lobes. The other defects were seen in the form ofpitting, spalling, wear grooves, macro and microcracks.

A sample from drilling mud was taken, dried at 120 �C and then its XRD pattern was taken as shown in Fig. 8. It is com-posed of different carbonate (CaCO3) and hard oxides compounds such as Al2O3, SiO2 and Fe2O3. These hard oxide particlescause different type of damage like, erosion and wear on rotor surface as mentioned in the earlier section. Moreover, drillingmud composition, hardness, temperature and characteristics keep changing, since it influxes with additives and formations.Hardness and solid content of drilling mud have major impact on rotor and stator life time. Effects of BHT and solid contentof drilling mud on life time of rotors for two different size drilling motors are presented in Figs. 9 and 10 respectively. In fact,life time should decrease with the increase of these two parameters. Higher solid content suppose to accelerate the rate of

(a)

(b)

Solid content (wt. %)

Fig. 10. Variations of rotor life time in terms of solid content of drilling mud for two different size drilling motors, (a) 4 4.3 inch size motor and (b) 6 3.4 inchsize motor. A lot of scatting can be observed in the results.

(a)

(b)

Fig. 11. Schematic drawings are made to illustrate the surface fatigue mechanisms by hard particles: (a) embedded particles in stator, and (b) wear causedby impingement of particles.

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erosion and wear, whereas raise in temperature should increase the rate of corrosion (reactions (1)–(3)). But the figures donot indicate such a direct relation. In the latter case, the recorded BHTs are not so high to degrade the chrome coating per-formance. In other words, the life time of rotors is dominated with other parameters than BHT.

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It seems, the failures of rotors are mainly due to the large and hard particles that cannot easily pass the pump seal lines,embed in the inner surface of the stator and rubbing against the rotating rotor. This is schematically illustrated in Fig. 11a.Once hard particle embeds into the elastomeric stator, abrasion occurs for every rotation of rotor. Depending on the size andhardness of embedded particles, the extent of damage will be shallow or deep. In other words, it acts like a cutting/machin-ing/drilling tool and thereby, surface fatigue crack can be initiated and magnified leading to spalling and disbonding of coat-ing. The local appearance of this effect is spalling which is shown in Fig. 3.

The other mechanism of rotor surface damage is erosion, due to the impingement of high velocity hard solid particles.This is schematically shown in Fig. 11b. Rotor life and its performance should be affected and decreased with increasing solidcontent of drilling mud. Nevertheless, in the present study, the collected data related to solid content showed a lot of scat-tering and perhaps it was too high and over ranged. As shown in Fig. 10 the range of solid content is very high and notacceptable, as it is recommended to be less than 2 wt.% [10]. It is also advised that, hard chrome plated rotors, not to be usedin an environment (mud system) contains salt, chlorides, and H2S. Solids and in particular sand content, is very crucial forrotor and stator performance. Even, high concentration of very fine powder-like sand can abrade rotor and stator. In otherwords, hard chrome is adequate for medium to low abrasive environments, otherwise, harder coatings [11,12] to be applied.

4. Conclusions

Premature failures of hard chrome coated rotors of the downhole drilling motors were investigated. It was found that:

1. The drilling environments for chrome plated rotors were found to be very abrasive and hostile, since they contained avery high solid content and salts.

2. The embedded large and hard solid particles in inner surface of stator were the main failure mechanism, resulted in sur-face fatigue, spalling and cutting of chrome coating.

3. An extensive microcracking of chrome coating was observed and led to easy detachment of chrome layer. The other dom-inant failure mechanism was erosion caused by impingement of high velocity solid particles.

4. Chemical reaction between chrome coating and the chloride containing drilling mud was the only active corrosion mech-anism. The acid gases produced due to influxes of drilling mud and the formation, caused the chrome layer to dissolve.

Acknowledgment

The authors are grateful for the financial support of this research by Iranian National Drilling Company (NIDC). Funding isdone under the Contract no. 33-06-5116-08.

References

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