INTRODUCTION

Polymers find applications as effective corrosion inhibitors for steel. The use of polymers as corrosion inhibitors have drawn considerable attention recently due to their inherent stability and cost effectiveness1 . Owing to the multiple adsorption sites, polymeric compounds adsorb more strongly on the metal surface compared with their monomer analogues 2 . Therefore, it is expected that the polymers will be better corrosion inhibitors. The Literature reveals that a wide range of polymeric compounds have been successfully investigated as potential inhibitors for the corrosion of metals in aqueous media. Polymers such as polyvinylpyrrolidone 3 4 , polyvinyl alcohol 5 , poly (o-anisidine-co-metanilic acid) 6 , poly anthranilic acid 7 , Polyacrylic acid 8 7 , Polyaniline9 .

Corrosion inhibitors are widely used as an economical method of corrosion control 10 , 11 . Environmental restrictions imposed on heavy metal-based corrosion inhibitors oriented scientific researchers towards the study of non-toxic and environmentally friendly corrosion inhibitors 12 . The influence of several organic compounds containing polar functions such as nitrogen, oxygen, phosphorous or sulphur on the corrosion inhibition behavior of mild steel in aqueous solutions has been documented 13 14 15 16 17 . These groups of atoms or bonds facilitate electronic interactions between organic corrosion inhibitors and metal surface thereby aid adsorption of the inhibitors onto metal surface. The quest for eco-friendly compounds as corrosion inhibitors has shifted research focus to exploring potential application of polymers as corrosion inhibitors in the past few years. The use of polymers as corrosion inhibitors has attracted considerable attention recently because: (i). they have low cost and are stable to metallic materials in aqueous media, (ii). possession of multiple adsorption sites and (iii) through their functional groups, they form complexes with metal ions, and on the metal surface, these complexes occupy a large area, thereby blanketing the surface and protecting the metal from corrosive agents present in the solution 18 19 20, 14 21 22 . The inhibitive power of these polymers is related structurally to the cyclic rings, heteroatom (oxygen and nitrogen) that are regarded as centers of adsorption. Some polymers have been reported to inhibit the corrosion of mild steel in various aqueous media 23 24 25 26 . Recently, polyacrylic acid reported to be very effective corrosion inhibitors for the protection of mild steel in aqueous media to promote eco-friendly environment. International 27 28 .

The aim of the present work is to determine the inhibitive effect of Polyacrylic acid (PAA)on the corrosion of carbon steel in 400 ppm acidic medium by chemical and electrochemical methods. Surface characterization techniques (SEM, EDAX) are also used to ascertain the nature of the protective film.

Experimental Part

2:1 Materials and sample preparation

Polyacrylic acid (PAA) and Steel coupons (with a diameter of 10 millimeters and the following chemical elements composition (wt.%): 0.046% Cr, 0.035% P, 0.01% S, 0.202% C, 0.161% Mn, 0.007% Mo, 0.065% Si, and 99.3% Fe) were supplied from a local market in Iraq .

Analytical grade 36M HCl was used in the preparation of 1M HCl dilute solutions.

Distilled water was exclusively used in the formulation of corrosion solutions. To this end, 1M HCl solutions containing concentrations of 0, 10, 50, 100, 200,300,400 ppm of polyacrylic acid were meticulously prepared. Prior to immersion, steel coupons were rigorously abraded using SiC papers of varying grits (120, 400, 800, and 1500 grades), degreased with acetone, and thoroughly rinsed with distilled water. The pre-treated steel coupons were then placed in the coupons holder and immersed in these corrosion solutions.

Results And Discussion

3.1 Surface Morphology Analysis

A comprehensive assessment of the surface chemistry and morphology of the steel coupons in 1M HCl solutions, both with and without polyacrylic acid polymer , was conducted using a Field Emission Scanning Electron Microscope (FE-SEM). Surface Morphology of Specimen Surface morphology of the mild steel sample was studied before and after corrosion in presence and absence of the inhibitors. The surface morphology supported the formation of adsorbed polymer film over the steel samples were shown in the figures below.

In the figure (1:1) (b) it is clearly shown in that the steel samples are getting cracks due to the acid corrosion. However, the presence of polyacrylic acid as inhibitor retard the corrosion and surface showed more smoothness in figure 1 (c) which is less smooth than figure 1(a).

3:2 Potentiodynamic Polarization Analysis

Figure(1:2) illustrates the potentiodynamic polarization plots (Tafel plots), representing the corrosion behavior of steel coupons in 1M HCl solution at 298 K, both with and without polymer. The data derived from potentiodynamic polarization, as depicted in Tafel plots (Fig.!:2), have been meticulously organized in Table(1:1). It presents crucial electrochemical parameters such as corrosion current density (i_Corr), corrosion potential (E_corr), and the cathodic and anodic Tafel slopes (β_c and β_a) for comprehensive analysis.

The introduction of polymer into the 1M HCl solution results in heightened anodic and cathodic over-potentials, alongside a noteworthy reduction in i_Corr. Notable changes in the cathodic and anodic Tafel slopes, as outlined in Table(1:1) , indicate that the adsorption of polymer alters the mechanism of anodic dissolution and cathodic hydrogen evolution. This alteration uniformly retards both cathodic and anodic polarizations, subsequently enhancing corrosion inhibition with increasing inhibitor concentration.

An examination of the data in Table (1:1) reveals a maximum variation in E_corr values in the presence of varying concentrations of polymer within the 1M HCl solution a. This interesting observation suggests that polymer can be classified as a mixed-type inhibitor when utilized in 1M HCl solutions 29 .

3:3 Corrosion Inhibition Efficiency

In the context of this study, inhibition efficiency (%IE) signifies the corrosion current density when inhibitor is introduced. inhibition efficiency can be defined by the following equation(1).

where, and stand the values of corrosion current density in the presence and absence of

inhibitor, respectively. As illustrated in Table(1:1), the utilization of polymer proves highly effective in mitigating the corrosion rate of steel coupons, regardless of the immersion solution.

Notably, as the concentration of the polymer inhibitor is progressively increased, a corresponding enhancement in corrosion inhibition efficiency is observed.

This efficiency rises to 89.9 %, reaching its peak at a polymer concentration of 400 ppm at 298K. Beyond this point, a marginal increase is observed when the concentration is further elevated to 400 ppm. It is postulated that increase in corrosion inhibition effectiveness at higher polymer concentrations may be attributed to high coverage and potential chemical interactions occurring between the adsorption layer which forming a passive film and the surrounding solution30 .

CONCLUSIONS

The effectiveness of polymer as a corrosion inhibitor for CS protection in a 1M HCl solution was investigated using a comprehensive range of electrochemical and surface analysis techniques. Based on the analysis results, these conclusions can be summarized into .

Polyacrylic acid acts as an efficient corrosion inhibitor for protecting CS in the corrosive solution of a 1M HCl solution. It influences both anodic and cathodic reaction rates, classifying it as a mixed-type corrosion inhibitor.

The inhibitory efficiency exhibits a strong dependence on its concentration. Notably, it achieves an inhibitory efficiency exceeding 89.9% at a concentration of 400 ppm signifying its practical applicability. For optimal protection, applying this inhibitor at higher concentrations is advisable.

The molecular adsorption of polyacrylic acid molecules onto the CS surface, leading to the formation of a protective film, represents a key mechanism underlying its inhibitory capabilities. The resulting film exhibits hydrophobic properties.

Surface morphology analysis, consistent with electrochemical findings, confirms the presence of a well-dispersed protective film across the CS surface. Furthermore, a noticeable reduction in surface roughness is observed in the solution with the inhibitor compared to the solution without an inhibitor.

DECLARATIONS

1.Authors’ contributions