Corrosion behaviour of HVOF sprayed SUS316L stainless steel in seawater

Corrosion behaviour of HVOF sprayedSUS316L stainless steel in seawater

Jin Kawakita *, Takeshi Fukushima, Seiji Kuroda,Toshiaki Kodama

National Institute for Materials Science 1-2-1, Sengen, Tsukuba-shi, Ibaraki-ken 305-0047, Japan

Received 9 October 2001; accepted 28 December 2001

Abstract

SUS316L stainless steel was coated onto the SUS316L plate by the high velocity oxy-fuel

(HVOF) spray technique. Its corrosion behaviour in seawater was investigated by the elect-

rochemical method and the microscopy. The coating had corrosion resistance inferior to the

bulk plate. The corrosion of the HVOF sprayed SUS316L coating was related to both its

porosity and its oxygen content. Depending on them, the corrosion took place at the small

pore and the boundary between the spray particles on the surface. � 2002 Elsevier Science

Ltd. All rights reserved.

Keywords: Metal coatings; Stainless steel; EIS; Polarization

1. Introduction

An offshore or a marine structure is located in the corrosive environment con-taining abundant sea salt, sea wave, and sunlight. To prevent the corrosion of thestructural steel under such a severe condition, various methods are applied corre-sponding to each site of the structure. Under the sea, the electrical protection usingthe sacrificial anode is effective. In the mild tidal zone, the steel is covered with theinsulating varnish or paint. In the severe tidal and the splash zones, the coatingrequires the sufficient physical strength in addition to corrosion resistance. There-fore, cladding of the anti-corrosion metal and alloy is adopted in these zones.

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Corrosion Science 44 (2002) 2561–2581

*Corresponding author. Tel.: +81-298-59-2124; fax: +81-298-59-2101.

E-mail address: [email protected] (J. Kawakita).

0010-938X/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0010-938X(02 )00030-6

mail to: [email protected]

In the Ultra Steels Research Project (STX-21 Project), the thermal spraying wassuggested as a coating method alternative to cladding and as a field-mending methodof the damaged clad steel. This research has been studied since 1997 [1]. The highvelocity oxy-fuel (HVOF) spraying is able to make a denser and less-oxidized coating,compared with other method such as the plasma spraying [2]. Furthermore, thisspraying system enables the metal and the alloy with the high melting point upto about 2000 �C to be deposited on the target substrate. These features are suitablefor an application to the corrosion resistant coating.

So far, several HVOF spray coatings have been subjected to the corrosion testin seawater. In addition to the cermets [3–5], the anti-corrosion alloys [6–8] wereadopted as the coating materials. These studies revealed that the HVOF method wassuperior to other spraying techniques to make the coating having higher corrosionresistance.

In general, the coating is formed on the steel, based on the assumption that thesprayed material has the same corrosion resistance as the original bulk material.Although this assumption may be acceptable, it is essential to reveal the corrosionbehaviour of the spray deposit itself. In this paper, SUS316L stainless steel wascoated onto the SUS316L bulk plate by the HVOF spray technique to avoid for-mation of the galvanic couple by combination of the noble coating and the less-noblesubstrate. SUS316L was one of the widely used stainless steel and was selected as aspray material. The corrosion behaviour of the sprayed coating in seawater wasinvestigated using the electrochemical measurements and the microscopic observa-tions.

2. Experimental

2.1. Specimen preparation

The HVOF spraying of SUS316L was carried out with a TAFA apparatus (JP-5000). The schematic illustration of this spraying system is shown in Fig. 1. Thespray process was described as follows; the mixture of an oxygen gas and a fuel(kerosene) was ignited by the spark plug, resulting in making a flame with the sonicspeed. Then, the powder was fed into the flame and was accelerated to become aflight particle. Immediately, the flight particle was impinged and deposited onto the

Fig. 1. Schematic illustration of HVOF spray system.

2562 J. Kawakita et al. / Corrosion Science 44 (2002) 2561–2581

substrate, leading to form the coating. The primary spray conditions were listed inTable 1. The powder of SUS316L (TAFA 1236F) was 20–53 lm in size and had thefollowing chemical composition; Fe bal, Cr 16.8, Ni 10.8, Mo 2.05, N 0.131, and O0.026 wt.%. The substrate was the SUS316L bulk plate with a dimension of 2t�50� 100 mm. It was blasted with alumina grit and was degreased by ultrasonicationin acetone. The varied spray parameter in this study was the combustion pressure toprepare three kinds of sprayed coatings. One coating was prepared under the stan-dard condition recommended by the manufacturer, which was termed as SP. Theothers were prepared under the higher and lower combustion pressure than SP,which were termed as HP and LP, respectively.

2.2. Characterization

The crystalline phase of the HVOF sprayed coating was characterized by theX-ray diffraction measurement using the Rigaku apparatus (RINT 2000) with theCuKa radiation.

The porosity of the coating was measured by the mercury porosimeter (Mi-cromeritics Autopore II 9220).

The electrochemical measurements were carried out in the conventional three-electrode system. The working electrode was a sprayed specimen, as shown in Fig. 2.A spray-coated plate was cut into square pieces with one side of 2.5 cm and wascleaned ultrasonically in acetone and ion-exchanged water, repeatedly. The surfaceof some of the pieces was polished and finished using Al2O3 (0.05 lm in diameter).The stainless lead was connected with the opposite side of sprayed face of thespecimen. The sprayed area of 2 cm2 left exposed and the rest of the specimen surfacewas insulated with the silicon resin. Two types of electrochemical cells were pre-sented in Figs. 3 and 4. In both cells, the Ag/AgCl electrode in the saturated KClsolution was used as the reference electrode. In the following, the electrode potentialof the sample is represented in volts versus the standard electrode potential of Ag/AgCl/saturated KCl. The electrolyte was artificial seawater of pH 8.3 at 300 K. To

Table 1

Spray conditions

Unit HP SP LP

Fuel flow rate dm3 min�1 0.37 0.32 0.24

Oxygen flow rate dm3 min�1 900 810 590

Combustion pressure MPa 0.81 0.66 0.43

Fuel/oxygen ratio – 0.7a

Barrel length mm 102

Powder feed rate gmin�1 60

Torch velocity mms�1 700

Spray distance mm 380

Powder feed gas – Nitrogen (N2)

Film thickness lm 400

a 1.0 corresponds to stoichiometric mixture ratio.

J. Kawakita et al. / Corrosion Science 44 (2002) 2561–2581 2563

avoid mixing artificial seawater and the KCl solution, and a salt bridge of agarcontaining KCl was adopted in addition to the reservoir of artificial seawater. Thecombination of the counter electrode and the aeration condition depended on themethod of the electrochemical measurement. In the case of the corrosion monitoring,as drawn in Fig. 3, the counter electrode was the same as the working one, andaeration continued during monitoring by air bubbling into the electrolyte. In the caseof the polarization measurement, as shown in Fig. 4, the counter electrode was a Ptplate (0:2t� 100� 100 mm) and de-aeration and aeration continued during theanodic and the cathodic scans, respectively. De-aeration was carried out by blowinga constant N2 gas into the electrolyte.

The corrosion monitoring was one of the electrochemical measurements by meansof the AC impedance, and this method determined the polarization resistance of thesample by subtracting one impedance value at the frequency of 10 kHz from anotherat 100 mHz. These values were obtained by the corrosion monitor (Riken Denshi,Model CT-5). The monitoring was carried out every 10 min for three days.

Fig. 2. Schematic illustration of working electrode of electrochemical cell.


The potentiodynamic polarization method gave the potential–current curve bymeasuring the current value when the electrode potential of the sample was scannedat the rate of 10 mV s�1 using a potentiostat with a function generator (HokutoDenko, HAB-151). The sample electrode was immersed in the electrolyte for 24 h toreach the steady state.

Fig. 3. Schematic illustration of electrochemical cell for corrosion monitoring.

Fig. 4. Schematic illustration of electrochemical cell for polarization measurement.


The surface and the cross-section of the specimens were examined by the visualobservation, by the optical microscope (Olympus, BX60M), and by the scannningelectron microsope (SEM, JEOL JSM-5400). The cross-section of the specimen wasprepared by embedding the specimen into the epoxy resin, part of which was re-moved by the abrading and polishing treatments.

3. Results and discussion

3.1. Physical properties of HVOF sprayed SUS316L coating

Fig. 5 shows the relation between the combustion pressure and the porosity of theHVOF sprayed SUS316L coating, superimposing the relation between the com-bustion pressure and the oxygen content of the coating. The content of other ele-ments of the coating was almost the same as that of the supply powder, regardless ofthe spray condition. As the combustion pressure increased, the porosity decreasedwhile the oxygen content increased. The comparatively high oxygen content indi-cated the presence of the oxide. Fig. 6 compares the XRD patterns of the sprayedcoatings with that of the supply powder of SUS316L. The LP coating had a similarpattern to the powder, indicating the retaining of the original austenite phase duringthe spray process under the LP condition. Some additional diffraction lines, however,appeared in the patterns of the SP and the HP coatings, as marked by arrows. Theselines were ascribed to the ferrite phase. Formation of both the ferrite phase and theoxide was caused by the thermal history of the spray particle during flying and de-positing under atmosphere including oxygen (O2). In addition to the oxygen content,

Fig. 5. Relation between combustion pressure and (a) porosity and (b) oxygen content of HVOF sprayed

SUS316L coating.


the relative intensity of the diffraction lines attributed to the ferrite phase increasedwith the combustion pressure. This result was mainly due to the increase in tem-perature of the spray particle during flying and depositing; according to our recentreport that the higher combustion pressure raised the temperature of the flightparticle [9].

A typical photograph of the surface of the HVOF sprayed SUS316L coating isshown in Fig. 7. In the visual observation, the surface of the sprayed coating was dulland rough, regardless of the spray condition. In the microscopic view, however, therewas an obvious difference in surface state among three types of coatings. The SEMimages of their surfaces were presented in Fig. 8. Almost the entire surface of the LPspecimen seemed to be composed of numerous spherical particles. As the combus-tion pressure increased, the number of the spherical particles decreased whereas thepart like a sandy area expanded. The combustion pressure was related to the flightspeed of the sprayed particle as well as its temperature [9]. At the higher combustionpressure, the fraction of the melted and the softened particles was large during flying,and the degree of plastic deformation was also large upon impinging to the substrate.As a result, flattened and fine particles were formed.

The accumulating state of component particles in the sprayed coating was re-vealed by the cross-sectional view of the coating. Fig. 9 shows the optical micro-scopic images of the cross-section near the surface of the coatings. It seems that everycoating is composed of flattened large and fine particles. In the case of the LPspecimen, there are small voids between the flattened particles. This void behaved as

Fig. 6. XRD patterns of HVOF sprayed coatings of SUS316L under (a) HP (b) SP (c) LP conditions and

(d) supply powder of SUS316L.


a small pore in the coating. Most of such voids could not be observed for the SP andHP specimens because of filling up by the fine particles, the fraction of which in-creased with the combustion pressure. This filling-up phenomenon was caused by theincrease in the temperature and the speed of the spray particles upon impingement,as cited above. As for every coating, there were smaller numbers of voids near theinterface between the coating and the substrate than near the surface, as shown inFig. 10. In particular, the decrease of the void was remarkable for the LP coating.This phenomenon can be explained by the deduction that the deposited particles inthe inner layer of the coating were subjected to the repeated impingement on ac-cumulation of the layer, leading to form a more closely packed structure.

3.2. Corrosion behaviour of HVOF sprayed SUS316L coating

Fig. 11 shows the anodic polarization curves of the HVOF sprayed SUS316Lcoatings. The shape of the curves possessed general features of the stainless steel, i.e.passive and pitting. The former feature corresponds to the gradual increase in cur-rent in the region from the starting potential to about 0 mV, and the latter to therapid increase in current over about 0 mV. In the case of the SUS316L bulk plate, thepassive current kept the value below 1 lA and the pitting started near 200 mV. Thisresult indicated that the SUS316L coatings were not as good as the original bulkplate with respect to corrosion resistance. The effect of the combustion pressure onthe polarization curve appeared clearly in the passive region rather than in the pittingregion. The current values in the passive region were comparatively high, implyingthat an electrochemical oxidation reaction proceeded differently from the ordinarypassive reaction. The average current in the range between the starting potential and0 mV was calculated, and Fig. 12 shows the relation between the combustion

Fig. 7. Photograph of surface of HVOF SUS316L sprayed coating.


Fig. 8. SEM images of surface of HVOF SUS316L sprayed coatings (a: HP, b: SP, and c: LP).


pressure and the average current. The average current of the as-sprayed specimensdecreased with the combustion pressure. It was considered that the anodic current,i.e. the reaction rate of electrochemical oxidation, depended on the porosity and on

Fig. 9. Optical microscopic images of cross-section near surface of HVOF sprayed SUS316L coatings (a:

HP, b: SP, and c: LP).


Fig. 10. Optical microscopic images of cross-section near interface between HVOF sprayed SUS316L

coating and substrate (a: HP, b: SP, and c: LP).


Fig. 11. Anodic polarization curves of HVOF sprayed SUS316L coatings in de-aerated artificial seawater

at 300 K, N: HP (as-sprayed), M: HP (polished), j: SP (as-sprayed), �: SP (polished), .: LP (as-sprayed)

and O: LP (polished). Scan rate is 10 mV s�1.

Fig. 12. Relation between combustion pressure and average current between the starting potential and 0

mV on anodic polarization curves of HVOF sprayed SUS316L coatings ( : as-sprayed and �: polished).


the oxygen content for the following reasons. The results of microscopic observation,mentioned above, showed that the coating with the lower porosity had smallernumber of the small pore in the outer layer near the surface. The small pore canbecome a starting point of oxidative elution of the metal constituent, mainly iron,because it provides the site with a low concentration of dissolved oxygen. On theother hand, the comparatively high oxygen content implied the presence of the oxideto some extent. Some of the effects of the oxide on the corrosion are the appearanceof an additional phase depleted of chromium and the inequality in thickness of theoxide near the surface of a sprayed particle. These can also bring about the oxidativeelution. In this case, the average current was dependent on the porosity rather thanthe oxygen content, resulting from the relation among the combustion pressure andthe porosity and the oxygen content (see Fig. 5). At each combustion pressure, theaverage current of the as-sprayed specimen was lower than that of the polishedspecimen. Furthermore, the variation of the polished specimen in average currentwith the combustion pressure was significantly smaller than that of the as-sprayedspecimen. This phenomenon was mainly due to exposure of the surface composed ofthe closely packed particles by peeling off the porous outer layer of the coating (seeFig. 9), in addition to the decrease in actual surface area.

Fig. 13 shows the cathodic polarization curves of the HVOF sprayed SUS316Lcoatings. The almost steady value of current is seen in the range between �550 and�700 mV, especially obvious in the polished HP specimen. In seawater, the reductionof dissolved oxygen molecules is the primary cathodic reaction on the specimen

Fig. 13. Cathodic polarization curves of HVOF sprayed SUS316L coatings in aerated artificial seawater

at 300 K, N: HP (as-sprayed), M: HP (polished), j: SP (as-sprayed), �: SP (polished), .: LP (as-sprayed)

and O: LP (polished). Scan rate is 10 mV s�1.


surface, and such a steady current is due to the limitation of oxygen diffusion. Thediffusion-limited current depends on the actual surface area. Fig. 14 shows the re-lation between the combustion pressure and the current value at �650 mV on thepolarization curve. The gap in current value between the as-sprayed and the polishedspecimens corresponded to the difference in surface area. Although the HP and theSP specimens had almost the same order of current, they were exceeded by the LPspecimen under both the as-sprayed and the polished conditions. This result wasprobably due to reduction of the oxide in addition to the large actual surface area ofthe LP specimen. In fact, the corrosion product was formed during immersion for 24h before the beginning of the polarization test. This assumption was confirmed bythe existence of a hump near �800 mV on the polarization curve of the as-sprayedLP specimen. From the standpoint of the cathodic reaction, the effect of the porosityand the oxide content of the coating on the corrosion was not clear.

After immersion in aerated artificial seawater for three days, the corrosion productwas observed on the surface of some types of HVOF sprayed SUS316L coatingsalthough the SUS316L bulk plate had no corrosion products on the surface. Fig. 15shows photographs of the surface of the coatings after immersion. Note that theoutside of the circle is a silicon resin used for insulation. In the visual observation,numerous spot-like corrosion products appeared on the surface of the as-sprayed LPspecimen. The number of the corrosion spots of the as-sprayed SP and HP specimenswas significantly smaller than that of the as-sprayed LP specimen. This result alsoindicated that the corrosion rate of the coating depended on its porosity, as men-tioned above. Although the HP specimen had lower porosity than the SP one, thereseemed to be more corrosion spots in the HP specimen. This was probably due to the

Fig. 14. Relation between combustion pressure and current at �650 mV on cathodic polarization curves

of HVOF sprayed SUS316L coatings ( : as-sprayed and �: polished).


effect of the oxygen content on the corrosion rate, as described above. The corrosionproduct decreased considerably by the polishing treatment, and the suppressing effect

Fig. 15. Photograghs of HVOF sprayed SUS316L coatings after immersion in aerated artifical seawater at

300 K for three days, a: HP (as-sprayed), b: HP (polished), c: SP (as-sprayed), d: SP (polished), e: LP (as-

sprayed) and f: LP (polished).


of polishing on the corrosion was notable for the LP specimen. This phenomenon wasmainly caused by peeling off the porous outer layer of the coating, as explained above.

Microscopic observation contributed to elucidate the corrosion mechanism of theHVOF sprayed SUS316L coating. From the microscopic viewpoint, the surface ofthe as-sprayed specimens was too rough to find out a corrosion site in the initialstage. On the other hand, it was easier to detect generation spots on the surface of thepolished specimens. Fig. 16 shows the optical microscopic images of the surface ofthe HVOF sprayed SUS316L coatings with the polishing treatment after immersionin aerated artificial seawater for three days. A corrosion domain was situated in thecenter of these images. As for the LP specimen, the corrosion took place at the smallpore among spray particles, resulting from their insufficient packing. On the otherhand, there were no obvious small pores for the SP and HP specimens. In this case,the corrosion was observed at the boundary among the sprayed particles, whichseemed to be packed closely in this field of vision. These results summarized that thesmall pore among the spray particles was attacked predominantly. This summarywas confirmed by the fact that SUS316L stainless steel was comparatively subject tothe crevice corrosion [10]. In the case of few or no small pores on the surface, thecorrosion took place on the boundary among the sprayed particles. This type ofcorrosion was presumably due to the presence of the alloy with poor corrosion re-sistance near the boundary, stemming from the change in chemical composition ofthe part of the spray particle.

The corrosion process was further elucidated by SEM observation. Fig. 17 showsSEM images of the surface of LP specimen with the polishing treatment after im-mersion in aerated artificial seawater for three days. Although these images were notobtained at the same site, they probably show the sequence of a corrosion processfrom the original (Fig. 17a) to the attacked surface containing the grown corrosionproduct (Fig. 17d). At the first stage of the corrosion, the needle-like corrosionproduct was formed at the small pore among the sprayed particles (Fig. 17b). Insuccession, the corrosion product grew and filled up the pore (Fig. 17c). At the finalstage, the pore was covered with the corrosion product (Fig. 17d).

Both the corrosion potential and the polarization resistance of the HVOF sprayedSUS316L coatings were lower than those of the SUS316L bulk plate, indicating thatthe coating was inferior to the bulk plate in terms of corrosion resistance. Thecorrosion potential and the polarization resistance of the HVOF sprayed SUS316Lcoatings are compared in Figs. 18 and 19, respectively. The curves in these figuresexpressed the changes of their values during immersion for three days in aeratedartificial seawater. As for the LP and SP specimens, both the corrosion potential andthe polarization resistance dropped rapidly at the initial stage of immersion, andthereafter increased gradually. These processes were explained by the initiation ofthe corrosion reaction, followed by its suppression. Once the corrosion took place ina localized small pore, the subsequent anodic reaction at this site lead to be sup-pressed because the small pore was filled with the corrosion product. On the otherhand, the corrosion potential of the HP specimen decreased gradually after rapiddropping, although the HP specimen had a similar behaviour to the LP and the SPspecimens with respect to the polarization resistance. The change in potential and


Fig. 16. Optical microscopic images of surface for HVOF sprayed SUS316L coatings with polishing

treatment (a: HP, b: SP and c: LP) after immersion in aerated artificial seawater at 300 K.


Fig. 17. SEM images of surface of HVOF sprayed SUS316L coating with polishing treatment (a) before

and (b, c and d) after immersion in aerated artifical seawater at 300 K for three days.

Fig. 18. Corrosion potential of HVOF sprayed SUS316L coatings as function of time in aerated artificial

seawater at 300 K, N: HP (as-sprayed), M: HP (polished), j: SP (as-sprayed), �: SP (polished), .: LP (as-

sprayed) and O: LP (polished).


resistance of the HP specimen might be caused by suppression of the cathodic re-action. The suppression mechanism was explained by the deduction that the effective

Fig. 19. Polarization resistance of HVOF sprayed SUS316L coatings as function of time in aerated ar-

tificial seawater at 300 K, N: HP (as-sprayed), M: HP (polished), j: SP (as-sprayed), �: SP (polished), .:

LP (as-sprayed) and O: LP (polished).

Fig. 20. Relation between combustion pressure and polarization resistance of HVOF sprayed SUS316L

coating at 72 h in aerated artificial seawater at 300 K, : as-sprayed and �: polished).


cathodic area decreased slightly because of formation of the corrosion productaround the comparatively numerous particle boundaries. Fig. 20 shows the relationbetween the combustion pressure and the polarization resistance of the sprayedcoatings at the immersion time of 72 h in artificial seawater. Under both the as-sprayed and the polished conditions, the SP specimen had a larger resistance than theothers. This result was caused by the fact that the SP specimen had the smallernumber of the pores than the LP specimen and that it had the smaller oxygen contentthan the HP specimen. The convex curves obtained in the monitoring measurementwere not consistent with the declining curves in the polarization measurement (seeFig. 12). This was explained by the difference in potential of the electrode on whichthe corrosion reaction took place, and in the time of immersion before the mea-surement.

4. Conclusion

The coating of SUS316L stainless steel deposited on the SUS316L substrate withthe HVOF technique possessed a similar characteristic to the ordinary stainless steelin artificial seawater. In addition to the comparatively high current value in thepassive region and the low starting potential of pitting in the anodic polarizationcurve, however, the low polarization resistance by the AC impedance measurementindicated poor corrosion resistance of the coating, compared with the SUS316L bulkplate.

The corrosion rate of the HVOF sprayed SUS316L coating was related to itsporosity and to its oxygen content. In the case of coatings with the higher porosity,the corrosion started and continued at the small pore on the surface of the coating,resulting from insufficient filling up by the sprayed particle. In the case of coatingswith lower porosity, the corrosion took place at the boundary between the sprayparticles, and this type of corrosion was noticeable for the sprayed coating with thehigher oxygen content.

There were larger numbers of small pores in the outer layer near the surface of thecoating than in the inner layer near the interface between the coating and the sub-strate. Such a pore can become the starting point of corrosion when seawater entersthe pore. The polishing treatment peeled the outer layer including the small pores aswell as decreasing of the actual surface area of the coating, leading to the consid-erable improvement of corrosion resistance.

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Documents

Corrosion behaviour of HVOF sprayed SUS316L stainless steel in seawater