Sea water Corrosion of Nickel based Plasma Spray Coating

Different types of erosion resistant coatings are applied/deposited on aero components, depending on the operating/working temperatures. Nickel based coating are applied on the air craft (compressor) components, which can sustain up to working temperature of 650°C. In the present investigation, to improve the compatibility between substrate (i.e. the machine component) and the top coat, application of bond coat is there. The application of Nickel based coating by thermal plasma spray technique has proven to be a satisfactory means of producing acceptable sealing surface with excellent abradability. Before the corrosion study, coated sample is subjected to hardness, thickness and porosity testing. Hence the result is being evaluated. The corrosion behavior of coating was studied by sea water immersion with a time period of 16 weeks. It is observed that, up to 9 weeks increase in weight of coating occurs in a sharp trend and then takes a decreasing trend. The weight gain of the samples has varied from 37.23% (with one week immersion in sea water) to a maximum of about 64.36% for six weeks immersion. Coating morphology and composition analysis of the coatings are studied using SEM and EDS. This behavior shows adsorption/deposition of the foreign particles with polygonal shape on the coating surface by sea water interaction. Foreign particles with polygonal shape deposited on the coating and with increase in immersion/treatment time, washing out of the deposited materials starts, which reflects the decreasing trend of weight gain of the specimen.


Introduction
Thermal plasma spray coating is one of the effectively developed modern technology to resist the surface from degradation. Coating made with plasma technique exhibit excellent wear, corrosion and thermal resistance. Nickel based abradable coating is used with thermal plasma spray techniques in air craft (Compressor components), which can sustain up to working temperature of 650°C, by improving engine efficiency. Characteristics of coating varies due to substrate surface preparation and critical plasma spray parameters [1][2][3][4]. Spraying  droplets flatten and solidify on the surface [5][6][7]. Till date, no effort has been taken on the characterization and evaluation of sea water corrosion behavior of different plasma spray coating used in aero engines. Present piece of work is intended to study corrosion behavior of Nickel base plasma spray coating, when treated with sea water at different time length.

Substrate Preparation
Nimonic plates were used for substrate surface (as this is been used in aero-components). Substrate surface was roughened using Al 2 O 3 powder of grit size 60 to produce a surface roughness of 4-6 micron Ra. [8]. The roughened surface was protected from contaminations up to spraying to avoid disintegration of coating.

Preparation of coating powder
Powders of Nickel, Copper Oxide, Cadmium Oxide and Hexagonal Boron Nitride powder are sieved, after baking at 130 0 -150 0 C for 2-3 hr. Nickel powder 54-68 %, Copper Oxide 10 -12%, Cadmium Oxide 3-5%, Boron Nitride 16-19%, Graphite 1-4 % and Aluminium Powder were mixed in a ball mill along with liquid glass. The mixture is rubbed over sieve and baked at 150 0 C and finally sieved to get the final before use..

Spray Deposition
Plasma spraying condition i.e. CPSP parameters were selected carefully for coating, where Argon (53 lpm) is used as Primary plasma Gas & Hydrogen (4 lpm) is used as Secondary Plasma-gas with Current(460 Amp).After coating the samples were heat treated at 650 0 C for half an hour.

Specimen preparation for corrosion test
With the help of EDM wire cut the coating specimens were cut into slices of 3-4 mm thickness and 3-6 gm weight. The cut samples were treated with sea water (Bay of Bengal) to study the corrosion behavior at different time length. The chemical composition was analyzed before corrosion test, is tabled below.

. Hardness test
Coated sample surfaces were polished using emery paper of distinctive grit size for hardness estimation. Brinell hardness test was performed at room temperature on 10 different areas and to get the average hardness value, and were observed to be in the range of 7-13 HB.

Corrosion Test
Corrosion behavior of coating was studied by sea water immersion with a time period of 16 weeks. Weight of specimens were taken before and after treatment at different time interval and the results are tabled below.  The graph shown in Fig. 1 shows that deposition of the foreign elements taking place on coating surface occurs during sea water interaction and with increase in immersion/treatment time, washing out of the deposited materials starts, which reflects the decreasing trend of weight gain of the specimen.  Table 4 showed the increase in ox Which implies, some kind of ion progress. The polygonal shaped par the increase/presence of Cl, which compound; as the cation (Cl) concen of raw & corroded samples, in Fig. 2 and Fig. 3.
Untreated specimen EDS and SEM spectra.
Treated specimen EDS and SEM spectra. ited at CPSP by plasma spray in Fig. 2 and Fig. 3, en particles .Coating deposited are smooth, more ected CPSP parameters. This might be the reason to of corroded sample in Fig. 3 shows small foreign coatings. EDS analysis data of corroded sample fro xygen in large amount where as very less increase nic reactions are taking place. Further detailed exa articles are deposited on coating surface and EDS a h is also another clue to confirm the formation of ntration is more in sea water than the anion concentr , show uniform e homogenous, o get a suitable n particles with om Table 3 and in Cl is there. amination is in analysis exhibit some chlorine ration.  Table 3. Coating elemental analysis before treated by EDS . Table 4. Elemental analysis of the coating after dipped in sea water for 10 weeks by EDS .

Conclusions
From this investigation it is found that, the Ni base abardable coatings used in aero component could be plasma sprayed with thermal plasma spray technique. The coating hardness is found to be 7-13HB. There is a weight gain of the coatings with initial time period of deeping in sea water but after few weeks there is a decrease in weight; may be due to washing out of compounds formed with interaction of the coating with sea water with prolonged time of treatment; implies weakening of the coating materials. From the SEM and EDS observations, the formation of polygonal shaped particles on the coating surface and increase in Cl component implies and confirms the formation of other compounds with sea water.