Application of MgO/ZrO 2 coating on 309 stainless steel to increase resistance to corrosion at high temperatures and oxidation by an electrochemical method

: 309 stainless steel is one of the steels that have the highest resistance to corrosion and thermal oxidation compared to other steel grades, but it should not be used at temperatures higher than 980°C if there are temperature ﬂ uctuations. Additionally, 309 stainless steel is not designed for use in wet environments and has the least corrosion resistance. This study aims to cover 309 stainless steel with MgO/ZrO 2 particles using a two-step electroplating deposition method and then sintering to increase its resistance to wet corrosion and oxidation in temperature ﬂ uc-tuations. The intermediate ZrO 2 coating makes the outer MgO layer about two times thicker and reduces the corrosion current density. The morphology of the coatings was determined using scanning electron microscopy and X-ray di ﬀ raction, and oxidation resistance was determined using cyclic oxidation tests and the wet corrosion test of the corrosion solution. 3.5% NaCl was used. The results showed that the coated samples, due to the use of TiO 2 middle layer and MgO protective layer in the electrochemical method, have a 2-fold increase in wet corrosion and an increase in hardness of about 77%, as well as an increase in oxidation resistance of 1.8% compared to the sample without coating. Also, the reason for using the electrochemical deposition method is for better surface smoothness and less porosity of the coating as well as its use on all metals compared to thermal spray methods.


Introduction
309 Stainless steel is austenitic steel, for which corrosion resistance at high temperatures is desired.This alloy resists oxidation up to 1,038°C.It can be used in environments containing sulfur up to 1,000°C because of its high Cr and low Ni content.Among the important uses of this steel are the manufacture of furnace parts, energy production systems, car exhaust parts, jet engines, and airplane parts.However, one of the problems with this steel that limits its applications is that it should not be used in temperature cycles of more than 980°C, and it is not designed for use in humid environments.Its high melting point, oxidation resistance, low thermal conductivity and hardness, and high resistance to thermal stresses make MgO a refractory material.They are physically and chemically stable at high temperatures [1][2][3][4][5][6].MgO exhibits good thermal and mechanical properties.Ceramic coatings are high-temperature coatings used on metals, ferrous alloys, and nonferrous alloys.These mineral coatings are first used by creating an oxide layer on the metal to prevent corrosion, wear, and chemical attacks [7].The coating methods are grouped into several types, and the selection of the appropriate method mainly depends on technology, the intended application, cost, material used, etc.In general, coatings are classified into two groups: nonmetallic types and metallic types.The nonmetallic includes a polymer, chemical conversion, and glass ceramics, whereas metallic types incorporate hard facing (thermal spraying, cladding, and welding) and vapor deposition (CVD, PVD).Apart from these, electrochemical depositions, sol-gel, chemical deposition, etc., are examples of miscellaneous methods.Thermal spray coating techniques are typically used to combat the hot corrosion of boiler tubes.
However, the presence of unmelted particles, element segregation, heterogeneous microstructure, and the existence of splat boundaries in some of the thermal spray processes results in high surface roughness and considerably deteriorates their performance.In addition, weak adherence between the coating and substrate and high porosity are also major problems in many applications [8].Cold spray is an emerging surface coating technology for rapidly developing thick coating, thin films, and additive manufacturing of parts at relatively low temperatures.The low cost and scalability of a cold spray method make it promising for diverse industrial applications, but only a limited range of materials can be coated [9].Electrochemical deposition includes electroplating deposition and electrophoretic deposition (EPD), both of which are widely used to coat substrates.In EPD, charged particles move toward the cathode using an electric field and are deposited on the substrate to form a uniform layer.Sintering is generally used to create a more compact and dense coating for this film.The use of EPD has been reported for the processing of ceramic films and various metal residues; however, in the electrolytic deposition method, the dissolved ionic components, which are mainly metal ions, are reduced at the cathode and obtained during the deposition of an electrolyte.Sediment layer composed of electrolytic solution.Anodes can also be soluble or insoluble .One of the important features of ceramic coatings is their good adhesion to metal substrates.One of the major problems in stainless steel coating is the formation of chromium oxide on the surface of stainless steel, which causes poor adhesion of the coating to the stainless steel substrate; however, the presence of this oxide layer protects stainless steel from oxidation.The use of an intermediate coating increases the adhesion of the coating [31][32][33][34].
Also, the use of coatings (HAp, TiO 2 , ZrO 2 , and HAp/ TiO 2 ) increases the corrosion resistance of steel [35].The main goal of this study is to fabricate and identify magnesium oxide (MgO) coating on 309 stainless steel alloys by applying deposition parameters and then optimal curing to achieve a uniform and crack-free coating with high hardness and good adhesion to increase resistance to corrosion and oxidation at high temperatures.

Materials and methods
Samples with dimensions of 15 mm × 25 mm and a thickness of 1.5 mm were prepared from 309 stainless steel (Figure 1).The compositions of the alloys are listed in Table 1.To obtain a MgO/ZrO 2 duplex coating on 309 stainless steel from a twostep electrolytic deposition fabrication process (OH) 4 and Mg (OH) 2 , annealing was then done, and graphite with square dimensions of 30 × 50 mm 2 was used as the anode.A scanning electron microscope (SEM) was used to examine the topography of the coatings, and a JEOL JDX 8030 X-ray diffraction analyzer was used to analyze the phase.For coating, first, the samples were sanded with 600, 2,000 grit, and after washing in acetone solution at room temperature, degreasing was done for 2 min.To activate the samples, electro-polishing was performed after washing in 15% sulfuric acid solution for 4 min with a constant current of 0.4 A. The samples were immediately washed with distilled water and immersed in the electrolyte bath of 0.005 M ZrO(NO 3 ) 2 solution.An MGATEK MP 6005 D series potentiostat was used as a power source.The deposition voltage was 1,600 mV, the duration of 10 min was chosen, and then the samples were placed in a solution of 0.1Mg(NO 3 ) 2 •6H 2 O for 30 min at 1,800 mV for deposition.After deposition, they were dried in the air for 12 h.Sintering was performed in a GSL 1500X50 MTI tube furnace under Ar gas.During the baking stage, magnesium hydroxide turns into MgO at a temperature of 300°C as a result of calcination.At 500°C, a higher baking temperature is required to ensure good adhesion of the coating to the substrate.However, it is difficult to increase the baking temperature above 1,000°C.Sintering above 1,000°C results in the formation of iron oxides, which may contribute to cracking of the coating.Since ceramics have a relatively high firing temperature, from 700 to 1,000°C, after various tests and analysis of the obtained coating, the firing  temperature of 900°C was chosen.After cooling for 24 h, the samples were examined by electron microscopy, X-ray diffraction, and polarization tests for phase and structure analysis as well as corrosion resistance.

Results and discussion
The microstructure related to higher applied voltages was investigated.As shown in Figure 2(b) and (c), the coating is formed on the substrate, but it is not uniform.Under the electrochemical coating condition of 1,800 mV for 30 min, a uniform coating without cracks with a good appearance was formed, as shown in Figure 1(d).However, it cracked at 1,800 mV in 30 min.The optimal voltage of 1,400 mV can be interpreted as follows: at voltages lower than the voltage at which the crack does not form, the current required to form the crack does not exist and is negligible.At voltages higher than 1,400 mV, the reason for the lifting of the coating from the surface of the sample can be considered related to the issue of launching, which may occur at high speeds, and the coating does not have the necessary adhesion.Hydroxide ions are required for the deposition of magnesium hydroxide on stainless steel layers.Hydroxides are mainly formed by the reduction reaction of water and oxygen.Hydroxide production from a water reduction reaction requires an oxygen supply, but the amount of oxygen in the solution is limited.Therefore, at the beginning of deposition, when the oxygen reduction reaction was dominant, the hydroxide increased and a finer and denser structure was formed.Thermodynamically, after the formation of the porous structure, the concentration of hydroxides increases locally between the pores of the structure.The hydroxide production rate determines the coating morphology [36].According to similar research on stainless steel, during firing, background elements such as chromium and spinel penetrate the MgO between the underlying MgO coating and form an intermediate layer that causes coating adhesion.

Phase analysis and the effect of temperature on the structure of MgO
As mentioned before, after forming a magnesium hydroxide coating on the samples to create an MgO coating and sintering operation, the samples were placed in an argon furnace at different temperatures.Figure 3 shows an image of the X-ray diffraction pattern of the coating on stainless steel before heat treatment.The presence of a peak of MgO in the sample before placing it in the oven indicates oxidation of the surface of the samples at ambient temperature.
To prevent the oxidation of the coating layer, the samples were subjected to heat treatment in an argon furnace.To achieve a more uniform and dense coating, as well as with the highest level of adhesion, the samples were subjected to heat treatment at temperatures of 700, 800, and 900°C.
Figure 4 shows the X-ray patterns of the sample after heat treatment in an argon furnace at different temperatures.The higher the temperature, the more MgO was formed.As can be seen, the sample before heat treatment had a lower MgO peak than other heat treatments.As shown in Figure 4, a temperature of 900°C has the highest peak of MgO, but above the temperature of 900°C, iron oxide is formed.This caused the samples to crack.

Coating hardness results
The hardness test was done using a universal hardness tester model UV1.The hardness of the coating that the device showed was 280 ± 20 HV.The maximum hardness of the 309 stainless was 150 HV.These results show that the hardness of the MgO coating was approximately 77% higher than that of uncoated 309 stainless steel.

Effect of applying MgO/TiO 2 coating on the oxidation resistance of 309 stainless steel at high temperatures
Cyclic oxidation was used to evaluate the oxidation resistance of the coatings.In this method, the samples were heated at a certain temperature and time in the furnace, after which they were removed from the furnace and weighed, and the cycle was repeated until the weight change was determined.The samples were tested, both coated and uncoated.For each test, coated and uncoated samples were placed in an oven and heated to 1,000°C using a weighing crucible for 1 h; then, the samples were removed from the oven and placed in an oven for 30 min.
The solution was then allowed to reach room temperature.Then, their weights were recorded, and the samples were placed in the oven at 1,000°C for another hour.This cycle was repeated intensively in the first few hours for 48 h, after which the weight remained almost unchanged as the exposure time increased.During the first few hours of the oxidation or corrosion test, the coated substrate was exposed to a corrosive environment and was severely corroded.As the test progressed, the corrosion products remained on the surface of the specimens, protecting them from further corrosion.Therefore, the corrosion products protect the substrate from further corrosion.Consequently, after a sharp increase in the first few hours, weight gain remained unchanged.Figure 5 shows that the oxidation resistance of the coated sample is better than that of the uncoated sample,  and the oxidation resistance of uncoated 309 stainless steel at 1,000°C after 50 h of cyclic oxidation was 1.5 mg•cm −2 , whereas that of the coated sample was less than 0.9 mg•cm −2 .Figure 6 shows the SEM images of the coating after oxidation tests.

Effect of MgO/TiO 2 coating on wet corrosion resistance
The TOEFEL polarization test was used to evaluate the insulation and corrosion properties of the coated samples.
The placement of steel used in wet environments causes water drops to be placed on their surfaces and may cause corrosion.For this reason, in this method, the corrosion behavior of steel and its coatings have been investigated.
Corrosion is checked on the surface to determine the insulating properties of the sample, as corrosion is a surface phenomenon.One of the data points obtained from the corrosion test is polarization resistance, which occurs due to potential changes on the surface.The most important part of the circuit resistance is the resistance of the cover, which has an insulating state, and its high value indicates insulation of the cover.In other words, polarization refers to the resistance to the passage of electricity from the surface to the substrate of a hard conductor and indicates the insulation of the coating.Information related to the corrosion polarization test is extracted from Figure 7 and summarized in Table 2.As mentioned, polarization resistance occurs owing to the potential change on the surface and it can be used to measure the insulation of the coating.As shown in Table 2, for the sample sintered after coating, the corrosion current is 0.0136 nA.The corrosion resistance of this sample was higher than those of other samples.Therefore, the electrochemical coating method can provide the desired conditions.A corrosion resistance of 227 kΩ is required, which is more than that of the uncoated sample, and the best insulation condition among the existing conditions in this project was obtained for this sample.The deposition method, along with a subsequent annealing method to prepare an MgO/ZrO 2 duplex coating on 309 stainless steel to increase corrosion and oxidation resistance at high temperatures, is presented in this work.The addition of a moderate electrolytic deposition of Zr (OH) 4 dramatically increases the adhesion strength by about two times, making the MgO outer layer coating thicker and longer.Hydroxide to MgO coating on steel in an argon atmosphere furnace at 900°C for 60 min results in the formation of the most uniform coating.Creating MgO coating under these conditions increases resistance to corrosion, oxidation, and hardness.

Figure 2 :
Figure 2: SEM image of the coating deposited at (a) the voltage of 1,200-1,300 mV for 90 min; (b) and (c) SEM image of the coating deposited at a voltage of 1,400 mV for 30 min; and (d) SEM image of the coating deposited at a voltage of 1,800 mV for 30 min.

Figure 3 :
Figure 3: Image of the X-ray diffraction pattern of the coated sample under optimal conditions before heat treatment.

Figure 4 :
Figure 4: Image of the X-ray diffraction pattern of the sample heattreated at 700-900°C in an argon furnace.

Figure 5 :
Figure 5: Weight gain versus oxidation time for coated and non-coated samples in corrosive and non-corrosive environments oxidized at 1,000°C.

Figure 6 :
Figure 6: SEM image of the sample after 50 h of cyclic oxidation testing at 1,000°C.

Figure 7 :
Figure 7: Potentiodynamic polarization curves of uncoated and coated samples in 3.5 wt% of NaCl solution.

Table 1 :
Chemical composition of stainless steel