The corrosion behaviour of HVOF sprayed stainless steel and nickel alloy coatings in artificial seawater

A J Sturgeon
Paper No.03245 presented at CORROSION 2003, NACE Conference, 16-21 March 2003, San Diego, CA, USA

Abstract

One of the principal aims was to determine if HVOF sprayed coatings of stainless steel and nickel alloys have similar corrosion properties in seawater compared to conventional wrought materials. The work reported here measured thelevel of corrosion performance that can be expected from coatings of corrosion-resistant alloys deposited onto a steel substrate using commercially available HVOF spraying systems. Two alloy types were considered, a stainless steelwith a composition similar to 316L (UNS S31603) and a nickel base alloy with composition similar to alloy 625 (UNS N06625). The cyclic potentiodynamic polarisation method was used to examine the corrosion behaviour of these coatingsand the same alloys in wrought form. A HVOF sprayed coating of nickel alloy 625 was found to be more corrosion resistant in seawater than a coating of 316L stainless steel. However, the nickel alloy coating did not match the corrosionresistance of the same nickel alloy in wrought form, but may have the ability to offer corrosion resistance (in seawater) approaching that of wrought stainless steel. The lower level of corrosion resistance of the nickel alloy coatingcompared to the same material in wrought form is believed to be due to microstructural differences and in particular related to preferential attack along the inter-particle (splat) boundaries of the coating.

Introduction

The use of thermal sprayed coatings of corrosion-resistant alloys such as stainless steels or nickel alloys to protect an underlying steel substrate has received much interest over the past few years. Such coatings are believedsuitable for applications where a barrier layer is needed to protect steel components or structures against corrosion in seawater or corrosive solutions such as mineral or organic acids. This interest is in part due to the expectationthat very low porosity coatings of these metallic alloys can be prepared using the High Velocity Oxygen Fuel (HVOF) spraying process. The presence of porosity or other defects in the coating is of concern because these can provide apath for seawater (or corrosive solution) to reach the substrate. Rapid attack of the substrate at this point could then occur. This may also lead to the situation where an electrochemical cell is set up with the coating acting as alarge cathode driving rapid corrosion at localised (anode) sites on the steel substrate. The requirement therefore is to deposit coatings with guaranteed low levels of porosity that is 'closed' in nature. The HVOF spraying process hasbeen shown to deposit coatings of several alloy types, including stainless steels and nickel alloys with both low levels of porosity (less than 2 vol%) and very low levels of oxide (less than 2 wt%). ^[1] This is a consequence of the higher particle velocities and relatively lower particle temperatures obtained with HVOF spraying compared to most other thermal spraying processes.

However, even with the attainment of such low porosity microstructures, HVOF sprayed coatings may not offer the same level of corrosion resistance as the corresponding wrought materials. The more inhomogeneous microstructurespresent in the sprayed coatings compared to the same material in wrought form is likely to affect corrosion properties. The coating microstructure is dominated by inter-particle (splat) boundaries, often depleted in alloy elements, andby the presence of thin oxide films at these (splat) boundaries. ^[2]

The corrosion performance of coating systems in aqueous environments can be difficult to evaluate. Reliance on immersion methods to compare corrosion behaviour requires long test durations (often 60 days or longer) and a qualitativevisual judgement of any corrosion attack. Electrochemical test techniques are used to provide a quicker and more quantitative tool for evaluating and comparing the corrosion behaviour of wrought alloy materials in aqueous environments.In seawater and some dilute acid environments, localised pitting and crevice attack of corrosion resistant alloys are usually of concern and can lead to the breakdown of corrosion resistance. The ASTM Standard G61 describes a procedurefor conducting cyclic potentiodynamic polarisation measurements to determine relative susceptibility to localised corrosion of wrought iron- or nickel- based alloys in chloride-containing environments. More recently, such techniqueshave also been applied to thermal sprayed metallic coatings ^[3,4,5] to provide a relatively quick method to rank their resistance to corrosion.

One of our principal aims was to determine if HVOF sprayed coatings of stainless steel and nickel alloys have similar corrosion properties in seawater compared to wrought materials. The work reported here compared the level ofcorrosion performance of coatings of corrosion-resistant alloys deposited onto a steel substrate using commercially available HVOF spraying systems. Two alloy types were considered, a stainless steel with a composition similar to 316Land a nickel alloy with composition similar to alloy 625. These coatings were prepared within a larger joint industry project, not reported here, that optimised their corrosion resistance in aqueous (sea water and dilute acid)environments. The cyclic potentiodynamic polarisation method was used to examine the corrosion behaviour of these coatings and the same alloys in wrought form.

Experimental procedure

Coating preparation and characterisation

Coatings of 316L stainless steel and nickel alloy 625 were sprayed onto low carbon (0.14% C) steel using three commercially available HVOF systems: the JP5000 (JP) and TopGun (TG) HVOF systems from Praxair Surface Technologies andthe Diamond Jet Hybrid (DJ) HVOF system from Sulzer Metco. The three HVOF systems differ quite significantly in terms of nozzle design and type of fuel gas used in the high-pressure combustion process. Detailed descriptions of thesedifference are described elsewhere. ^[1] Coatings were prepared using different spraying parameter settings within a 'design of experiment' approach (not reported here) to maximise their corrosion resistance. Gas atomised powders were used which were spherical inshape. The HVOF system, fuel type and powder type used to prepare the coatings reported in this paper are given in Tables 1 and 2.

Table 1 HVOF sprayed 316L stainless steel coatings

Table 2 HVOF sprayed nickel alloy 625 coatings

Test pieces of low carbon steel with dimensions 50 mm x 50 mm were coated to a thickness of about 300µm. Cross sections of each coating were prepared and examined by optical and scanning electron microscopy. The level ofporosity was measured from optical images of the cross-section using quantitative image analysis equipment. For each coating three measurements were made and a mean value calculated. The oxygen content was measured on samples ofcoating detached from the substrate and ground to a powder. The level of oxygen was analysed by the inert gas fusion technique, using Leco TC 136 equipment. Estimates of the oxide level in the coatings were calculated on the assumptionthat the detected oxygen was associated with the presence of Cr ₂O ₃.

Measurement of coating adhesion

Adhesion was measured in accordance with ASTM C633-79 (Re-approved 1993) 'Standard test method for adhesion or cohesive strength of flame-sprayed coatings'. This test attempts to measure the adhesion (bond strength) of a coating toa substrate, or the cohesive strength of the coating, in tension normal to the substrate surface. A high strength structural adhesive was used to bond the loading fixture to the coating. For each coating type, the adhesion of five testpieces was measured and the average failure stress and standard deviation calculated.

Electrochemical testing

The corrosion behaviour of the coated test pieces was evaluated using an electrochemical test method similar to that described by ASTM G61 for wrought iron- and nickel-based alloys. The coating surface was tested in the as-sprayedcondition, immediately following a detergent wash, water rinse, acetone degrease and drying in air. The electrochemical corrosion test comprised cyclic anodic polarisation in artificial seawater solution (3.5% NaCl) with a pH of 8.2and at a temperature of 25°C. The seawater solution was de-aerated by purging with nitrogen.

A 50x50 mm coated test piece was clamped to the bottom of an Avesta-type cell arrangement, with a 1 cm ² diameter area of the coating in contact with a 250 ml volume of de-aerated seawater solution. The cell seal design allowed flushing with purified water to minimise crevice corrosion problems associated with thecell edge contacting the coating. After a stabilisation period of 2 hours, the corrosion potential was measured relative to a Saturated Calomel reference Electrode (SCE). The coated sample was then anodically polarised from thecorrosion potential at a rate of 10 mV.min ^-1, whilst measuring the corrosion current to a platinum counter electrode. On reaching an anodic current density of 10 mA.cm ^-2, the applied potential was reversed and scanned back down to the corrosion potential. A plot of anodic current density in mA.cm ^-2 from the 1 cm ² test area was obtained as a function of the applied potential (mVsce). Polarisation plots were also obtained for the low carbon steel substrate and wrought samples of 316L stainless steel and nickel alloy 625,both with similar composition to their respective coating materials. The surface of the wrought alloy samples was abraded prior to testing using 600 grit SiC paper and then degreased as described above for the coatings.

In the electrochemical polarisation test, the anodic current density is a measure of material dissolution from the surface being tested. A rapid increase in anodic current during the test is often associated with the initiation andpropagation of localised corrosion due to the formation of pits, the presence of crevices or a breakdown of the oxide film. For the low porosity coatings prepared in this work, it was assumed that the measured anodic current originatesprimarily from the coating. In an actual service environment the coating may be exposed to anodic potentials that could reach 400 mV for natural seawater with the presence of a bio-film, and possibly up to 600 mV in a chlorinatedseawater environment.

The corrosion potential at the end of the stabilisation period and the anodic current density at potentials of 100, 400 and 600 mV _SCE for the forward scan were used as measures to compare the corrosion performance of the different coatings and wrought alloy materials. Coatings with higher (more noble) corrosion potentials, and lower anodiccurrents at the selected potentials, were taken to have better resistance to corrosion and expected to provide greater protection to the underlying steel substrate. When interpreting the polarisation plots for corrosion resistantalloys in wrought form, the presence of an anodic current above typically 0.01 mA.cm ^-2 is often taken to indicate the onset and progression of localised pitting or crevice corrosion attack at the surface of the material being tested.

Results and discussion

Coating Microstructures

Measured porosity and oxide levels for the 316L stainless steel coatings are shown in Table 3 and in Table 4 for the nickel alloy 625 coatings. These results show that coatings were prepared with low levels of porosity, at or below 4 vol%. There was a larger variation in the level of oxide in the coatings, ranging from about18 wt% down to less than 1 wt% depending on material type, HVOF system and powder size.

Table 3 Properties of HVOF sprayed 316L stainless steel coatings

¹ Oxide level calculate from measured oxygen content and assumption that the oxide is Cr ₂O ₃

Table 4 Properties of HVOF sprayed nickel alloy 625 coatings

For the stainless steel coatings, the TG HVOF system produced coatings with the highest oxide levels and lowest amount of porosity. A cross section through one of the stainless steel coatings prepared using the TG system is shown inFigure 1. This image shows that the powder was highly melted during the spraying process to give a lamella type microstructure with oxide stringers (darker contrast phase) aligned parallel to the substrate surface. Thestainless steel coatings prepared using the JP and DJ HVOF systems both had much lower levels of oxide, but higher amounts of porosity than the TG coatings. Cross sections through these coatings are shown in Figures 2 and 3. The microstructure obtained with the JP and DJ HVOF systems appear to consist predominately of well stacked, partially deformed particles. This microstructure type suggests the powder particles were at a lowertemperature on impact with the substrate, possibly below their melting point, when sprayed using these two HVOF systems. The results in Table 3 also indicate that a smaller powder size range (15 to 45 µm) gave coatings with a slightly lower porosity. Similarly, the oxide level in each coating type is dependent on the powder size range, with the smallersize giving a noticeably higher oxide content in the prepared coatings.

Again for the nickel alloy coatings, the TG HVOF system produced coatings with the highest oxide content. Coatings with considerably lower oxide levels were prepared with the JP and DJ HVOF systems. However, unlike the stainlesssteel coatings, there was little difference in the porosity levels. With all three HVOF systems porosity levels of about 2 vol% were obtained. Cross sections through nickel alloy coatings prepared by each HVOF system are shown in Figures 4 to 7

For all the coatings the measured values of adhesion were considered good, and in most cases were about 80 MPa with failure occurring at the coating to substrate interface. The exception was the stainless steel coatings preparedusing the JP system, which gave lower adhesion values of below 60 MPa with the coating being removed from the substrate.

Corrosion behaviour

As reported above, different coating microstructures were obtained for both the stainless steel and nickel alloy coatings, depending partly on the HVOF system used to prepare the coating. In all cases the coating microstructures aredifferent than those seen for these materials in wrought form. The accelerated electrochemical corrosion test was used to compare the corrosion behaviour of the HVOF coatings to wrought material.

The measured corrosion potentials and anodic current density at 100, 400 and 600 mV _SCE are given in Table 3 for the stainless steel coatings, Table 4 for the nickel alloy coatings and Table 5 for the same materials in wrought form. In addition, values are also given for the low carbon steel substrate alloy. The forward scan of the polarization curve for some of the stainless steel coatings and for wrought316L stainless steel are shown in Figure 8. Likewise the forward scan for selected nickel alloy coatings and wrought nickel alloy 625 are shown in Figure 9. Only the forward scan is shown for clarity.

Examination of these results reveal that for the two coating types, the stainless steel coated test pieces gave the highest anodic current densities, measured at 4.3 to 8.4 mA.cm -2

The results also show that for both the stainless steel and nickel alloy coatings, the TG HVOF system produced coatings with much higher anodic current densities when polarized above the rest potentials. This was taken to indicatethat these coatings were experiencing higher levels of corrosion. The difference in corrosion behaviour is associated with the different coating microstructures. A microstructure consisting of well stacked partially deformed particles,typically obtained with the JP and DJ HVOF systems, appears more resistant to corrosion that the lamella type microstructure with higher oxide obtained using the TG system.

Comparison with wrought alloys

In the same electrochemical test, the wrought 316L stainless steel showed a plot typical for this material ( Figure 8). On anodic polarization from the rest potential, the anodic current density initially increased slowly and did not exceed 0.01 mA.cm ^-2. On reaching a potential of about 275 mV _SCE a rapid increase in corrosion current occurred, associated with the onset of pitting. All the HVOF sprayed coatings of 316L stainless steel exhibited a significantly lower rest potential and showed a much morerapid rise in anodic current density as the potential was increased. The coatings also showed an apparent breakdown potential at about 0mV _SCE, considerably lower than that seen for the wrought alloy.

The wrought nickel alloy 625 exhibited a gradual increase in anodic current density as the potential was raised from the rest potential. This is shown in Figure 9. All the nickel alloy 625 coatings showed higher anodic current densities at potentials below about 400 mV _SCE than the nickel alloy in wrought form. The more heavily oxidised coatings of this material prepared by the TG HVOF system had a considerably more negative corrosion potential and much higher anodic currentdensity.

A part of the forward scan obtained for one of the better nickel alloy coatings (JP11) together with those for wrought 316L stainless steel, wrought nickel alloy 625 and for the uncoated carbon steel substrate are shown in Figure 10. These results indicate that the nickel alloy coating on a carbon steel substrate has a much lower anodic current density and consequently is believed to have better corrosion resistance than the uncoated substratematerial. However, the corrosion resistance of the nickel alloy coating does not match that of the same alloy in its wrought form. The nickel alloy coating gives a much higher anodic current density than the wrought nickel alloy, andhas values slightly higher than those measured for wrought 316L stainless steel (below its pitting potential). A steel substrate with a HVOF sprayed nickel alloy coating is believed to have the ability to offer corrosion resistance inseawater similar to wrought 316L stainless steel, but not that of the wrought nickel alloy.

The nature of corrosion attack of the nickel alloy coating during the electrochemical test is being examined. In one test a coating prepared in a similar manner to the coating labelled DJ37 was held at +300 mV SCE

Fig. 12. Cross section through coating after exposure to test solution for 20 hours to illustrate corrosion along inter-particle boundaries

Conclusions

1. A steel substrate coated with HVOF sprayed nickel alloy 625 may have the ability to offer corrosion resistance (in seawater) approaching that of wrought 316L stainless steel, but not that of the nickel alloy in wrought form.
2. HVOF sprayed coatings of nickel alloy 625 can provide better resistance to corrosion in seawater than coatings of 316L stainless steel at equivalent cost.
3. The lower level of corrosion resistance of the nickel alloy coating compared to wrought material appears to be related to preferential attack along the inter-particle (splat) boundaries

Acknowledgements

The author would like to acknowledge the participation and support of EWI, BP, Shell, Petrobras, Marathon Oil, US Navy, UK Navy, IHI, and Sulzer Metco.

References

1. Kreye, H., Gartner, A., Kirsten, and Schwetzke, R, 'High Velocity Oxy-Fuel Flame Spraying', Proceedings of the 5th HVOF Spraying Colloquium, Erding, Pub. GTS, (2000).
2. H. Edris, D.G. McCartney and A.J. Sturgeon: Microstructural characterisation of high velocity oxyfuel sprayed coatings of Inconel 625, J.Mat.Sci 32, 863-872 (1997).
3. S. Kuroda et al: Microstructure and corrosion resistance of HVOF sprayed 316L stainless steel and Ni base alloy coatings, Thermal Spray - Surface Engineering via Applied Research, Ed. CC Berndt, Pub. ASM International, (2000).
4. C. M. Eminoglu et al: Potentiodynamic corrosion testing of HVOF sprayed stainless steel alloy, Tagungsband Conference Proceedings, Ed. E. Lugscheider, P.A. Kammer, Pub. DVS, Germany (1999).
5. A. J. Sturgeon and D. Buxton: The electrochemical behaviour of HVOF sprayed coatings, Thermal Spray - Surface Engineering via Applied Research, Ed. CC Berndt, Pub. ASM International, (2000).