Corrosion behavior of mild steel immersed in different concentrations of NaCl solutions Mousa May Material Engineering Dep., Faculty of Engineering, Sebha Uni., Sabha - Libya
Abstract: The current study deals with the effects of various immersion times and different concentrations of NaCl solutions on corrosion behavior of mild steel. The kinetics of mild steel corrosion in NaCl media has been investigated by weight loss at room temperature. Generally, the weight loss of mild steel at room temperature ( ͂ 25◦C) has been found to be quite significant, indicating poor corrosion resistance. Based on the data results, the corrosion process in different NaCl environments, may attributed to the present of dissolved oxygen. Also, the presence of Cl- as an aggressive ions play an importance role in corrosion process. Chloride anions in the solution could help to remove the metal cations accumulated on the anode by forming soluble compounds, and this contributes to an accelerated anodic reaction and thus faster rusting of the metals. The corrosion rates of the samples were also calculated by using an average weight loss measurements. In corroding media, (NaCl solutions), corrosion of carbon steel depends on the amount of oxygen dissolved in the media and concentration of the media. The higher the concentration, the more film build up and the lesser the corrosion rate. While the smaller the concentration the higher the corrosion rate. Keyword:
NaCl
solution,
Corrosion
Behavior,
Mild
Steels.
1. Introduction Corrosion is a process that involves deterioration or degradation of metal. The most common example of corrosion is the formation of rust on steel. Most corrosion phenomena are of electrochemical nature and consist of at least two reactions on the surface of the corroding metal. One of the reactions is the oxidation (e.g., dissolution of iron) also referred to as the anodic partial reaction. The other is a reduction reaction (e.g., reduction of oxygen), and is referred to as the cathodic partial reaction. The products 1
of the electrochemical reactions can react with each other nonelectrochemically to form the final product (e.g., rust) [1]. Gradients of metallic and electrolytic ion concentrations, temperature, ambient pressure, and the presence of other metals, bacteria, or active cells, all these factors influence the corrosion rate [2]. Carbon steel is the most widely used engineering material, accounts for approximately 85%, of the annual steel production worldwide. Despite its relatively limited corrosion resistance, carbon
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Corrosion behavior of mild steel…………………………………..………… Mousa May
steel is used in marine applications, nuclear power and fossil fuel power plants, transportation, chemical processing, petroleum production and refining, pipelines, mining, construction and metal-processing equipment [3]. Because carbon steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, it is easy to understand that the corrosion of carbon steels is a problem of enormous practical importance. It well known that corrosion involves the transfer of electrons along the surface of the metal under the influence of a potential difference [4]. Corrosion is accelerated by acids or by contact with less active metals such as copper or lead. In addition, certain salt solutions also accelerate corrosion, not only because they are acidic by hydrolysis, but also because of specific catalytic effects or reactions of the anions [5]. Therefore, there is effective collision of particles which affect corrosion rate. Environmental factors like oxygen concentration in water or atmosphere, the pH of the electrolyte, temperature, concentration of various salts solutions, play a significant role in the rate of corrosion of metals even if such metallic materials are completely homogeneous in nature [6]. Meanwhile, hydrogen evolution from an acidic environment is responsible for the sustenance of corrosion of metal [7]. Higher concentration of a solution will cause more hydrogen gas evolution. The stability of the halide in the surface complex determines the effect of corrosion kinetics of the metal/alloy. According to Norio Sato [8], metallic 2
corrosion in aqueous solution consists of the anodic dissolution of metals and the cathodic reduction of oxidants present in the solution. These reactions are charge-transfer processes that occur across the interface between the metal and the aqueous solution. The cathodic process is carried out by the reduction of hydrogen ions and/or the reduction of oxygen molecules in aqueous solution. These two cathodic reductions are electron transfer processes that occur across the metal– solution interface, whereas anodic metal dissolution is an ion transfer process across the interface. The aim of the present study is to investigate the corrosion behaviour of mild steel over a wide range of NaCl concentrations and test immersion periods. The weight loss results have also been supplemented by surface study investigations. The simplest way of measuring the corrosion rate of a metal is to expose the sample to the test medium (e.g. sea water) and measure the loss of weight of the material as a function of time. 2. Experimental work 2.1 Specimen Preparation The effect of different immersion times and NaCl concentrations on corrosion behavior of mild steel substrates were investigated. Mild steel exposed to different corrosive environments and left for a stipulated period of four weeks with a weekly interval of collection, weighing and re-immersing into the various environments. Carbon steel samples of 50 x 40 x 2 mm size were cut from a sheet. Samples were machined and
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abraded sequentially with silicon carbide papers of grades 180, see Fig 2.1. Specimens were then washed, cleaned with ethyl alcohol and dried up. After taking the initial weight, (to
an accuracy of three decimals) before the immersion tests in order to conduct weight loss experiments. The samples placed in the NaCl solutions of different concentrations.
Figure 2.1 mild steel samples used
2.2 Measuring and Weighing After surface preparation, the dimensions of the specimens should be carefully measured to permit calculation of the surface area. The following formula (2-1) used for calculating the corrosion rate [9]. 𝑪𝑹 (𝒎𝒑𝒚) =
𝟓𝟑𝟒 𝑾 𝑨×𝑫×𝑻
2-1
Where; W = weight loss (mg) , A = surface area (in2), D = mild steel density (g/cm3), T = immersion time (hr). The original area is used to calculate the corrosion rate throughout the test. After measuring, the specimen is degreased by washing 3
in a suitable solvent such as ethanol, and then weighted.
2.3 Corrosion Tests 2.3.1 Preparation of Corroding Media Different concentrations of aqueous NaCl solutions were used as a corroded media to conduct the corrosion behavior of mild steel substrates. The samples were exposed to 1, 3, 5, 7 and 10 % NaCl solutions throughout different immersion times, at room temperature (25±2 °C), see Fig 2.2. The concentration of NaCl
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solutions were prepared according to the following formula (the conc. NaCl % = the amount of NaCl (g) / the volume (1L)); i.e 1% NaCl = 10 g of sodium chloride dissolved in 1000 ml of H2O. In addition, some samples immersed into pure water to evaluate the corrosion rate. The selected
immersion times to calculate the changes in samples weight loss were 1, 7, 14, 21 and 28 days. Throughout the experiments, NaCl solutions in all samples were levelled with distilled water to maintain the same concentrations during the immersion times.
.
Figure 2.2 immersed samples in different NaCl concentration
2.3.2 Determination of Weight Loss
2.4 Morphology
At the end of each interval testing period the samples were taken out from the corrosion media, washed with tap water, distilled water and then with ethanol. After that, the samples were weighed for weight loss calculation. The process of washing, drying, weighing, determination of weight loss and recording was repeated consistently.
Scanning microscopy analyses is a beneficial tool to characterize the corrosion behavior of metallic substrates immersed in corrosion media. An optical microscope with high digital camera was used to identify the effects of exposing of mild steel substrates to different NaCl concentrations. Images were obtained for all samples in different immersion times to characterize the corrosion product that formed on the samples.
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Corrosion behavior of mild steel…………………………………..………… Mousa May
3. Result and discussions 3.1 Sample weight vs immersion times The experimental results show that a continuous decrease in weight with increased the immersion times. The relationship between weight loss and immersion times are illustrated, see Fig 3.1 and 3.2. Generally, the weight loss of mild steel at room temperature ( ͂ 25◦C) has been found to be quite significant, indicating poor corrosion resistance. After immersion in pure water up to 28 day, the steel substrate showed a clear weight reduction. Same results was indicated when steel coupons immersed in 1% NaCl media, see Fig 3.1. The behavior may attributed to the presence of chloride ions (Cl−) in corrosion media. Based on the results data, the corrosion process in different NaCl environments, may attributed to the present of dissolved oxygen. Water solutions rapidly dissolve oxygen from the air, and this is the source of the oxygen required in the corrosion process. It well known that, the most familiar corrosion of this type is the rusting of iron when exposed to such environment [9]. In addition, under the condition of high amount of Cl−, the existence of Cl− is conducive to the formation of FeOOH [9]. (Fe2+, Fe3+)+ Cl− + OH− →
FeOOH
Moreover, and according to the adsorption theory [11], Cl- adsorbs on 5
the metal surface in competition with dissolved O2 or OH-. Once in contact with the metal surface, Cl- favors hydration of metal ions and increases the metal ions migration from the metal surface to the bulk of the solution. However, the effect of adsorbed oxygen is opposite, which decreases the rate of metal dissolution [12]. From Fig. 3.1, it revealed that as increased the immersion time in 3 % NaCl, the reduction in weight loss weight was slightly increased. However, there is a little increase in weight of samples with the immersion time in 5 % NaCl solution, see Fig 3.2. These results may be due to the increase in conductance of the solution as a result of continuous addition of Fe2+ caused by the corrosion of mild steel. Similar results were found by Singh and Mukherje [13] for the mild steelNaCl solution system. The decrease in corrosion rate at higher concentrations is assumed to be a consequence of the increase in the viscosity of solutions due to the formation of a dimer, resulting in a decrease in the mobility of the ions. Also, a slightly increase in weight of samples immersed in 5 % NaCl, see Fig 3.2, may be either due to a decrease in solubility of the electrolyte as it slowly becomes saturated with the corrosion product or the formation of some surface film which retards the corrosion of mild steel. The formation of a surface film is unlikely, as evident from the significant dissolution of the samples. Moreover, carbon steel immersed in solutions containing an appreciable concentration of Cl-, may be not readily passivated anodically [14].
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Breakdown of passivity by Cl- occurs locally rather than generally over the surface.
Figure 3.1 sample weight via immersion times in pure water, 1 & 3% NaCl
The changes in corrosion behavior of mild steel immersed in 7 & 10 % NaCl showed similar results, see Fig 3.2. It can be seen that initially with increasing time of exposure from 1 to 14 days, the weight of mild steel increased. This may be due to very large internal resistance offered by the solution. However, on further increase of exposure time, the reduction in sample weight was observed. This result may referred to a deposited layer of metallic substrate starts breaking beyond 14 days as a uniform attack by NaCl solution. As results, minute anodes of active metal such as steel substrates are formed surrounded by large cathodic areas of metals.
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This is due to a potential difference between the two areas, and thus resulted cell, called a passive-active
cell. This cell will lead to pitting corrosion. Chloride ions do not chemically react with the metals. Chloride ions only assume a role as a medium or catalyst in the electrochemical process. Chloride anions in the solution could help to remove the metal cations accumulated on the anode by forming soluble compounds, and this contributes to an accelerated anodic reaction and thus faster rusting of the metals [15]. The metal dissolution in various concentrations as immersed times was observed see Figure 3.3. Moreover, the high concentration of
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Corrosion behavior of mild steel…………………………………..………… Mousa May
chloride and consequently the low oxygen concentration may also enhance the corrosion rates and initiate the pit. Chloride ions from the
bulk solution will cause an increase in acidity inside the pit formation, promoting corrosion.
Figure 3.2 sample weight via immersion times in 5, 7 & 10 % NaCl
Fig 3.3 corrosion products as an immersed time in NaCl
3.2 Corrosion rate results
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The values of weight loss and corrosion rate calculated from the
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Corrosion behavior of mild steel…………………………………..………… Mousa May
equation (2-1) at different concentrations of NaCl solutions at 25 °C in various times, have been recorded in following tables. The obtained results indicated that corrosion rates decreased when carbon steel immersed in high NaCl concentrations as compared with that in pure water and low NaCl concentrations. This may occurred due to the very high resistance of the solution at higher concentrations. In corroding media (NaCl solutions), corrosion of carbon steel depends on the amount of oxygen dissolved in the media and concentration of the media. The higher the concentration, the more film build up and the lesser the corrosion rate. While the smaller the concentration the higher the corrosion rate. It was reported [15] that the effect of chloride ions on the corrosion of steel surfaces may vary as the changes in NaCl concentrations. It can be seen (from Table 1 & 2) that the corrosion rate of steel in pure water is slightly increased as the immersed times increased. Similar results noted for
carbon steel exposure to 1% NaCl. This can mainly attributed to the chloride ions because of its ability to destroy the passive surface of steels and accelerate their corrosion. However, the decrease in corrosion rate with exposure time in 3, 5 and 7 % NaCl may be due to a decrease in solubility of the electrolyte as it slowly becomes saturated with the corrosion product or the formation of some surface film which retards the corrosion of mild steel. This result was supported by other authors [16] who pointed out that the formation of corrosion product acts as a physical barrier against oxygen diffusion and thus decreases the corrosion rate. In addition, the reduction in corrosion rate may attributed to a decrease in the mobility of the ions within the solution. Besides this, an increased concentration may increase the electrostatic ion-ion interactions in solution and therefore decrease in the degree of dissociation.
Table 1 weight loss and corrosion rate for pure water, 1 % & 3 % NaCl Immersion Time, days
Pure water
Corrosion media 1% Nacl
3% NaCl
Weight loss gr
CR mpy
Weight loss gr
CR mpy
Weight loss gr
CR mpy
1
0.0011
0.464
0.0032
1.350
0.00027
0.114
7
0.0056
0.337
0.0096
0.578
0.0085
0.512
14
0.0347
1.046
0.0177
0.533
0.0174
0.525
21
0.0420
0.849
0.0381
0.770
0.0400
0.808
28
0.0572
0.862
0.0523
0.788
0.0456
0.687
CR = Corrosion Rate, (mpy)
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Corrosion behavior of mild steel…………………………………..………… Mousa May
Table 2 weight loss and corrosion for 5 %, 7 % & 10 % NaCl Immersion Time, days
Corrosion media 7% Nacl
5% Nacl
10% NaCl
Weight loss gr
CR mpy
Weight loss gr
CR mpy
Weight loss gr
CR mpy
1
0.011
4.643
-0.0004
-0.036
-0.0005
-0.211
7
-0.0033
-0.198
-0.0026
-0.156
-0.0124
-0.747
14
-0.0358
-1.079
-0.0095
-0.286
-0.0113
-0.340
21
-0.0017
-0.034
0.0434
0.872
0.018
0.361
28
0.0239
0.360
0.0517
0.779
0.0209
0.315
CR = Corrosion Rate, (mpy)
3.3 Surface Observations The surface morphology of the specimens was carried out by using an electronic microscopy after removing the corrosion product. The microphotographs illustrated in Fig. 3.5 a, b and c. Figure 3.5a represented the corroded sample exposure 21 days in pure water. The images produced at a magnification of 10x. The Figure 3.5b showed the sample immersed for 21 days in 3% NaCl, which corresponded to high corrosion rates. Figure 3.5c appeared the corroded sample after 21 days of exposure in 7% NaCl. The surface morphology indicated a uniform attack by corrosion media. It can be seen that the corroded portion to total
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surface areas is small on sample immersed in high concentration (7% NaCl) compared with that in pure water and low NaCl concentration. The results attributed to a uniform and localized attack of carbon steel in presence of Cl- ions within the solution. In addition, carbon steel immersed in solution with low chloride content most likely would be most disastrous when compared to a higher concentration of it. Seemingly the higher the concentration, the more the film build up and the lesser the corrosion reaction. This can be explained by the fact that the material deposited on the surface acts as a barrier leaving less active area on the surface.
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Corrosion behavior of mild steel…………………………………..………… Mousa May
Figure 3.5 the surface morphology of corroded specimens
4. Conclusions The main conclusion to be drawn from this work can be summarized as the following. 1.
2.
3.
10
The obtained results showed a continuous decrease in weight of steel samples with increased the immersion times. The overall results indicated that corrosion behavior of carbon steel depends on the chloride ions (Cl-) concentration in corrosion media. Mild steel immersed in 7 & 10 % NaCl showed a little increase in
4.
weight, and the behavior may related to very large internal resistance offered by the solution or a decrease in solubility of the electrolyte as it slowly becomes saturated with the corrosion product. The results indicated that corrosion rates decreased when carbon steel immersed in high NaCl concentrations as compared with that in pure water and low NaCl concentrations. The behavior may attributed to the formation of corrosion product which acts as a physical
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Corrosion behavior of mild steel…………………………………..………… Mousa May
uniform and localized attack of carbon steel surface in presence of Cl- ions within the solution.
barrier against oxygen diffusion and adsorption to the metal surface and thus decreases the corrosion rate. The surface morphology showed a uniform attack by corrosion media. This may referred to a
5.
سلوك التاكل للفوالذ الكربوني الطري مغمور في تراكيز مختلفة من محلول كلوريد الصوديوم مويس يم هذا العمل يدرس اتثري اختالف زمن الغمر وتركزي حملول لكوريد الصوديوم عيل سلوك التالك للفوالذ الكربوين .معليات التالك لهذا املعدن درست بواسطة تقنية فقدان الوزن يف درجة حرارة الغرفة .وبشل عام ،لوحظ ان فقدان الوزن للفوالذ الكربوين كبري نسبيا ما يشري ايل مقاومة جد صغرية لهذا املعدن بوسط التالك .واستنادا ايل نتاجئ ادلراسة ،ميكن ان يعزي هذا النقص يف وزن العينات ايل وجود الاكسجني املذاب وكذكل ايل ايون اللكور ابحمللول حيث يلعب دورا همام يف معليات اتلك املعدن .ان ايوانت اللكور يف احمللول ميكن ان تساعد عيل ازاةل ايوانت املعدن Fe+2املتجمعة عيل املواقع الانودية عن طريق تكوين مركبات ذائبة ،وهذا يسهم يف ترسيع التفاعل الانودي ومن مث زايدة معدل التالك .كام ان معدل اتلك املعدن ( الفوالذ الكربوين ) يعمتد عيل مكية الاكسجني املذاب ابحمللول وتركزيه ،فلكام اصبح الرتكزي ( لكوريد الصوديوم ) اكرب تكونت طبقة محية عيل سطح املعدن تقلل من معدل التالك ،والعكس حصيح التالك. معدل يزداد حيث الصغرية ابلرتاكزي References
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