Surface and Coatings Technology 162(2002) 6–10

0257-8972/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00560-1

Influence of the spraying processes on the properties of 316L stainlesssteel coatings

Lidong Zhao*, Erich Lugscheider

Materials Science Institute, Aachen University of Technology, Juelicher Street 344a, D-52070 Aachen, Germany

Received 31 January 2002 ; accepted in revised form 26 July 2002

Abstract

The 316L stainless steel coatings produced by different thermal spraying processes are usually used to improve the corrosionbehaviour of steel surfaces. In this study the coatings of 316L stainless steel were produced using atmospheric plasma spraying,shrouded plasma spraying and high velocity oxy-fuel spraying(HVOF). The influence of the thermal spraying processes andspray parameters on the properties of the coatings was investigated. The coatings were studied in terms of their microstructure,oxidation and corrosion behaviour. The corrosion behaviour of the coatings was evaluated using electrochemical polarisationmeasurements in 0.1 N H SO and 0.1 N C H OØH O acid solutions. The experimental results revealed that dense coatings could2 4 6 8 7 2

be produced using all the three processes. The coatings with low oxidation could be produced by HVOF. The HVOF coatingswere harder than other coatings. The HVOF coating showed also the best corrosion behaviour.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Spraying processes; Stainless steel coatings; Corrosion behaviour

1. Introduction

The stainless steel coatings produced by thermalspraying processes provide an alternative to protect steelcomponents in corrosive environmentsw1–3x. However,the stainless steel coatings have in some cases provedto be incapable to protect the steel surface in aggressiveenvironmentsw1x. The coatings, mainly produced byatmospheric plasma spraying(APS) or by wire arcspraying, contain porosity and plenty of oxidesw3–5x.In the worst cases, the porosity and the crevices betweenlamellae in the coatings can ease the penetration ofaggressive media through the coating onto the interfacebetween the coating and the substrate, leading to thedestruction of the bonding between the coating and thesubstrate and finally to a separation of the coating fromthe substratew1–3x. In addition, the spray material isstrongly oxidised during APS and arc spraying. Theoxidation of alloying elements such as chromium

*Corresponding author. Tel.:q49-241-16602-34; fax:q49-241-16602-17.

E-mail address: zhao@msiww.rwth-aachen.de(L. Zhao).

impaired the corrosion behaviour of the coating bychanging the electrochemical potentials in the coatingw4x. Therefore, a low porosity and a low oxidation werevery important for a corrosion resistant stainless steelcoating.In this study the coatings of 316 L stainless steel

were produced using different spraying processes, APS,shrouded plasma spraying(SPS) and high velocity oxy-fuel spraying(HVOF). The influence of the sprayingprocesses and spray parameters on the coating micro-structure, the oxidation of the spray material and thecoating corrosion behaviour was investigated. The coat-ing corrosion behaviour was evaluated using electro-chemical polarisation measurements in 0.1 N H SO2 4

and 0.1 N C H OØH O acid solutions.6 8 7 2

2. Experimental procedure

In this study the spray powder of 316L stainless steelwith a size distributiony53q20 mm was used toproduce the coatings. Commercial mild steel St 37 wasused as the substrate.

7L. Zhao, E. Lugscheider / Surface and Coatings Technology 162 (2002) 6–10

Table 1Spray parameters for plasma spraying processes

Process Plasma gas Current Distance Feed rate(SPLM) (A) (mm) (gymin)

APSySPS AryH2 400–550 115–135 28–4235–42y2–6

Table 3Oxygen content, porosity and microhardness of the coatings

Process APS SPS HVOF

Oxygen(wt.%) 2.22–4.43 1.31–1.94 0.50–1.64Porosity(%) -1–2 -1–2 -0.5Hardness(HV )0.3 241–262 244–257 293–312

Table 2Spray parameters for HVOF spraying process

H2 O2 Ca gas(N )2 Feed rate Distance(SLPM) (SPLM) (SPLM) (gymin) (mm)

680–740 165–205 20–30 40–70 260–300Fig. 1. Cross-section of an APS coating.

APS and SPS were conducted using a spray systemof the company Sulzer Metco with the plasma torch F4.The plasma jet was shielded by a argon gas shroudcoming from a special cap adapted on the plasma torchof the APS equipment during SPS. Both the APS processand the SPS process applied a gas mixture of argon andhydrogen as plasma gas. HVOF spraying was carriedout using a DJ 2600 system of Sulzer Metco withhydrogen as fuel gas. The spray parameters are shownin Tables 1 and 2.Metallographical investigation was carried out using

light microscopy and scanning electron microscopy(SEM). Oxygen contents of the coatings were measuredby means of an OyN-analyser(LECO TC136) aftertheir separation from the substrate. The microhardnessof the coatings(Vickers scale) was measured using amicrohardness tester made by Buehler Ltd, USA.Corrosion behaviour of the coatings was evaluated by

polarisation measurements in 0.1 N H SO and 0.1 N2 4

C H O ØH O acid solutions by means of a potentiostat6 8 7 2

using platinum as counter electrode and a saturatedcalomel electrode as reference. The measurements wereconducted using a scanning rate of 15 mVymin.

3. Results

Table 3 shows the oxygen content, porosity andmicrohardness of the coatings. The HVOF coatings showdifferent oxygen contents. The lowest oxygen contentof the HVOF coatings is only 0.50 wt.%, which is verylow regarding that the spraying process took place atthe air atmosphere without any additional protectingmeasures. The lowest oxidation was realised by usingthe lowest oxygen to hydrogen ratio during HVOFspraying. The ground for it was that the melting degreeof the spray powder decreased by reducing the OyH2 2

ratio during HVOF sprayingw6–8x. In the selectedparameter area, the lower the OyH ratio, the more the2 2

unmolten powder particles. Since the oxidation on suchunmolten particles was dominated by a diffusion mech-anism, the very short in flight time of the particles

limited the oxidation on such particles. With raising theO yH ratio more and more particles were melted during2 2

spraying. The convection dominated oxidation on themelted particles led to more oxidation. It is reported inw8x that a coating formed from totally melted powderparticles could have a oxygen content of more than 8wt.%. Comparing the APS coatings with the SPS coat-ings, it is clearly seen that the oxidation during plasmaspraying at the air atmosphere can be lowered by usingargon shield gas. Both during APS and during SPS theoxidation of the spray material increased with raisingthe plasma power, namely with raising the hydrogenflow rate and the current. The reason was that theincrease of plasma power could lead to a better meltingof the powder on the one hand, but could also lead tooverheating the spray powder on the other hand. Theoverheating of the powder particles was favourable forthe oxidation of the spray particles. Compared with theHVOF spraying in this study, the spray powder wasmelted much better during the plasma spraying pro-cesses. Therefore, the spray powder was oxidised moreduring SPS despite using argon shield gas than duringHVOF spraying.Figs. 1–3 show the cross-sections of an APS, a SPS

and a HVOF coating. Both the APS coating and theSPS coating show a typical lamellar structure indicatingthat the spray powder was melted well during spraying.The dark particles and dark thin lamellae are the oxidesformed during spraying. Fig. 4 shows a SEM photograph

8 L. Zhao, E. Lugscheider / Surface and Coatings Technology 162 (2002) 6–10

Fig. 2. Cross-section of a SPS coating. Fig. 3. Cross-section of a HVOF coating.

Fig. 4. SEM micrograph and mapping picture of the APS coating.

of the APS coating and the oxygen, chromium andmolybdenum distributions in its mapping picture. It canbe seen that the concentrations of chromium and molyb-denum in the oxides vary. It is reported inw8x that themicro electron probe analysis of the oxides in highalloyed steel coatings showed that the oxides are mixediron oxides containing different contents of Cr, Ni andMo. Compared with the APS coating, less dark oxidesare seen in the SPS coating. The HVOF coating shows

no lamellar structure. Instead, there are a lot of deformedparticles, which did not melt during spraying. Thedeformation of the particles was caused by the highkinetic energy of the particles while their impacting onthe substrate. Some small dark oxides are also seen inthe HVOF coating which mainly resulted from themelted particles.The porosity measurements revealed that dense coat-

ings could be produced using all the three spray pro-

9L. Zhao, E. Lugscheider / Surface and Coatings Technology 162 (2002) 6–10

Fig. 5. Polarisation curves of the coatings in 0.1 N H SO .2 4

cesses. The densest coatings are HVOF coatings. Insome HVOF coatings no pore can be seen. The groundis the very high kinetic energy of the particles with highplastic performance during HVOF spraying. The APSand SPS coatings are some more porous than the HVOFcoatings. But the densest coatings of them have also aporosity of less than 1%.The HVOF coatings are harder than other coatings,

which resulted from the reinforcement caused by thedeformation of the unmolten particles and the peeningeffect of the in flight particles on the before sprayedlayers. The APS coatings are almost as hard as the SPScoatings.Fig. 5 shows the polarisation curves of a HVOF

coating containing 0.54% O, a SPS coating containing1.34 wt.% O and an APS coating containing 4.32 wt.%in 0.1 N H SO solution. All the three coatings showed2 4

a very similar corrosion potential in this solution. All ofthem had a passivity in the solution. The HVOF coatinghad a lower maximum current density in the activeregion than those of the SPS and APS coatings. Themaximum current density in the active region of theAPS coating was much higher than those of othercoatings. In the passivity region the HVOF coatingshowed the lowest densities. The SPS coatings showedsome higher current densities than the HVOF coating,but much lower current densities than the APS coating.The results of the polarisation measurements show thatthe HVOF coating provided the best corrosion behav-iour. The SPS coating was not so corrosion resistant asthe HVOF coating, but much better than the APScoating. The lower corrosion resistance of the APS andSPS coatings than the HVOF coating could be attributedto the following two factors. Firstly, the HVOF coatingswere denser than the SPS and APS coatings. The real

corrosion surface could be reduced thereby. Secondly,the SPS and APS coatings contain more oxides andcorrespondingly had more burnout of alloying elementsin the coating, leading to lack of chromium at somelocations where the corrosion occurred more stronglyw1x. Because the SPS coating was as dense as the APScoating, the significant difference in their corrosionbehaviour must result from their different oxidationdegrees. Therefore, the oxidation during spraying is thecrucial factor influencing the corrosion behaviour. Basedon the above results, it is suggested that the corrosionresistant stainless steel coating of high quality shouldbe produced using HVOF process.Fig. 6 shows the polarisation curves of the coatings

in 0.1 N C H O ØH O solution. All the three coatings6 8 7 2

showed a passivity in this solution. The HVOF coatingshowed a little higher corrosion potential and a littlelower maximum current density in the active regionthan other coatings. The SPS and APS coatings showeda similar corrosion potential and a similar maximumcurrent density in the active region. The HVOF coatingshowed lower current densities in the passivity regionthan other coatings. It is very remarkable that the currentdensities of the APS coating were not much higher thanthose of other coatings until a potential of approximately350 mV in this solution. In general, the differences inthe corrosion behaviour in this solution among thecoatings were much lower than those in the strongerH SO acid solution. Compared with the curves in Fig.2 4

5, it is seen that the current densities of the APS coatingin 0.1 N C H O ØH O solution were already lower than6 8 7 2

those of the HVOF coating in the 0.1 N H SO solution.2 4

Therefore, it can be said that the corrosion behaviour ofthe coatings depends strongly on the corrosive environ-ments. The application conditions for a protective stain-

10 L. Zhao, E. Lugscheider / Surface and Coatings Technology 162 (2002) 6–10

Fig. 6. Polarisation curves of the coatings in 0.1 N C H OØH O.6 8 7 2

less steel coating is essentially important to decide whatspray process should be used to produce the coatingwhich are suitable regarding both the cost and thecoating quality. In case of high requirements on thecorrosion resistance, HVOF process should be the bestcandidate, while in case of lower requirements APSprocess may already provide suitable coatings.

4. Conclusions

In this study APS, SPS and HVOF spraying processeswere used to produce 316L stainless steel coatings. Theoxidation of the spray material could be significantlyreduced using HVOF process with suitable parameters.The argon shield gas applied during SPS could alsolower the oxidation of the spray material. The HVOFcoatings were denser and harder than the SPS and APScoatings. The HVOF coating also showed the bestcorrosion behaviour among all the coatings. The corro-sion behaviour of the coatings depends strongly on the

corrosive conditions. The HVOF coating in the H SO2 4

solution was already corroded much more than the APScoating in the C H OØH O solution.6 8 7 2

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