Corrosion Characterization in Nickel Plated 110 ksi Low Alloy Steel and Incoloy 925: An Experimental Case Study

Incoloy 925 is an age hardenable Nickel-Iron-Chromium alloy with the addition of Molybdenum, Copper, Titanium and Aluminium used in many applications in oil and gas industry. Nickel alloys are preferred mostly in corrosive environments where there is high concentration of H2S, CO2, chlorides and free Sulphur as sufficient nickel content provides protection against chloride-ion stress-corrosion cracking. But unfortunately, Nickel alloys are very expensive. Plating an alloy steel part with nickel would cost much lesser than a part make of nickel alloy for large quantities. A brief study will be carried out to compare the performance of nickel plated alloy steel with that of an Incoloy 925 part by conducting corrosion tests. Tests will be carried out using different coating thicknesses of Nickel on low alloy steel in 0.1 M NaCl solution and results will be verified. From the test results we can confirm that Nickel plated low alloy steel is found to exhibit fairly good corrosion in comparison with Incoloy 925 and thus can be an excellent candidate to replace Incoloy materials.


Introduction
Incoloy 925 is used in high strength piping systems and petroleum industry generally. It is a high performance alloy having high creep and oxidation resistance. The resistance to sulphide stress cracking and stress corrosion cracking in sour crude oil and natural gas makes it preferable for manufacturing downhole and surface gas well components including tubular products, valves, hangers, landing nipples, tool joints and packers. They are used in environments where there is high temperature, mechanical stress and surface stability is required. It is a precipitation -hardenablenickel-iron-chromium alloy. It can be age hardened by adding Aluminium and Titanium. Copper and Molybdenum are added to increase its resistance to corrosive media. Its chemical composition underlines that Nickel is the major constituent in Incoloy 925 with 44 %. The disadvantage of Incoloy 925 is the high cost involved in its procurement.
Nickel is considered useful for plating metal as it provides superior ductility, corrosion resistance, and hardness. Coating low alloy steel with Nickel will provide excellent corrosion characteristics and by varying the coating thickness we can use it as a substitute for Incoloy 925. It is extremely important to evaluate chemical stability of materials under potential environments and therefore study of corrosion characteristics is important. concentrations of Cland SO 4 2showed that the concentration range had no substantial effects on the anodic behaviour of the alloy. After polarization no localized corrosion was found in the samples [1]. Results obtained by exposing C-ring specimens of Incoloy 925 to a solution containing hydrogen sulphide, sodium chloride and acetic acid proved that the alloy resisted sulphide stress cracking. Test results of corrosion tests on Incoloy 925 on free sulphur environments show a very low corrosion rate of 0.028-0.030mm/annum. Electrochemical polarization study has been carried out to study the corrosion behaviour of nickel coating and the polarization curves have revealed that the current density used during pulse electro-deposition plays a vital role in characteristics of Nickel coating. Thus the aim of the present work is to study the corrosion behaviour of nickel plated low alloy steel and Incoloy 925 in a saline medium by varying the thickness of nickel coating and understanding how coating thickness affects the corrosion rate.

Experimental details
The procedure for electroless nickel plating is as follows. The alloy was first degreased and cleaned. The activation of surfaces to be plated shall be performed after cleaning to ensure that the surfaces are ready for plating. The activation step removes films and oxide layers that may interfere with the plating process. Activation techniques are somewhat material dependent and include alkaline, acid and/or current reversals with electrocleaners.
The nickel plating process for plating thickness of 0.0005",0.001" and 0.002 " shall be in accordance with ASTM B733 with a phosphorus content in the 9% -12% range [2][3]. The plating shall be qualified to a Class 2 heat treatment in accordance with ASTM B733. The plater shall have a plating procedure that defines bath operating conditions, bath monitoring frequency and tests, bath composition and bath replenishments. A bake or heat treat cycle is required of the plated product to (1) remove hydrogen from susceptible (to hydrogen embrittlement) substrates and (2) harden the nickel phosphorus plating to about 65 HRC. The heat treat process shall be to ASTM B733 Class 2. This requires a 500°F -750°F (260°C -400°C) heat treat cycle for typically 2 to 4 hours.
The corrosion behavior of nickel plated 110 ksi low alloy steel samples having different plating thickness (0.0005",0.001" and 0.002 ") and Incoloy 925 (composition given in Table 1) was evaluated using Versastat 3 potentiostat. Test was carried out at room temperature in a three electrode cell having Ag-AgCl reference electrode and platinum gauge counter electrode. Prior to electrochemical measurements the specimen was washed in distilled water. For testing in neutral medium the electrolyte used was aqueous solution of 0.1M NaCl solution. The experiments were conducted at a scan rate of 0.0167 mV/s. Nickel plated low alloy steel samples having thicknesses of 0.0005 ",0.001" and 0.002" were exposed to an area of 0.44 cm 2 ,0.85 cm 2 and 0.63 cm 2 respectively. For Incoloy 925 the exposed area was 2.465 cm 2 . The working electrode was allowed to obtain a stable open circuit potential (OCP) in the test solution. All samples achieved OCP in 30 minutes.

Results and Discussion
The potentiodynamic polarization measurements were performed on Incoloy 925 and 110 ksi Low Alloy Steel evaluate its corrosion behavior. Polarization results indicate that the corrosion process in the present system is under anodic control as reflected by the Tafel slopes. Corrosion current density (icorr) and corrosion potential (Ecorr) values were determined by applying Tafel extrapolation of polarization curves.   Fig. 1 shows polarization curves for nickel plated 110 ksi Low Alloy Steel in 0.1M NaCl solution. The current density shows significant resistance after the Ecorr is reached indicating sign of passivity in the sample. The values of i corr and E corr for samples in NaCl solution are shown in Table 2. Though the nickel plated low alloy steel with plating thickness of 0.0005" shows resistance to material degradation once E corr is reached, overall corrosion rate of this alloy in NaCl solution is high.
Similarly corrosion behaviour of nickel plated low alloy steel with plating thickness of 0.001", 0.002" and Incoloy 925 is shown in Fig. 2, 3 and 4 respectively. The corrosion current density is not increasing once the E corr is reached, showing significant resistance of samples to material degradation. Further the i corr and E corr values for these samples are reported in Table 2. In order to compare the corrosion behaviour of nickel plated with varying thickness with Incoloy, a polarization curves were plotted together in Fig. 5. From Fig. 5, it can be clearly seen that Incoloy exhibits highest corrosion resistance with lowest corrosion current density and lowest corrosion. Nickel plated with 0.0005" shows least resistance to corrosion and with increase in thickness of nickel, corrosion rate was observed to decrease. From the above results, it is evident that as coating thickness increases corrosion rate is decreasing. Although the corrosion rate for the sample with highest coating (0.002") is not as low as Incoloy, still the reduction in corrosion rate with increase in plating thickness is a positive trend. The current density value of 0.002" nickel plated low alloy steel is more compared to that of the other two samples. This clearly indicates that increase in thickness of nickel plating will provide more corrosion resistance. Although the corrosion resistance observed for the nickel plated samples is not as much as that observed for Incoloy 925, the positive trend shows that it can be used as a promising candidate in oil and gas industry.
Further, cost comparison studies has been carried out between Incoloy and nickel plated low alloy steel. Incoloy 925 material is 6 times or more expensive than that of a 110 ksi alloy steel material for the same dimensions. So when an Incoloy part is requested, if we know the exact application then we could just do nickel plating on the 110 ksi alloy steel and manufacture the product which would