Improvement of Corrosion Resistance of Low Carbon Steels by Surface Modification

doi:10.21203/rs.3.rs-3952164/v1

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Improvement of Corrosion Resistance of Low Carbon Steels by Surface Modification

https://doi.org/10.21203/rs.3.rs-3952164/v1

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This research aims to investigate the corrosion resistance of surface layers of pack chromizing, titanizing, and chromtitanize low-carbon steel in a chloride-containing environment at room temperature. The parameters affecting the diffused coat, such as the coating time, coating temperature, and the effect of pack compositions, were also studied. Tests are performed to measure wet corrosion behavior using the Potentiodynamic polarization corrosion test. The microstructures and elemental analysis using a scanning electron microscope (SEM) attached to an EDX unit were obtained. Phase analysis is determined using EDX. X-ray diffraction examination indicated that in the chromizing, titanizing, and chromotitanizing processes at different coating times and temperatures of 900°C, 1000°C, and 1100°C. The outer layer is composed basically of Cr,Ti, Cr_1.9Ti, FeTi, Al₂O₃, Cr₂O₃, TiO₂, Cr_1.36Fe_0.52, (Ti0.86)3.58, and Ti_0.86 phases in addition to the elemental iron.

The thickness of the coating increases with the increase of the deposition time. The coating is composed of a thick outer layer and a thin inner layer. The formation of the coating depends on the inward diffusion of Cr and Ti atoms and the outward diffusion of Fe atoms.

The linear polarization examination showed that the corrosion resistance of steel is enhanced by coating it in a 3.5% NaCl aqueous solution at room temperature.

Potentiodynamic polarization examination showed a lower corrosion rate of the coated specimens than the uncoated ones, and the coated specimens exhibit nobler behavior than the uncoated ones in a 3.5% NaCl aqueous solution at room temperature.

The corrosion resistance of the coating is proportional to the deposition time and temperature.

Cr-Ti

and Ti

pack cementation

XRD

SEM

corrosion resistance

Pack cementation is an effective and convenient approach to fabricate protective diffusion coatings [1]. In general, the specimen is buried in a powder mixture composed of insert fillers, master alloy elements and activators. The metal halide generated at high temperature diffuses into the specimen surface and forms a protective layer through interdiffusion effect with the substrate [2]. This kind of in-situ chemical vapor deposition process has been extensively used in the fabrication of aluminizing, chromizing, and siliconizing coatings due to the low cost and the tight adhesion between the coating and the substrate at high temperature [2, 3]. It has been reported that the addition of Ti can improve the corrosion resistance of metallic materials by enhancing the protectiveness of passive films [4].

Low carbon steel (0.25% C max.) is usually applied in numerous industries, including automotive, bridges, ships, storage tanks, large pipes, etc., for its low price and excellent mechanical properties, machinability, weldability, and formability [5]-[6]. Low carbon steel has low resistance to corrosion, which is considered a major economic problem worldwide. Because low carbon steel equipment and machine parts are highly corrosive in chloride or seawater environments, which have varying corrosive levels depending on ion concentration, evaluating corrosion problems in carbon steel is critical [7]-[8]. However, corrosion can be controlled in several ways, including the use of a protective coating, which is the recommended solution for developing materials for resistance to corrosion [9]-[10].

The powder process (pack) is an inexpensive and relatively easy way to obtain chromizing coatings with the possibility of processing pieces with different geometries.9 The process involves placing the substrate in a powder mixture containing a source of chromium, an inert component (usually Al₂O₃) and an activator. The package containing the mixture and the sample is heated to 900–1100ºC for up to 7 hrs. to form the coating.[11]

This work is aimed at protecting low carbon steel surface from corrosive ingredients present in the working environment and enhancing the corrosion resistance through the deposition of the layers of coating of Ti, Ti/Cr and Cr using the pack cementation method. The influence of pack chromizing, titanizing and chromtitanize parameters of low-carbon steel (temperature, time) on the morphology and composition of the produced layers was studied.

Low carbon steel sheet is used as the substrate in this work. The nominal composition of this steel is shown in Table 1, where W is the mass fraction. Specimens was cut into discs of 20 mm in diameter and 2mm in thickness. Before the pack process, the specimens are ground with 2000 grit SiC papers, followed by rinsed in distilled water, degreased with ethanol again washed in distilled water and finally dried with acetone.

Table 1

The chemical composition of the substrate (wt %)
Element	C	Si	Mn	P	S	Ni	Fe
W%	0.0507	0.016	0.295	0.024	0.018	0.001	bal

The specimens were embedded in the cementation box filled with the cementation mixtures which were composed of five mixtures as shown in table (2), Ammonium chloride was used as activator and Alumina powder was used as inert material to prevent the caking, melting and collapse of the ferrochromium and ferrotitanium.

The powders are weighed according to this ratio and thoroughly mixed by tumbling in a ball mill for about 30 min.

Table (2) Chemical composition of the cementation mixture for treatment with Chromium and Titanium

Type of mixture	Chemical composition in wt%
	COATING material	ALUMINA	AMMONIUM CHLORIDE
Mixture A	48%FeCr	50% AL₂O₃	2% NH₄CL
Mixture B	36% FeCr- 12% FeTi	50% AL2O3	2% NH4CL
Mixture C	24% FeCr- 24% Fe Ti	50% AL2O3	2% NH₄CL
Mixture D	12% FeCr- 36% Fe Ti	50% AL2O3	2% NH₄CL
Mixture E	48% FeTi	50% AL2O3	2% NH₄CL

The specimens are placed in an away that the distance between them and the walls of the cementation container not less than 15 mm. Container is closed with its airtight lid. A cement refractory clay, asbestos and sodium silicate is smeared along the edges of the lid to ensure a voiding oxidation of the contents during heating.

The sealed boxes are held in the heating furnace at the required coating temperature of 900ºC, 1000ºC, 1100ºC, for different coating times 3, 5, 7 hours. At the end of the diffusion process the containers were removed from the furnace and after cooling, they unpacked. The coated specimens then cleaned gently to remove any appearance dirt on its surface to carry out the tests. The collected powder can be reused. The chemical composition of ferrochrome powder used in the present study is shown in Table 2.

Table 3

The chemical composition of ferrochrome powder (wt %)
Element	C	Si	P	S	Cr	Fe
W%	8.9	1.2	0.021	0.05	61	bal.

The chemical composition of ferrotitanium powder used in the present study is shown in Table 3.

Table 4

The chemical composition of ferrotitanium (wt %)
Element	Si	Ti	O	Al	Fe
W%	3	27	8.3	6	bal.

Characterizations

Microstructure and X-ray diffraction

XRD analysis was carried out using an x-ray diffractometer, type XRD-D8 Advanced, Germany, with Cu-Kα radiation and a 0.154 nm angle of incident. Also, a microstructure examination was carried out using a JEOL JSM 5410, Japan, with an accelerating voltage range of 200 v to 30 kv and a power magnification range of 6x to 100000x.

Electrochemical performance

Linear polarization, Tafel and Potentiondynamic tests were performed on the uncoated and coated specimens. All tests were carried out using Autolab Potentiostat (PGSTAT 30) specified for electrochemical measurements. The working electrodes were coated and uncoated specimens with an exposed area of 1cm². All potentials were measured with respect to saturated calomel electrode.

Linear polarization, Tafel and Potentiondynamic tests were performed on the uncoated and coated specimens to evaluate the coating's ability to protect and prevent low-carbon steel from corrosion. The experiment was conducted using Autolab Potentiostat (PGSTAT 30) specified for electrochemical measurements. The conventional three-electrode electrochemical cell was used to measure the test output. The electrochemical cell includes, (Pt) as a counter electrode, saturated Ag/AgCl as a reference electrode, and a sample holder as the working electrode. The cell parameters were the scanning of electrode potential between − 1.6 and + 0.6 V at a scan rate of 0.01 V.s^-1 and 1.0 cm² was the area of exposed work to the aqueous solution. All specimens were examined at room temperature and using an aqueous solution of 3.5% NaCl. It's a simulated marine environment which is the most corrosive medium for steel substrate and coated specimens.

XRD analysis

The XRD technique was used to identify the phases present during the pack cementation process. Figure 1 shows the XRD pattern that was conducted for the chromizing, chromotitanizing, and titanizing-coated low carbon steel samples.

Figure (1) (a) shows the X-ray patterns for coated specimen chromized at 900^°C for 5hr (48%FeCr) together with the positions of all Bragg reflections for the identified phases. For the samples treated at 900ºC, there is the major presence of Cr_1.36Fe_0.52 intermetallic type, and in smaller quantities Cr₂O₃, C = Cr, like those indicated by the intensity of diffraction peaks. Al₂O₃ peaks were also found as a residue from the filler material used on the pack mixture.

Figure (1): X-ray diffraction pattern for some coated specimens

The X-ray diffraction pattern of the treated samples for chromotitanized with different percentage of ferrochrome and ferrotitanium powders at 900, 1000, and 1100^°C for 3, 5, and 7hrs., resulted in the formation of Cr_1.9Ti, Ti Fe intermetallic type and Cr₂O₃ and TiO₂ oxides beside Al₂O₃. This suggests that the temperature in pack chromatinization is an important parameter controlling the formation of oxide and/or carbon compounds. The formation of (Cr,Fe)₂N_1-x is possible due to the nitriding reaction of chromium and iron after the decomposition of NH₄Cl, which is added to the chromising mixture as an activator.[11] The ferritematrix is not revealed in all analyses due to the reduced penetration of the X-rays, which is in agreement with the study by Arai and Moriyama.8

Figure (1) (d) shows the X-ray patterns for coated specimen titanized at 900^°C for 5hr (48%FeTi) The titanizing-coated low carbon steel samples shows TiFe phase which is known to be stable thermodynamically, the peak corresponding to TiFe phase appeared in the XRD results. It was reported that isolated TixFe particles were detected by SIMS imaging for the Ti-0.078 wt% Fe system, but they might be too tiny to be detected by the XRD method [12–13]. Other phases such as Ti O₂ and Ti_0.86 were detected.

SEM

Figure 2 shows the morphology of the low carbon steel surface, which was chromized, chromotitinized, and titanized to form a coated film at different bath composition.

Figures 2(a) show the morphology of the low carbon steel surface, which was chromized, the surface morphology after Chromizing treatments at 900°C shows a dense structure for the layer of expanded austenite, accompanied by a uniform chromized layer with a very smooth finish of the surface, and no visible cracks were observed. It was due to the intensive chromized precipitation that occurred and expanded austenite was also detected, as was evidenced in XRD test results. The surface morphology for the chromotitanized coating on the low carbon steel surface is shown in Figs. 2(b) deposition of Cr/Ti at a low deposition time will result in a surface that is characterized by an amorphous matrix that contains a fine nanocrystal fiber structure that is visible in SEM images of this sample. Also, a longitudinal micro void and rough surface can be detected in this image that formed in the coating's surface. which caused a increase in the amount of deposited coating. Figures 2(c) shows the deposed for Ti coating increased to 5hrs., a higher deposition rate occurred, which resulted in a change in the morphology towards a denser microstructure, more uniform coating, a much coarser surface finish than the former surface layer, and visible micro voids. Also, a nano-crystalline fiber structure can be detected.

For the chromized and chromotitanized surfaces the particles formed could be Cr oxides such as Cr₂O₃ and carbides [14]. Thus, the darker region can present the chromium-rich region (Fig. 2(a)) distributed across the treated surface. Moreover, no cracks were visible on the surface of the Cr chromized. However, it is known that the deposition of Cr₂O₃ in some parts of the Cr coated may lead to the apparition of cracks. Moreover, some pores have been observed and according to the studies conducted by Fan et al. [15], the kirkendall effect was found to be responsible for pores and voids formation in chromizing coatings. Deposition of Cr₂O₃ can also increase the roughness of the coated substrate [16].

Figure (2) SEM of a specimen Chromized at 900^°C for 5 hr, (a) the Chromized mixture contains 48 wt%FeCr ferrochromium, 50 Wt% Al₂O₃ powder and 2 Wt% NH₄Cl. (b) Specimen chromotitanized at 900^°C for 5 hr, the chromotitanized mixture contains 24 wt%FeTi ferrotitanium + 24 wt%FeCr ferrochromium, 50 Wt% Al₂O₃ powder and 2%Wt NH₄Cl. (c) Specimen titanized at 900^°C for 5 hrs., the titanized mixture contains 48 wt%FeTi ferrotitanium, 50 Wt% Al₂O₃ powder and 2%Wt NH₄Cl.

Corrosion behavior

Figure 6 shows the Tafel polarization curves for coated and uncoated low carbon steel. It provides several significant parameters, such as corrosion current density (icorr), corrosion potential (Ecorr), and corrosion rate (CR).

The Mils Penetration per Year can be calculated by the given formula [17, 18]:

Mils Penetration per Year (MPY) = 0.13 * i_corr * EW / Density

Additionally, to determine the precise calculation of the Mils Penetration per Year (MPY) of each sample that corresponded to current density of corrosion rate (i_corr) from the graph, (EW) is the equivalent weight. All results are tabled as presented in Table 3. The MPY results decreased from 0.3023 to 0.0127 as the corrosion resistance properties of high carbon steel increased. As Cr content escalated, volume fraction of retained austenite increased and its stability improved, which minimised the grain boundary corrosion, as less iron atoms moved that accelerated corrosion reaction, thus improving the corrosion resistant properties.

4.6 Electrochemical Corrosion Test:

4.6.1. Linear Polarization Experiments

Table (3) shows the electrochemical parameters derived from linear polarization experiments for blank and coated low carbon steel. The table includes the different process parameters.

Figure (3) shows the effect of ferrochromium percent on the corrosion rate of the coated specimen at temperature of 900°C, 1000°C and 1100°C at different coating times in 3.5% NaCl aqueous Solution at room temperature. The corrosion resistance of the coated samples was better than without coatings. When the chromizing temperature was 1100°C, there was no percolation layer on the surface of the sample and the corrosion rate was the highest. When the chromizing temperature was 900°C, the corrosion rate was the lowest. With the increase of the chromizing temperature, the corrosion rate increased, and the corrosion resistance decreased. When the chromizing temperature was too high, holes appeared on the surface of the sample, the compactness decreased, the corrosion of the chromized layer began to increase, and the corrosion transitioned from uniform to pitting, resulting in increased corrosion. The corrosion rate decreases with increasing ferrochromium percent in the pack and increasing time. But the corrosion rate at 1000°C and 1100°C increases with increasing time and the corrosion rate at 1100°C has the highest value. This may be due to the bad effect of cracks and voids appearing at high temperature and longtime of coating.

The chromising of steel and ferrous alloys produces a surface layer composed mainly of Cr, Fe and C, which significantly increase the hardness, wear and corrosion resistance of the substrate. [19, 20]

Figure (3): Effect of ferrochromium percent in pack on the corrosion rate at different temperature and different time in 3.5% NaCl Solution at room temperature

Figure (4): Effect of ferrotitanium percent in pack on the corrosion rate at different temperature and different time in 3.5% NaCl Solution at room temperature

Figure (5): Effect of the pack cementation composition on the corrosion rate at different temperatures and time in 3.5% NaCl solution at room temperature.

Figure (6) SEM micrographs from a cross-section of a sample uncorded and corroded chromized sample.

Figure (6) shows the microstructure of the chromized coated and corroded coated samples after corrosion test in 3.5%Nacl solution. The chromizing temperature is 1100°C and 5hrs. Before corrosion, the coating surfaces are uneven Fig. 6 (A), while Fig. 6(B), shows cracks and holes on the corroded surface.

For the tetanized samples, Ti-rich precipitate particles are observed in the microstructure of designed steel (Fig. 2c). The titanium-containing corrosion products are extremely dense and free of pores and cracking, which is beneficial for corrosion resistance. Other investigations show that the addition of titanium element increases the crystallite size of geothite and produces double domain particles consisting of the particles core and porous crystallite shell [21]. During this study it is observed that the corrosion rate of the coated tetanized steel is higher than chromized samples.

After corrosion, irregular-shaped corrosion products are unevenly distributed on each coating.

Corrosion occurs mainly in coating defects such as tiny cracks and holes. The corrosion process passes through three stages. Firstly, pitting corrosion occurs in the defects of the coating. Secondly, the pitting corrosion grows into depth and spreads over the surface. Lastly, the coating is corroded.

4.6.1. potiondynamic polarization experiments

In Fig. 7, the potentiodynamic polarization curves obtained in artificial sea water (3.5% NaCl) for some pack coated samples, the low carbon steel substrate coated and uncoated samples are compared. After corrosion testing, the pack cemented layers of Ti, Cr/Ti and the hard chrome provided superior corrosion resistance when compared with the low carbon steel substrate.

The electrochemical reactions that occur on the surfaces of the coated samples are far less destructive than those which occurred on the unprotected carbon steel substrate. This is shown by the potentiodynamic polarization curves of the three coatings and the hard chrome that exhibit a passivity region in which the rate of corrosion reaction is low. Moreover, the corrosion currents are shifted to the left, which indicates minor current values relative to the substrate.

Among the pack chromizing conditions, the ones treated at 900ºC for 7 hrs resulted in better corrosion behavior.

The increase in corrosion resistance for the samples treated at 900ºC can be attributed to the fact that these samples are externally composed mainly of (Cr_1.36Fe_0.52, Cr, and Cr₂O₃), as shown by the X-ray diffraction analysis (Fig. 3b). The polarization curves also show that the protection against corrosion increases with the pack cementation time and temperature, which is the most significant parameter.

It is well known that the higher Cr concentration that the coating possesses, the better corrosion resistance that the coating exhibits. This fact suggests that at 900ºC, for 7 hrs. the Cr concentration on the layer is higher, which results in better corrosion protection (0.0127).

Figure (7), the potentiodynamic polarization curves obtained in artificial sea water (3.5% NaCl) for some pack coated samples

Table 5 shows the electrochemical parameters, such as corrosion potential Ecorr, corrosion current Icorr and polarization resistance Rp, of the tested samples collected from the polarization curves. Rp is calculated by the quotient V(Ecorr)/I(Icorr), which indicates that if Rp decreases, corrosiveness of the medium increases.

The decreasing order of corrosion resistance of the samples is hard chrome, followed by the sample chromized at 900ºC for 7 h, 900ºC for 5 h, 900ºC for 3 h and the substrate which undergoes major corrosion rate.

The protection against corrosion increases with pack treatment time and temperature, with temperature being the most significant factor. All chromium coatings are more corrosion resistant than the low carbon steel substrate.

Table (5) shows the electrochemical parameters derived from linear polarization experiments for blank and coated low carbon steel alloy. The table includes the different process parameters.

Table (5): Result of linear polarization experiments at different temperatures for different times in 3.5% NaCl solution at room temperature

Specimen No	E_corr, V	Rp, Ω.	I_corr, A/cm²	Corr. Rate, mm/year	%Cr	%Ti	Temp. (^ºC)	Time (h)
Blank	-0.622	1.217E + 00	6.89E-04	0.3023	--	--	--	--
1	-0.443	3.382E + 01	1.734E-06	0.02502	100	0	900	3
2	-0.51	7.702E + 01	2.56E-06	0.0369	75	25	900	3
3	-0.452	2.897E + 02	2.974E-06	0.0429	50	50	900	3
4	-0.441	9.806E + 01	4.685E-06	0.05307	25	75	900	3
5	-0.502	8.503E + 0	4.968E-06	0.07168	0	100	900	3
6	-0.567	2.882E + 01	2.245E-06	0.03239	100	0	900	5
7	-0.48	2.539E + 01	3.48E-06	0.05021	75	25	900	5
8	-0.463	3.866E + 01	4.401E-06	0.0638	50	50	900	5
9	-0.521	2.704E + 01	7.423E-06	0.08406	25	75	900	5
10	-0.444	7.463E + 00	6.481E-06	0.09351	0	100	900	5
11	-0.454	4.8E + 01	2.515E-06	0.0127	100	0	900	7
12	-0.527	9.503E0 + 1	1.875E-06	0.0212	75	25	900	7
13	-0.392	1.88E + 02	8.80E-07	0.02681	50	50	900	7
14	-0.455	7.06E + 01	2.412E-06	0.02732	25	75	900	7
15	-0.511	4.518E + 01	2.804E-06	0.04046	0	100	900	7
1	-0.401	4.12E + 01	2.201E-06	0.03175	100	0	1000	3
2	-0.540	1.387E + 01	2.095E-06	0.04199	75	25	1000	3
3	-0.480	2.539E + 01	3.48E-06	0.05021	50	50	1000	3
4	-0.463	3.866E + 01	4.401E-06	0.0638	25	75	1000	3
5	-0.521	2.704E + 01	7.423E-06	0.08406	0	100	1000	3
6	-0.467	6.782E + 01	2.415E-06	0.03484	100	0	1000	5
7	-0.344	6.507E + 01	5.213E-06	0.05904	75	25	1000	5
8	-0.571	2.087E + 01	5.297E-06	0.07643	50	50	1000	5
9	-0.471	9.787E + 0	8.416E-06	0.09523	25	75	1000	5
10	-0.409	1.095E + 02	1.007E-06	0.09532	0	100	1000	5
1	-0.397	1.767E + 01	7.259E-06	0.1047	100	0	1100	3
2	-0.457	1.518E + 01	8.611E-06	0.1242	75	25	1100	3
3	0.1242	6.947E + 00	1.39E-05	0.1574	50	50	1100	3
4	-0.387	1.235E + 01	1.498E-05	0. 1697	25	75	1100	3
5	-0.473	2.213E + 01	1.465E-05	0.211	0	100	1100	3
6	-0.459	1.704E + 01	7.485E-06	0.108	100	0	1100	5
7	-0.521	5.123E + 0	1.329E-05	0.1918	50	50	1100	5
8	-0.312	2.797E + 00	2.797E + 00	0.2516	0	100	1100	5

The increase in corrosion resistance for the samples treated at 900°C can be attributed to the fact that these samples are externally composed mainly of Cr and Cr_1.36Fe_0.52, as shown by the X-ray diffraction analysis (Fig. 1a). The polarization curves also show that the protection against corrosion increases with the pack chromising time but decreases with temperature, which is the most significant parameter.

It is well known that the higher Cr concentration that the coating possesses, the better corrosion resistance that the coating exhibits.[22]. This fact suggests that at 900°C, the Cr concentration on the layer is higher, which results in better corrosion protection.

From the experiments, it can be concluded that pack chromizing treatment is an effective way to obtain layers with high chromium concentration, high hardness and good wear and corrosion resistance. An increase in activator concentration from 6 to 12% did not generate significant changes in the thickness and morphology of the produced layers, which were found to increase with treatment temperature and time. [23–24]

The sample pack chromized at 900°C for 7 h yielded a corrosion resistance close to the hard chrome coating, which is a promising condition for replacing the commercial hard chromium coating [25].

Thus, chromizing at 900°C for 7 h provides an optimum condition to obtain higher corrosion and wear resistance.

There are only a few mentions concerning the solution tetanized layers, consisting of a solid solution of Ti in α-Fe, on low carbon steels, because of a low titanium content in these layers, and consequently, their poor quality [26, 27].

The thickness of the coated later increased gradually with the increase of time or temperature. The higher the coating temperature or the longer the holding time, the greater the tendency to form holes in the coat, as well as the greater the chances of melting or fusion bonding on the surface.

The results obtained denote that the corrosion resistance of steel was enhanced significantly by coating. This can be followed shifted of E_corr in the noble direction (more positive values) for all coated specimens rather than the blank. I_corr for coated specimen decreased than the blank. At all coating times and coating temperatures, the corrosion rate was increased with increasing ferrotitanium percent in the cementation pack.

It can be noted that the corrosion resistance at 900°C of the coated specimen better than the corrosion resistance at 1000°C and 1100°C, it means that:

C.R at 900°C < C.R at 1000°C < C.R at 1100°C.

To address this challenge, turning to the process pack cementation, wherein the addition of dopant elements and substitutions to the mixture cementation pack can alter its properties and improve surface characteristics. [28]

4. CONCLUSIONS

The immersion experiment was carried out to study the seawater corrosion behavior of pipeline steel used in ocean environment by investigating the microstructure of designed steel, corrosion kinetics, corrosion phases, macroscopic/microscopic surface morphology. In addition, the role of chromium and titanium elements in the formation of corrosion products is also studied. According to the experimental results, some conclusions are summarized in following:

The microstructure of designed coated steel consists of Fe, Cr₂O₃, TiO₂, Cr_1.36Fe_0.52, Al₂O₃, Cr_1.9Ti, Ti Fe.

the Ti-rich precipitate particles and Cr-rich particles are observed in the steel substrate.
The corrosion rate of the early corrosion stage follows the exponential format. The corrosion behavior of designed steel is divided into two stages. At the first stage, the main corrosion process is the formation of Cr-rich and Ti-rich compounds. While, at the second stage, the one is the formation of ferrous corrosion product.
The chromium element changes into the Cr-rich compounds or replaces the iron atom in the corrosion products of goethite. Titanium element promotes the formation of compact and dense rust free of pores.
C.R at 900°C < C.R at 1000°C < C.R at 1100°C.
Corrosion rates decreased as Cr, Cr/Ti, and Ti deposition time increased, except for Cr at 900°C.
The chromium element changes into the Cr-rich compounds or replaces the iron atom in the corrosion products of goethite. Titanium element promotes the formation of compact and dense rust free of pores.
The surface of the corroded specimen was rough and peeled off. Chloride ions penetrated the metal through the porous protective film and reacted with the substrate. It developed various compounds causing crack initiation and propagation in the film layer.
The pack cementation was employed to improve the electrochemical corrosion resistance of low carbon steel via Cr, Ti, and Cr/Ti modified coatings. Continuous coatings were formed on low carbon steel surface by this method. A series of electrochemical experiments were carried out to investigate the corrosion resistance of low carbon steel, Ti, Cr/Ti, and Cr coatings. The sample surface was investigated by scanning electron microscopy (SEM). The phases of sample surface were detected by X-ray diffraction (XRD). It was concluded from all the outcomes that the corrosion resistance of the samples could be sorted in the following sequence: Cr Coatings > Cr/Ti coatings > Ti coating > low caebon steel.

Author Contributions All authors reviewed the manuscript

Conflict of Interest： The authors declare that they have no known competing fnancial interests or

personal relationships that could have appeared to influence the work reported in this paper.

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Improvement of Corrosion Resistance of Low Carbon Steels by Surface Modification

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Abstract

Figures

Introduction

Experiment

Characterizations

Microstructure and X-ray diffraction

Electrochemical performance

Results and Discussion

XRD analysis

SEM

Corrosion behavior

4.6 Electrochemical Corrosion Test:

4.6.1. Linear Polarization Experiments

4.6.1. potiondynamic polarization experiments

4. CONCLUSIONS

Declarations

References

Additional Declarations

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