Adsorption And Corrosion Inhibition Of Several Organic Inhibitors On Carbon Steel In Simulated Concrete Pore Solution

Reinforcement concrete is currently the most widely used construction material in the world. The corrosion of reinforcement is the main reason caused the damage of concrete structures. Adding corrosion inhibitor has been regarded as an effective method to prolong the service life of materials and control corrosion in concretes. Nowadays, most of the inhibitors used in concrete environments are inorganic substances which show some problems such as the toxic. The studies of environmental organic inhibitors are relatively scarce which is due to the complex mechanism and the difficulty observation of the inhibition behaviors of organic inhibitors on local corrosion. By using the in-situ spectroscopy systems combined with traditional techniques and quantum chemistry, detailed results on the corrosion behavior and the inhibition mechanism may be obtained. In this paper we introduced succin imide (SI), L-proline (LP), calcium lignosulfonate (CLS), sodium carboxymethyl cellulose (CMC) and sodium oleate (SO) as five kinds of organic inhibitors. The main content of this paper is study the adsorption behaviors and the mechanism of corrosion inhibition of these five inhibitors for Q235 carbon steel in simulated concrete pore solution. Before the research, we characterized the chemical structure of these five organic inhibitors.And explored a suitable simulated concrete pore solution(SCP solution) for detecting the inhibition behavious of organic inhibitors on local corrosion, that is, the saturated Ca(OH)2 solution containing 0.1 mol/L NaCl with the pH value of 12.5. The research results of this paper can be divided into the following several aspects:(1) The corrosion behaviours of succin imide (SI), L-proline (LP), calcium lignosulfonate (CLS), sodium carboxymethyl cellulose (CMC) and sodium oleate (SO) for Q235 carbon steel in saturated Ca(OH)2+0.1 mol/L NaCl solution was studied by means of weight loss measurement, dynamic-polarization curves, scanning electron microscopy/energy dispersive spectro-metry (SEM/EDS) and X-ray photoelectron spectroscopy (XPS). The results illustrated that the inhibition effect of these five inhibitors are as follows: CLS>SO>CMC>SI>LP. With the adding amount of CLS was 0.001 mol/L, the inhibition efficiency of CLS on carbon steel is over 98% and the passivation region of polarization curves is about 960 mV. The results showed that CLS was an effective inhibitors for carbon steel in SCP solution for both general corrosion and pitting corrosion. The inhibition efficiency of 0.0008 mol/L SO and 0.0008 mol/L CMC were 77% and 61%, respectively. SI and LP showed poor inhibition effect for carbon steel. With the adding amount of LP increased, the corrosion of carbon steel in SCP increased.(2) The inhibition mechanism of CLS and SO for Q235 carbon steel in SCP solution were studid. The XPS studies reveal that the adsorption between CLS and calcium presents as Ca-O-S bonds, which was a kind of physisorption. Also the benzene ring groups of CLS could adsorb on the passive film by sharing the lone pair or π-electron with free d-orbital of metal, which was a kind of chemisorption. The adsorption of CLS on carbon steel surfaces occurs by both physisorption and chemisorption probably result in the adsorption energy between CLS and the surface was strong and the adsorption film formed by CLS was dense and uniform. Due to the adsorption behaviors, CLS showed good inhibition effect in both general corrosion and pitting corrosion. When dissolved in the solution, the carboxyl groups of SO was with negative charges and tended to diffuse to the surface of carbon steel which had the excessive positive charge. The oleic acid groups adsorbed on the surface of steel while the long-chain arranged to the outside of the surface. The adsorption of SO molecular formed a barrier layer at the surface and effectively prevent the diffusion of Cl- ions on the surface, which is supported by the well known adsorption mechanism of organic molecules at the metal/solution interface.(3) In our research, we used the fluorescence microscope to observe the structure of the film formed on the surface of samples which were immersed in SCP solution with and without inhibitors for 10 h. The "Adsorption film-Passive film-Matrix" double-layer structure has been verified by the fluorescence microscope.(4) The long-term inhibition efficiency of five inhibitors was measure by a 7200 h long-term weight loss measurement. The results showed that the inhibition rate of these five inhibitors was following CLS>SO>CMC>SI>LP, which had a good agreement with the 720 h weight loss measurements. SI, LP, and CMC failed in the long-term test. Only when the adding amount of SO increased to 0.0008 mol/L, the inhibition rate was over 75%. While CLS still showed a good inhibition effect for carbon steel in long-term test with the adding amount higher than 0.0002 mol/L. With the adding amount increased to 0.0008 mol/L, the inhibition rate of CLS was increased over 93%. CLS is an organic corrosion inhibitor which is suitable for long-term use.(5) For the first time, we used microscopy infrared imaging systems to observe the adsorption behavior of five organic inhibitors on carbon steel in SCP solution. And we firstly observed the characteristic adsorption of inhibitors on pitting corrosion. The sinapyl alcohol monomer of CLS showed a characteristic adsorption on pits formed on the surface, which is supported by the Hard-Soft-Acid-Base (HSAB) theory.(6) The quantum chemical indices of five organic inhibitors were calculated to predict their inhibition performance by the regulation of the relationship between structure and inhibition by previous studies. According to the forecast results of the quantum chemical calculation, the inhibition rate of inhibitors should following the rules of SO>CLS>CMC>LP>SI. But the experiment results was in an order of CLS>SO>CMC>SI>LP. The inconsistency of the calculation results and experiments result was occurred between inhibitors with the similar AE value and one of them showed the performance of characteristic adsorption. The results illustrated that the characteristic adsorption may enhance the inhibition efficiency of the organic inhibitors. The quantum chemical calculation was more suitable to inhibitors used in general corrosion or inhibitors with a similar structure or with a big AE gap.