RESEARCH BULLETIN OF THE
HIROSHIMA INSTITUTE OF TECH.
VOL. 42(2008)PP. 7-14

P A P E R

Improvement of Corrosion Fatigue Strength of WC CoatedSteel by Adjusting Powder Composition and Spraying Pressure
Mitsuo KIDO ・Tarou TOKUDA ・Yoshiko SHINHARA
Rongguang WANG ・Kozo OHOTANI ・Yuu SATAKE(Received Oct.1,2007)

Abstract
Two types oftungsten carbidepowders,WC/Co and WC/NiCrMo,werethermallyspraying coatedon Mild steelbyaHVOF process using two types ofsprayguns,models JP-5000 and DJ-2700. The
spraying pressure for the JP-5000 gun was greater than the DJ-2700 gun. The corrosion fatigue
behavior were investigated in 0.05kmol/m of Na SO aqueous solution for each type of coatedspecimen. Thecorrosion fatiguestrength oftheWC/NiCrMo specimen is higher than both theWC/
Co specimens. The larger residual compressive stress was existed in the specimen prepared by thehigh spraying pressure than by the low spraying pressure. The corrosion resistance of the WC/
NiCrMo specimens is higher than both the WC/Co specimens. The corrosion fatigue strength of
the WC/NiCrMo specimen is the highest, which results from both the lower effective stress due tothe larger residual compressive stress and the high corrosion resistance of the coating.
Key Words: WC thermallyspraying coatings;HVOF process;spraying pressure;corrosion fatigue;
residual stress;corrosion;passive film

1 Introductionchanging the additives in the spraying powders (Ref 3). As aTungsten carbides (WC) are usually used to coat steel

result, the corrosion fatigue strength was improved by thethrough a high velocity oxygen-fuel flame spraying (HVOF)

addition of chromium in the WC/Co powder. However, theprocess to improve wear resistance(Ref 1,2). However,cracks

improvement effect was small and appeared only in the low

can occur in the coating that sometimes induces delaminationstress zone. Furthermore,thespraying pressurecan changethe

fromthecoating/substrateinterfaceunder staticor cyclictensile

distribution of the residual compressive stress in the coatedloads. Furthermore, aqueous solutions can penetrate to thespecimen (Ref 6).

coating/substrate interface through the defects or cracks in the

In this work, two types of tungsten carbide powders were

coating, which induces corrosion fatigue fracture of the sub-thermallyspraycoated on Mild steel bya HVOF process using

strate. Until now,there have been fewreports on the corrosion

two types of sprayguns with different spraying pressures. The

fatigue behavior of WC coated steel (Ref 3-5, 7). Particularly,

fatigue behavior, the corrosion behavior and the corrosion

the influence of the spraying pressure and the adjusting of thefatigue behavior were investigated and discussed.

powder composition in the HVOF process has not been clearly

2 Experimental Procedures

researched.
In a previous report, the authors discussed the improvement

2.1 Specimensof the corrosion fatigue strength of WC coated specimens byCommercial supplied rolling steel for common structure use,

Department of Mechanical System Engineering, Faculty of Engineering, Hiroshima Institute of Technology
Department of Computer Science, Faculty of Applied Information Science, Hiroshima Institute of Technology
Graduate student, Department of Mechanical Systems Engineering, Faculty of Engineering, Hiroshima Institute of Technology

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Mitsuo KIDO・Tarou TOKUDA・Yoshiko SHINHARA・Rongguang WANG・Kozo OHOTANI・Yuu SATAKE

2.2 Fatigue Tests in Air and in Solution
In the fatigue test, a fatigue machine (SERVOPET Lab,
Shimadzu Co., Japan) with a capacity of 49kN was used to
produce a sinusoidally cyclic stress wave with a stress ratio ofR(σ /σ )＝0 and a frequency of ＝14Hz in air and in an
aerated solution of 0.05kmol/m Na SO with pH＝6.5. The
maximum cycles were N＝1.0x10 cycles in air and 2.0x10

cycles in solution. The temperature was 295±3K and thehumidity was 60±10%RH during all tests. In the corrosion
fatigue test,the solution was cycle flowed bya pump with afluxof0.67mL/s and was refreshed when thespecimen was changed.

Fig.1 Shape and size of specimen (a)Fatigue test specimen,(b)Corrosion test specimen

2.3 M easurement of Residual Stress
Mild steel (Low carbon steel), was used as the substrate. The

The residual stress σ along the axis of the WC coatedshapeand sizeofthespecimen is shown in Fig.1. Thesubstratespecimen was measured by an X-ray diffraction apparatus

was first blast processed byalumina grids and then two types of(M21X, Mac Science Co.,Germany;XRD). The specimen was

powders, WC-12mass%Co and WC-15mass%NiCrMo, werefirst polished with adiamond paste(mean size:9μm)to acertain

thermally spray coated by the HVOF process. The composi-

depth fromthesurfaceand then theX-raypeak of(211) forthetions of the powders are shown in Table 1. The spraying

substrate and the peak (102)conditions are shown in Table 2. The sprayed specimens,(Ref 3). Residual stress values were obtained using the parallel

including both the substrate and the coating, are respectively

beam slit method,with diffraction angles determined bythe halfcalled, WC/Co-A, WC/Co-B and WC/NiCrMo-A specimens.

value breadth method. The stress value was calculated from theTwo spraying guns,JP-5000 and DJ-2700,wereused to prepare

slope M ofsin ψcurve. ψ was set for six angles at intervals of

the WC/Co-A and WC/Co-B specimens respectively,while the10°(deg) between 0°and 50°. The x-ray source was Cr-

WC/NiCrMo-A specimen was prepared by the JP-5000 gun.

Kαoperated at 30kV and the anodic current was 20mA.

for the coating were recorded

The spraying pressure is greater for the JP-5000 gun than the
DJ-2700 gun(Table2)(Ref8,9). Thethickness ofeach coating
was about 200μm.

2.4 M easurement of Anodic Polarization and Potential

An autograph machine (AG-250kND,

Distribution
The specimens were polished by #800 emery paper andShimadzu Co., Japan) was used to obtain tensile curves at atensile rate of 1.67x10 m/s.

cleaned in acetone for 1.8ks before the tests. The area for thepolarization was about 1x10 mm and the solution was aerated

Table 1 Chemical compositions of powder (mass%)with 0.05kmol/m of Na SO with a pH＝6.5. A potentiostat(HZ-3000, Hokuto Co., Japan) connected to a platinum plate(counter electrode) and a saturated calomel electrode (S.C.E.,
reference electrode)was used. The polarization test was carriedout after a 0.6ks cathode reduction and 1.8ks immersion in the

Table 2 HVOF spraying conditions

solution for each specimen. The current density was recordedwith an increasing rate of potential of 0.83mV/s.
The potential distribution on the specimen surface in the
Na SO solution during an open circuit was measured using ascanning vibration electrode tester (SVET HV-301, Hokuto
Co.,Japan). The micro vibration electrode was madeofPt witha tip diameter of20μm. The distancebetween theelectrodeandthe specimen surface was set at 8μm and the scanning area was
1mm x 1mm. Beforethetest,aVickers indent with cracks fromthe coating surface to the substrate was introduced to the8Improvement of Corrosion Fatigue Strength of WC Coated Steel by Adjusting Powder Composition and Spraying Pressure

specimen as an artificial defect (Ref 10).

smaller than the substrate. In general, the increase in theroughness of the substrate surface by the blast process for the

2.5 Surface Observation and Composition Analysis

WC coated specimens always induces a decrease in the fatigue

Themorphologies ofthecoating and thefractureofspecimens

strength. The fatigue strength ofWC/Co-A specimen is a little

after the fatigue test were observed by a scanning electron

larger than theWC/Co-B specimen. Thetensilecurves near the

microscope (JSM-5900LV, JOEL Co., Japan; SEM). The

yield field of specimens are shown in Fig.3. The yield point for

porosity p of the coating was obtained by calculating the area

the WC/Co-A specimen is larger than the WC/Co-B specimen,

ratio of open pores on the observation zones of the coating.

which is a direct reason to for thehigher fatiguestrength for the

The specimen after the SVET test was cleaned in acetone for

WC/Co-A specimen than the WC/Co-B specimen. Of course,

1.8ks and immediately analyzed by an X-ray photoelectron

the porosity in the coating will also influence the fatigue

spectroscopy (AXIS-ULTRA, Shimadzu Co., Japan; XPS).

strength. The porosityof each coating will be described in Fig.

The x-ray source in the XPS analysis was Mg-Kα.

8.

The

specimen surface was sputter-etched by argon ions to get the

Figure4 shows thedistribution ofresidualstress σ along thecomposition change in the depth.

axis of specimens from the specimen surface to the deep of thesubstrate. The vertical axis shows theresidual stress with“＋”

3 Results and Discussion−”for the tensile one, and thefor the compressive one and “

3.1

horizontal axis shows the depth from the surface to the deep ofCurves in Air

Figure 2 shows the S-N curves for four types of specimens

the substrate. A compressive stress is detected for each typeof

in air. The stress amplitude σ is half the amplitude of thespraying coated specimens. The value is larger for the WC/

applied nominal stress on the cross-section of the specimen.

Co-A and the WC/NiCrMo-A specimens than the WC/Co-B

The fatigue strength of each WC coated specimen is muchspecimen. Especially, the larger compressive stress exists not
only in the coating but also in the substrate for the WC/Co-A
and the WC/NiCrMo-A specimens. The residual compressive
stress in the spraying specimens will result in a decrease in theeffectivestress levelcomparing with thenominalstress (Ref13).
It will be the main reason for the increase in the yield point andthe fatigue strength of the WC/Co-A specimen. The residualcompressive stress was formed during the thermal spraying of
the WC particles. Higher spraying pressure causes higher
compressive stress in the specimen due to the collision of theFig.2 Relation between stress amplitude and number ofcycles to failure in air.

Fig.4 Distribution of residual stress in radius (depth)
direction from surface

Fig.3 Relation between nominal stress and nominal strain

9

Mitsuo KIDO・Tarou TOKUDA・Yoshiko SHINHARA・Rongguang WANG・Kozo OHOTANI・Yuu SATAKE

Fig.5 Relation between stress amplitude and number ofcycles to failure in 0.05kmol/m Na SO solution.

spraying particles to the substrate (Ref 11, 12).
3.2

Curves in Solution

Figure 5 shows the S-N curves obtained in aqueous Na SO
Fig.6 SEM observation of near the crack initiation point on
corrosion fatigue fracture surface (Arrow indicates corrosionpits) (a)Mild steel (σ＝115.5MPa, N ＝8.20x10 ), (b)WC/
NiCrMo‑A (σ＝126MPa, N ＝1.70x10 ).

solution. The corrosion fatigue strength of the WC/Co-B
specimen is much smaller than thesubstrate,whilethecorrosion
fatigue strength of WC/Co-A is almost the same as the substrate. Since the powder composition for the WC/Co-B specimen and the WC/Co-A specimen is the same, the different
corrosion fatigue strength for the two specimens is clearly
resulted from the difference spraying process. That is, thehigher spraying pressure brings about low porosity, higher
compressive stress and higher yield point for the WC/Co-A
specimen, which improves the corrosion fatigue strength of the
WC/Co-A specimen. According to Fig. 5, the corrosion
fatigue strength ofthe WC/NiCrMo specimen is thelargest one
among specimens, and the value is much larger than the substrate. The result should be related to both the spraying
pressure and the composition of the spraying powder.
Figure 6 shows micro-pits on fractured surfaces of WC
coated specimens after corrosion fatiguetest. Cracks areinitiated from corrosion pits (Ref 14). The pits were formed by the
corrosion reaction between the substrate and the solution

Fig.7 Polarization curve of Mild steel and WC cermet
sprayed coatings in 0.05kmol/m Na SO solution

penetrated through defects or cracks in the coating. The formation of pits is related to the corrosion resistance of the WC
coated specimens including the corrosion resistance and theNiCrMo-A specimen is much smaller than the WC/Co speci-

porosity of the coating. Figure 7 shows the polarization curves

mens. The highest corrosion resistance of the WC/NiCrMo-Aof the WC coated specimens in the Na SO solution. The

specimen should be attributed to the formation of a compact

corrosion current density of each WC coated specimen ispassive film on the coating surface (Ref 15-17). On the othersmaller than thesubstrate,meaning that thecorrosion resistance

hand, the influence of the spraying pressure cannot be found onof the WC coating is higher than the substrate. The current

the corrosion resistance of the WC/Co-A and WC/Co-B speci-

density of the WC/Co-A specimen is almost the same as themens. In general, defects such as pores and micro-cracks exist

WC/Co-B specimen and the current density of the WC/

in the coating, thus aqueous solution can penetrate to the inter10

Improvement of Corrosion Fatigue Strength of WC Coated Steel by Adjusting Powder Composition and Spraying Pressure

porosity for the WC/Co-A and WC/NiCrMo-A specimens issmaller than the WC/Co-B specimen,which is attributed to thehigher spraying pressure for the two specimens. Thus, thepenetration of aqueous solution to the interface will become
difficult for the WC/Co-A and WC/NiCrMo-A specimens thanthe WC/Co-B specimen. It should have influence the corrosion
behavior of the WC/NiCrMo-A specimen.
Although it is believed that a compact passive film can beformed on theWC/NiCrMo-A specimen,theformation behaviorof the passive film has not been clear. In order to clarify this,the potential distribution in the solution near the specimen
surface at open circuit was investigated by using a scanningvibration electrode method and the result is shown in Fig.9.
Before the test, a Vickers indent (P＝490N,30s)with artificial
cracks from the coating surface to the interface was introduced
to simulate defects in the coating. The formation of cracks bythis method has been confirmed in previous papers (Ref 18).
The vertical axis (Ax)shows the amplified output of the poten-

Fig.8 Open porosity of WC cermet sprayed coatings surface

tial gradient in thesolution near thespecimen surface,wherethe
face through the defects to cause pitting corrosion. Figure 8

plus value shows the anode area and the minus value shows theshows the mean value of porosity (p ) of the WC coatedcathode area. X and Y-axis show the scanning area of 1mm x

specimens and their distribution in size(p ). As the result, the1mm. On the WC/Co-A and the WC/Co-B specimens, anodic

Fig.9 Corrosion potential change around the indentation of WC cermet sprayed coatings.

11

Mitsuo KIDO・Tarou TOKUDA・Yoshiko SHINHARA・Rongguang WANG・Kozo OHOTANI・Yuu SATAKE

largely after Ar etching, and disappeared after E ＝300s
etching. Accordingly, the amount of the WO in the film issmall and thin. On theother hand,theNi 2p peak appeared near852.7eV and 855.6eV (Fig. 10(b)), corresponding to Ni andNi(OH) .

After E ＝300s etching, Ni(OH) peak largely

decreased leaving onlythe Ni peak,indicating the hydroxide ofnickel is also small and thin. On the other hand,the Cr 2p peakis shown in Fig. 10(c)near 576.7eV, corresponding to Cr O .
Different from that ofNi 2p and W 4f,the peak ofCr 2p almost
not changed even after E ＝1.2ks etching. Furthermore, Mo
3d (Mo )appeared after E ＝300s etching and increased after
E ＝1.2ks etching. Therefore, the passive film formed on thespecimen surface should be mainly the Cr O and Mo with asmall amount of Ni(OH) . Such film plays an important role on
the suppression of the corrosion of the specimen, which isrelated to the improvement of the corrosion fatigue strength of
the WC/NiCrMo-A specimen.
3.3 Factors Related to Corrosion Fatigue Strength
As described above, the corrosion fatigue fracture is caused
byboth themechanical and chemical destructions,i.e.,corrosion
pit forms in thesubstratedueto theelectrochemicalreaction andfatigue cracks originate from the bottom ofthepit mainlydueto
the stress concentration (Ref 14, 19).
Comparing with the WC/Co-B specimen, the larger spraying
pressure for preparing the WC/Co-A specimen and the WC/
NiCrMo-A specimen brings about the larger compressive stressin the specimens and smaller porosityin thecoatings. Thelager
compressive stress resulted in the improvement of the yieldFig.10 Spectra of Material WC/NiCrMo‑A in 0.05kmol/m
Na SO solution (a)W 4f spectra, (b)Ni 2p spectra, (c)Cr 2p
spectra.

point and thefatiguestrength ofthespecimens in air becausethe
slip deformation of the specimens is difficult and the effective
stress applied on the specimen can be decreased. The porosity

zones appeared near the indent after immersion (t 0s) and thealso plays a role in the improvement of the yield point and thevalues kept stable even after t 9ks. On the WC/NiCrMo-A

fatigue strength.

specimen, the anodic zones disappeared after t 9ks immersion,

The influence of the spraying pressure for the WC/Co-A

which should be attributed to the formation of passive film on

specimen and the WC/Co-B specimen did not appear in thethe specimen surface during corrosion. The compact passive

corrosion resistance. It means that the compressive residual

film suppresses the anodic reaction of the substrate throughstress and the smaller porosity for the WC/Co-A specimen

defects (cracks).

cannot largelyinfluence the corrosion behavior of the specimen.

The composition of the passive film after the open circuit

On the other hand, the largely enhancement of corrosion resis-

immersion test was immediatelyanalyzed byXPS. The spectratance of the WC/NiCrMo-A specimen is attributed to both thefrom the WC/NiCrMo-A specimen are shown in Figure 10.

larger compressive stress in the specimens and the formation of

The etching time of the surface by Ar is indicated as E

compact passive film with the main composition of Cr O andAccording to Fig.10 (a), W 4f peak appeared near 31.4eV and

Mo. The compact passive film should the mainly inhibit the

35.5eV, corresponding to the WC and WO respectively.

penetration of solution to the coating/substrate interface andHowever, the WO peak is much small (Ref 18), decreases

suppress the formation of the corrosion pits.
12

Improvement of Corrosion Fatigue Strength of WC Coated Steel by Adjusting Powder Composition and Spraying Pressure

Fig.11 Various factor on corrosion fatigue strength of thermal sprayed coatings

The above factors related to the corrosion fatigue strength ofis attributed to the larger residual compressive stress and thespecimens in this work are concluded in Fig.11. First, thesmaller porosityin thespecimen prepared bythehigher spraying

compact passive film, few open pores and compressive residualpressure (JP-5000 gun) than by the lower spraying pressure

stress work as the plus material factors, the asperity works as(DJ-2700 gun).

the minus environment factor (Ref 3). Next, the stress ampli-

specimen is higher than both the WC/Co specimens.

tudeand frequencywork as theminus mechanical factor,and the(2) The corrosion resistance of the WC/NiCrMo specimens iscorrosion environment works as the minus environment factor.

higher than both the WC/Co specimens. This is due to the

However,the corrosion fatigue strength is finallydetermined byformation of compact passive film containing Cr O and Mo on

the relative intensity of the above factors. As the experiment

the WC/NiCrMo specimen surface.

result, the corrosion fatigue strength of the WC/NiCrMo-A(3)Thecorrosion fatiguestrength oftheWC/NiCrMo specimen

specimen is the highest one, which is attributed to the decreaseis the highest,which results from both thelower effectivestressof the effective stress by the large compressive stress, thedue to the larger residual compressive stress, and the highformation of the compact passive film on the surface and thecorrosion resistance of the coating.

The fatigue strength of the WC/NiCrMo

smaller porosity in the coating (Ref 20).

Acknowledgements
4 Conclusion

Part ofthis work was supported byMEXT.HAITEKU,2004〜.

Two types of tungsten carbide powders, WC/Co and WC/
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