Paper 171 presented at ITSC 2002 International Thermal Spray Conference, 4-6 March 2002, Essen, Germany.
Coatings of corrosion resistant alloys are used for the protection of steel component surfaces exposed to seawater and other corrosive solutions such as mineral and organic acids, or alkaline solutions. The coating acts as a barrier layer and needs to be free of any connected porosity to prevent penetration by the corrosive solution. In the work reported here a Diamond Jet HVOF spray gun has been modified to allow deposition of stainless steel and nickel alloy coatings with very low levels of both oxide and porosity. The corrosion performance of these coatings was examined in a seawater solution using accelerated tests based on electrochemical methods and compared with coatings prepared using the conventional Diamond Jet (DJ2600) system, and with material in bulk form. In addition, cross-sections of the coatings before and after corrosion testing have been examined to identify any attack of the coating or underlying steel substrate.
Corrosion resistant coatings of stainless steels and nickel alloys deposited using the HVOF spraying process have received much interest over the past few years. This is in part due to the expectation that very low porosity coatings of these metallic alloys can be prepared. Such coatings may act as a barrier layer and protect an underlying steel substrate against corrosion in seawater or corrosive solutions such as mineral or organic acids. However, HVOF sprayed coatings do not appear to offer the same level of corrosion resistance as the corresponding bulk materials.  This may be due to the more inhomogeneous microstructures present in the sprayed coatings compared to the same material in bulk form. The coating microstructure is dominated by inter-particle (splat) boundaries, often depleted in alloy elements, together with the presence of porosity and oxide films.
The HVOF spraying process has been shown  to deposit coatings of several alloy types, including stainless steels and nickel alloys with low levels of porosity (less than 2%) and low levels of oxide (again less than 2%). 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. Modifications to current HVOF systems allow further improvement in the quality of metallic coatings, in particular a further lowing of porosity and oxide levels.
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 and a qualitative visual 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 bulk alloy materials in aqueous environments. In seawater and other chloride 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 procedure for conducting cyclicpotentiodynamic polarisation measurements to determine relative susceptibility to localised corrosion of bulk iron or nickel based alloys in chloride containing environments. More recently such techniques have also been applied to thermal sprayed metallic coatings [1,3,4,5,6] to provide a relatively quick method to rank their resistance to corrosion.
The work reported here examined the corrosion properties of stainless steel and nickel alloy coatings prepared using the conventional Diamond Jet (DJ2600) HVOF system and a Diamond Jet system modified to produce coatings with reduced oxide and porosity levels. Two alloy types were considered, stainless steel 316 and a nickel alloy with composition similar to 625 alloy. Cyclic potentiodynamic polarisation and potentiostatic electrochemical methods were used as accelerated corrosion tests.
2 Experimental procedure
2.1 Coating preparation and characterisation
Coatings of stainless steel 316 and nickel alloy 625 deposited onto a 1018 (AISI) steel substrate were supplied by Sulzer Metco Inc. These coatings were prepared using a conventional Diamond Jet HVOF system with hydrogen as the fuelgas, referred to as DJ2600 coatings, and using a modified Diamond Jet HVOF system. Details of the modifications are confidential, but were made to reduce coating oxide levels whilst still attempting to achieve minimum porosity levels. These coatings are referred to as modified DJ coatings.
Cross sections of each coating were prepared using standard metallographic methods and images collected by optical microscopy.
2.2 Electrochemical testing
Only the corrosion behaviour of the 316 coated test pieces has been examined to-date. The corrosion behaviour was first examined using a potentiodynamic electrochemical test similar to that described by the ASTM standard G61 forbulk iron and nickel based alloys. This corrosion test comprised of cyclic anodic polarisation in artificial seawater solution (3.5% NaCl) purged with nitrogen, with a pH of 8.2 and at a temperature of 25°C. Details of the testare described elsewhere.  A plot of corrosion current in mA from a 1cm 2 test area was obtained as a function of the applied potential (in mVsce). In the electrochemical polarisation test, the corrosion current density is a measure of material dissolution from the surface being tested. A rapid increase in corrosion current during the test is often associated with the initiation and propagation of localised corrosion due to the formation of pits or the presence of crevices. Coatings with a lower corrosion current at a selected potential, were taken to have better resistance to corrosion. A rapid increase in corrosion current density, reaching values above 0.01 mA/cm 2, is often taken to indicate the onset and progression of significant localised pitting or crevice corrosion attack at the surface of the material being tested.
A further test was undertaken in which the potential was held constant whilst the corrosion current density was measured over a time duration of 20 hours. This potentiostatic test had the same equipment set-up as described before.  The potential was selected from examination of the polarisation scans and was a potential that produced a corrosion current density in excess of 0.01 mA/cm 2. The selected potential was -100 mVsce for the stainless steel coatings. Corrosion currents densities of greater than 0.01 mA/cm 2 were expected to produce significant damage in the coating over a period of 20 hours. Cross sections through the coatings were prepared after the 20 hour test and evidence of corrosion attack identified using optical microscopy.
The coating surface was tested in the as-sprayed condition, immediately following a detergent wash, water rinse, acetone degrease and drying in air.
The same corrosion tests were also undertaken on bulk samples of stainless steel (316 type). The surface of the bulk alloy was first roughened using 600 grit SiC paper and then degreased as described above for the coatings.
3 Results and discussion
3.1 Coating microstructures
Cross sections of the coatings are shown in Fig.1. The DJ2600 coatings of stainless steel 316 and nickel alloy 625 have a layered structure consisting of well consolidated metallic layers separated by a darker contrast phase believed to be oxide. Each layer represents one pass of the spray gun over the test sample. Despite the noticeable oxide content, both coatings have little porosity and the powder particles are well deformed with little evidence of un-melted or partially deformed particles. The modified DJ produced coatings have very little oxide visible in the cross section. These coatings consist mostly of partially deformed particles densely stacked to give very low levels of porosity. The nickel alloy coating in particular appears fully dense with almost no visible porosity.
Fig.1. Cross sections of as-sprayed coatings:
a) stainless steel 316 using DJ2600 b) using modified DJ c) nickel alloy 625 using DJ2600 d) using modified DJ
3.2 Corrosion behaviour
The potentiodynamic plots for the stainless steel 316 coatings and for the same alloy in bulk form are shown in Fig.2. The bulk 316 alloy shows the expected behaviour for this alloy in a chloride solution. Initially the corrosion current remains very low, at values below 0.01 mA/cm 2, as the potential is raised from the rest potential. In this region the bulk alloy is considered passive and resistant to corrosion. Above a potential of about 280 mVsce (referred to as the breakdown potential),the corrosion current density rapidly increases due to the onset of localised corrosion as pits form on the surface being tested.
The coatings also show such a breakdown potential above which there is a rapid increase in current density. However this potential, at about 0 mVsce for the modified DJ coating, is much lower than the breakdown potential for the bulk alloy. In addition the current density below the breakdown potential for both coatings is much higher than that for bulk stainless steel. These results suggest that both coatings are more susceptible to corrosion than the same alloy in bulk form. There is some difference in corrosion behaviour between the two coatings. The modified DJ coating has a slightly higher breakdown potential and a lower corrosion current density below this potential. At a potential of -100 mVsce the modified DJ coating has a corrosion current density of about 0.025 mA/cm 2 compared to 0.22 mA/cm 2 for the DJ2600 coating. This indicates that the modified DJ coating is more resistant to corrosion than the DJ2600 coating, but not as resistant as bulk stainless steel 316.
Fig.2. Potentiodynamic polarisation plots for stainless steel 316 coatings and bulk alloy, forward scan only
To further identify differences in corrosion performance between the stainless steel coatings, potentiostatic corrosion tests were undertaken at a potential of -100 mVsce. These tests were performed on new samples. The results ofthe test are shown in Fig.3. Both the modified DJ coating and DJ2600 coatings initially have a very low corrosion current density of about 0.0001 mA/cm 2. This rapidly increases for the DJ2600 coating up to a value of about 1 mA/cm 2 after 2 hours, and then stays at this value for the remainder of the test duration. The modified DJ coating exhibits a slower increase in corrosion current density to reach a value of about 0.01 mA/cm 2 after 6 hours. Again this value is maintained for the remaining test duration. These results suggest that the modified DJ coating is more resistant to corrosion attack giving a lower corrosion rate. The corrosion current density is low for this coating and indicates that at -100mVsce potential, very little corrosion is occuring. The corrosion current density of 1 mA/cm 2 obtained for the DJ2600 coating at this potential suggests a significant amount of corrosion attack is occurring. The bulk alloy shows quite different behaviour with the corrosion current density, and correspondingly corrosion rate, dropping during the test to a much lower value.
Fig.3. Potentiostatic plots for stainless steel 316 coatings and bulk alloy
Examination of the coatings after the 20 hour duration revealed that localised corrosion attack had occurred on the coating deposited using the DJ2600 system. This coating had a dark grey circular test area with the presence of red coloured spots on the coating surface, Fig.4.
These features were not observed on the coating prepared with the modified DJ system. This coating had a light brown staining over the circular test area. By comparison the bulk stainless steel surface was only lightly stained after the test with no evidence of any localised corrosion.
Fig.4. Appearance of stainless steel 316 coating surface after exposure to the potentiostatic corrosion test:
a) DJ2600 b) Modified DJ
A cross section through the DJ2600 coating showed several locations where corrosion attack of the coating had occurred, Fig.5. These appear as a pits located in the upper portion of the coating and in some cases extended down through the coating to reach the substrate. In such locations corrosion of the steel substrate was observed.
Fig.5. Cross sections of stainless steel 316 coating after potentiostatic corrosion test:
b) Modified Diamond Jet
The cross section through the modified DJ coating did not reveal any significant corrosion features within the coating, Fig.5. Although no substantial corrosion of the steel substrate was observed, there was possible evidence of corrosion along the interface with the underlying steel substrate. This appears as an intermittent thin dark layer atthe interface.
4 Summary and conclusions
The modified DJ HVOF system was able to deposit coatings of stainless steel and nickel alloy with considerably reduced oxide levels, whilst still retaining very low levels of porosity.
The improved quality of the stainless steel coating achieved using the modified DJ system led to an improvement in corrosion resistance, compared to the same coating deposited using the DJ2600 system.
Despite the attainment of coatings with very low levels of porosity and oxide, the coatings were more susceptible to corrosion than the same alloy in bulk form. This is in agreement with similar work undertaken with the JP5000 HVOFsystem. 
Further work is currently underway looking at the corrosion behaviour of the nickel alloy coatings. This work will be presented at a future date.
This work was supported by Sulzer Metco (US) Inc, who provided the DJ2600 and modified DJ coatings. The author would like to thank colleagues at TWI Ltd, Cambridge, UK for their services in undertaking this work.
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