Evolution Study of the Surface States of Low Carbon Microalloyed Steel before and after Corrosion in NaCl Solution

. Surface topography is of paramount interest in science, technology, and industry. In this work investigation of the evolution of the surface with multiparameter characterization was carried out using optical profiler. Surface profile is measured two - dimensionally applying Gaussian filter of cutoff 0.8 mm. R a , R q , R sk , R ku , R p , R v and R z values according to the standard ISO 4287 were measured. Following ISO 25178, 3D roughness parameters: S a , S q , S sk, S ku , S p and S v values were reported. The appearance of the texture of our microalloyed steel has been indicated by evaluating the texture aspect S tr . Analysis of Abott Firestone curves and related parameters S mr and S dc was presented. Some significant characterization parameters such as surface hardness, weight loss and corrosion rate were analysed and compared.


Introduction
Currently, the micro-alloyed steels have emerged as an important class of materials and drawn lots of research focus. They are called micro-alloyed steels because they contain only a small number of alloying elements such as: vanadium, niobium or titanium [1-2]. In addition, they have a ferritic matrix with extremely fine grained structure due to the effects of the alloying elements [3]. Consequently, micro-alloyed steels are generally characterized by high strength and ductility resulting from the fine grains and ferrite matrix. They are thus widely used in the oil and gas industry due to their beneficial mechanical properties. In recent years, there is a growing awareness of the necessity to evaluate the surface quality. The surface roughness is recognized as being one of the most important properties. A wide range of roughness parameters can be used to describe a given surface [4][5]. The most common parameters are calculated from line profiles according to the ISO 4287 standard [6], but due to an increased use of 3D profilers a set of complimentary area roughness parameters have been defined in the ISO 25178 standard [7].
In the present study, an effort has been made to determine the surface properties of micro-alloyed steels by analysing the change in several 3D parameters. In addition, microstructural evaluation, surface hardness measurements, weight loss, and corrosion rate were performed and considered in order to assess the suitability of such steels for application to gas transmission pipelines.

Experimental details 2.1 Materials
The steel used in this study is a low carbon ferritic steel with a ferrite-pearlite fine-grained microstructure. In delivery condition, the steel manifests approx. 86.64 ferrite and 13.35 pearlite, with a measured average grain size of ASTM 8 (see Table 1) [8].

Metallographic preparation
In order to perform the gravimetric measurements, pipeline tests of Ø12 diameter were machined into a rectangular form of 25 mm × 15 mm × 4.45 mm. Specimens were subjected to a mechanical polishing at a speed of 500 rpm using the silicon carbide abrasive papers sequentially of up to 1000 grades, the polished surface was then thoroughly rinsed with distilled water and degreased in acetone before used and dried with an electric drying machine. All experiments were carried out under thermostatic conditions 25°C (±0.1°C).

X-ray diffraction
The XRD analysis was performed using X'Pert3 Powder Diffractometer. Diffraction patterns were recorded at 45 kV and 40 mA in 0.013 o /s detector's movement speed, the scanning was carried out with a step length of 3.35° (2θ) and a step time of 29.07 s. Flat diffracted beam pyrolitic graphite monochromator was used to remove fluorescent X-rays. It becomes very relevant when analyzing iron compounds with CuKα radiation. Noise that emerges because of the fluorescence (without monochromator) would be so large that only the most intensive diffraction maximums (e.g. from ferrite) would appear in the diffraction pattern. The diffraction patterns were recorded automatically by a data acquisition system. The peaks obtained were identified with those available in PDF-2 data base.

Surface roughness measurements
An optical profilometer, AltiSurf® 500, was used to characterize the roughness of the sample. Using the software, Phenix V2, cartographies of the disc surfaces were done with 5x5 mm dimensions. An acquisition with steps of 2 micron, a speed of 400 micron/sec and a double frequency 400/800 Hz. Roughness profiles of a 4.9 mm length were extracted from the diagonal of the cartographies and their parameters (R a , R z , etc.) were calculated with Altimap topography XT software. Optical images were captured by a CCD camera and then recorded.

Weight loss measurements
The samples were tinted with anti-rust paint all over the sides except around one facade that is to be buried in the ground at pH from 7 to 9. After weighing with sensitive electronic balance, samples were immersed in 10 ml 0.9% NaCl solutions. Weight loss of metal samples was noted at every 24 h time interval. The experiments were carried out in duplicate and the average values were reported. After the completion of the test, the corrosion rate is calculated by the following equation:

X-ray diffraction analysis
X-ray diffraction patterns ( Fig. 1)   It can be seen that there were not any significant changes in the amount of each phase formed, and as the time exposure is extended, the intensity of the peaks corresponding to ferrite increases slightly, this indicates that the interplanar space was smaller. Using a simple 2D profile, as shown in Fig. 2, may not provide a complete picture of the sample surface. This profile can provide accurate height, width and roughness data, but may miss detailed surface features that can be measured by adding 3D analysis capabilities. According to ISO 4287, 2D amplitude parameters values are shown in Table 3.

Optical profiler analysis
In te ns ity , a. u.

Diffraction angle 2θ, deg.
For sample immersed for 144 h, it is important to note that the sum of R P and R V is equal to R Z , which is lower than the value R t . This result is in good agreement with the literature given by the ISO 4287 standard.  Following ISO 25178, 3D surface roughness parameters are indicated in Table 4.  It is observed from Fig. 4 that an increase in arithmetic mean height S a is accompanied by an enhancement of the corrosion. Interestingly, this trend is similar to that in the variation of corrosion rate with corrosion period (Fig.  8). It can be concluded that the corrosion rate increased with an increase of surface roughness. The reason of this phenomenon is that by increasing the roughness the active sites increased and deeper grooves played an active role as suitable places for localized corrosion. The depth of the valleys that influenced the diffusion of active ions during NaCl corrosion. By contrast, in a smooth surfaces weak points are less activate than on rough surface and the formation of stable passive film on smooth surfaces is more likely to occur.
A bearing ratio analysis was performed on both types of samples as seen in Fig. 5. A bearing ratio curve indicates the percentage of a surface that falls above/below a particular depth. These curves quantified the percentage of valley area that tended to lead to corrosion.   From these data prsented in Table 5, it was determined that the deeper valley structure tended to hold processing NaCl solutions, allowing rusting to occur.
Surface anisotropy is also observed with the 3D roughness parameter S tr ratio of texture aspect. The value can lie in the interval from 0 to 1. It can be concluded that the appearance of the surface texture after 144 h of immersion into NaCl solution is of the anisotropic type while the steel as-received is essentially isotropic before corrosion.

Weight loss and corrosion rate measurements
The results obtained for the variation of weight loss and corrosion rate with exposure time, with varied concentrations of NaCl solution are presented in Table 7. Table 7. Weight loss and corrosion rate variations with immersion period and NaCl concentration.
It must be pointed out that in the initial condition (steel as-received) no weight loss was seen. However, there is a significant variance between the weight losses of the samples tested in different chloride concentrations that has noteworthy effect on the weight loss of microalloyed steel. It is seen from the attributed to difference in the samples grain size [9]. It was equally revealed that concentration of NaCl affects rate of corrosion and posited that the bigger the grains of material, the more are its resistance to corrosion [10]. Hence, the as-received sample with the biggest grain size distribution revealed less weight loss as compared to the samples immersed in NaCl solution.
The NaCl medium with 10 mg/ml concentration had the least corrosion effect of the immersed samples by the end of the 24th hour achieving a weight loss value of 4.24 mg. At this value it could be considered to be fairly protective. The NaCl medium with 50 mg/ml concentration recorded the highest weight loss with a value of 41.66 mg showed that the NaCl exhibited very good corrosion at the end of the 168th hour. Additionally, it was further observed that the exposure time had greater significant effect on the corrosion rate. Ferritic steels having a softer matrix can be easily and rapidly corroded by corrosion reactions depending on the time. On the other hand, hardness of the steel matrix provides good chemical and mechanical properties. Chloride ions induce localized corrosion on carbon steels and can as well lead to the formation of significant amount of corrosion products on steels [11]. Our results show that the corrosion rate of the steel increases sharply and then decreases to stabilize at a value close to 0.07012 (mg/h.cm²) due to the presence of a passive film. Most corrosion-resistant metals and alloys are in the passive state [12], which confirms our results.
Due to the presence of the passive film, the passive dissolution which corresponds to a certain potential is slower than the active dissolution. It depends, inter alia, on the properties of the film, in particular its solubility in the electrolyte. During passivation, in other words during the transition from the active to the passive state, the dissolution rate decreases [13], which is in good agreement with our results. The increasing degradation of micro-alloyed steel due to chloride as detected in this study compares well with outcomes on corrosion performance of carbon steel. However, micro-alloyed steel may be supposed to be basically well-suited for gas transmission pipelines application since its corrosion rates are characteristically low.

Conclusions
Based on the experimental results, the following conclusions can be drawn from the present investigation: i. The XRD technique enables to identify the phases formed and the intensity of those peaks defines the amount of the steel phases. The microstructure of the samples consists of fine ferrite. The amount of ferrite reduced with hardness due to corrosion in NaCl solution.
ii. Weight loss of the samples is determined as a function of immersion period into NaCl solution. The results show that the microstructure and micro-hardness at different exposure time directly affect the corrosion rate, as the micro-hardness decreases the corrosion rate increases. The samples displayed higher corrosion rate. The presence of a passive film provides a higher protection characteristic.
iii. Sodium chloride concentration of up to 50 mg/ml increases the weight loss. Chloride further increases the corrosion rate. In the absence of chloride, no weight loss and corrosion rate were observed. iv. Many parameters have been established regarding the measurement and assessment of surface roughness. Surface parameters have been shown to exert a great influence on corrosion and corrosion rate where the amplitude parameters have the most significant effect. v. Micro-alloyed steel is supposed to be well-suited for gas transmission pipelines application due to its very low corrosion rate.