Surface and Coatings Technology 162 (2002) 610
0257-8972/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972 02 .00560-1
Influence of the spraying processes on the properties of 316L stainlesssteel coatings
Lidong Zhao*, Erich LugscheiderMaterials Science Institute, Aachen University of Technology, Juelicher Street 344a, D-52070 Aachen, Germany
Received 31 January 2002 ; accepted in revised form 26 July 2002
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 O H O acid solutions. The experimental results revealed that dense coatings could2 4 6 8 7 2be 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
The stainless steel coatings produced by thermalspraying processes provide an alternative to protect steelcomponents in corrosive environments w13x. However,the stainless steel coatings have in some cases provedto be incapable to protect the steel surface in aggressiveenvironments w1x. The coatings, mainly produced byatmospheric plasma spraying (APS) or by wire arcspraying, contain porosity and plenty of oxides w35x.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 substrate w13x. 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: email@example.com (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 4and 0.1 N C H O H O acid solutions.6 8 7 2
2. Experimental procedure
In this study the spray powder of 316L stainless steelwith a size distribution y53q20 mm was used toproduce the coatings. Commercial mild steel St 37 wasused as the substrate.
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Table 1Spray parameters for plasma spraying processes
Process Plasma gas Current Distance Feed rate(SPLM) (A) (mm) (gymin)
APSySPS AryH2 400550 115135 28423542y26
Table 3Oxygen content, porosity and microhardness of the coatings
Process APS SPS HVOF
Oxygen (wt.%) 2.224.43 1.311.94 0.501.64Porosity (%) -12 -12 -0.5Hardness (HV )0.3 241262 244257 293312
Table 2Spray parameters for HVOF spraying process
H2 O2 Ca gas (N )2 Feed rate Distance(SLPM) (SPLM) (SPLM) (gymin) (mm)
680740 165205 2030 4070 260300Fig. 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 4C H O H O acid solutions by means of a potentiostat6 8 7 2using platinum as counter electrode and a saturatedcalomel electrode as reference. The measurements wereconducted using a scanning rate of 15 mVymin.
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 O yH2 2ratio during HVOF spraying w68x. In the selectedparameter area, the lower the O yH ratio, the more the2 2unmolten 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 2spraying. 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. 13 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) 610
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 in w8x 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) 610
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 4a very similar corrosion potential in