Paraformaldehyde as a corrosion inhibitor for carbon steel in (%15) HCl

The effect of paraformaldehyde in the solution of 15% HCL as an inhibitor of C-steel corrosion has been studied using the weight loss method. It was found that the efficiency of inhibitor increased by increasing the inhibitor concentration and decreasing temperatures. Also, the action of paraformaldehyde as a corrosion inhibitor has been attributed to the insoluble complexes was adsorbed on the C-steel surface. The iron complexes were identified by FTIR spectrum and the mechanism of inhibition was explained as Langmuir adsorption. The thermodynamic parameters of the adsorption process were calculated and explained.


Introduction
Corrosion is generally called the undesirable damage that affects metals and metal alloys due to their interaction with the medium. The meaning of this definition is very wide which is mean interaction with water, electrolytic solutions and liquid mixtures as well as includes air, some gases and some fuses [1], since the productivity of oil reservoirs decreases constantly with a period of time and the process of acidification by hydrochloric acid is one of the oldest and most important methods used to improve the productivity of oil wells through improving the permeability of the bearing layer. This method has been applied for years when the layer was treated with acid injected through the well tubes to a certain depth that dissolve the rocks (calcium carbonate rocks). It also affects the cement bonding between the rock granules or the use of acid to clean the walls of the well from the effects of drilling fluid and other pollutants. This process corrodes the equipment's that is used in drilling and productions [2]. Hence, it is very important to study the corrosion and This work is licensed under a Creative Commons Attribution 4.0 International License. corrosion inhibition of carbon steel in hydrochloric acid. The first adsorption theory of inhibitor action was proposed in 1923 by researchers in [3] who studied alkaline and aromatic amines in aqueous sulfuric acid and hydrochloric acid. (M. Abdallah) study the effect of adding Amino pyridine derivatives as an inhibitor of corrosion to carbon steel in acid solutions HCl, which is one of the organic inhibitors that adsorb on the surface of the metal [4]. The researchers in [5] also used polyethylene imine to reduce the wear of malleable steel in an HCl solution that creates an adsorbent layer on the steel surface so that the inhibition efficiency reaches 95% by increasing the inhibitor concentration. The relationship between the coefficient of disability and the concentration of the inhibitor takes the form of adsorption isotherm. On this basis, (Sieverts and Lueg) suggests the concept of the adsorption mechanism of the inhibitor. A rise in temperature impedes the work of corrosion inhibitor and high temperatures increase the rate of corrosion, as the corrosion is doubled by increasing the solution temperature (10) degrees Celsius [6].
Substances that are able to interact with the metal surface and form only insoluble or slightly soluble compounds are effective corrosion inhibitors [7]. The addition of corrosion inhibitors effectively protects the metal against an acid attack [8][9][10]. The aim of the research is using Paraformaldehyde as an inhibitor in (%15) HCl at different concentrations and temperature.

Weight loos measurements:
In this work, paraformaldehyde was used as a corrosion inhibitor for coupons of (C-Steel) alloy. Tables (1) Where W add and W free are the weight loss of C-steel in the presence and absence of inhibitor. Tables (1), (2) show that the corrosion increases with increasing temperature and decreases with increasing inhibitor concentration.

The effect of temperature on adsorption
The effect of temperature on the corrosion rate of C-steel in the acid (15%) HCl was studied in the absence and presence of different concentrations of paraformaldehyde over a temperature range from (40 -90) ºC. Using weight loss measurement, the corrosion rate (R corr ) was calculated from the following equation [13]: show that the corrosion rate (R corr ) and efficiency (E) increase with increasing the concentration of the inhibitor and decrease with increasing temperature (T). The values of activation energy (Ea) of the corrosion process were calculated from the Arrhenius equation [14]: Where R corr is the corrosion rate of weight loss, A is Arrhenius constant, R is the gas constant and T is absolute temperature. Figure  increases with increasing in temperature but decreases with increasing the concentration of the inhibitor due to the corrosion inhibitor covering the metal surface [15]. We also noticed that the rate of corrosion increased with the increase in the exposure certain time at the temperature (40 and 60) ºC and it decreased at the temperature (80 and 90) ºC relatively.
Due to the increasing in the dissolution of paraformaldehyde at high temperatures and its coverage of more surface area on the metal.

Adsorption isotherm
The surface covered values by the corrosion inhibitor ( Ɵ ) on samples of C-steel inside hydrochloric acid (15% HCl) listed in Tables (1) and (2) Tables (3 to 6) and negative value for adsorption heat reflect the exothermic nature of the C-steel dissolution process.

Thermodynamic data
The Langmuir equation for isotherm adsorption can be written as follows: where K is the equilibrium constant for adsorption process. Through the curve of log C / Ɵ against log C, a straight line was obtained Figure (3), indicate the adsorption of paraformaldehyde on C-steel surface follows Langmuir, from this result it can be assumed that there is no interaction between the adsorbent materials, so the equilibrium constant (K eq ) calculated from equation (7) [11] and the values are listed in Tables (3 to 6 the surface of C-steel and the ΔG values range between (15.07 -19.58) kJ, this is means that the adsorption on C-steel is physical adsorption because the values are less than (40) kJ [17]. The ΔS values changed from less random to more random and it reversed depending on the paraformaldehyde concentration and temperature at each period time as long as exist uncovered surface area on C-steel, and the adsorption in all cases of the adsorption process were spontaneous on the surface of C-steel.   (7) and Figure (6) shows the IR spectrum of paraformaldehyde before the adsorption process.

Conclusions
1-Paraformaldehyde can be used as acid corrosion inhibitor.

2-The efficiency of inhibitor depends on the concentration of paraformaldehyde and to
the temperature of medium.
3-Paraformaldehyde makes insoluble and stable complexes on C-steel surface.
4-The adsorption of paraformaldehyde on C-steel surface follows Langmuir equation.