Self-healing polymer coating with efficient delivery for alginates and calcium nitrite to provide corrosion protection for carbon steel

Highlights

• Self-healing polymer coatings with alginates and calcium nitrite were developed.

• Cellulose nanofibers added to the coating acted as a pathway for the transport and release of corrosion inhibitors.

• The positions of alginates and calcium nitrite layers in coatings were key point.

• The coating with 5% alginate, 8% calcium nitride and 1% cellulose nanofiber showed the highest resistance.

• A two-step formation of thin and thick healing films was effective in improving the self-healing.

Abstract

This study described the development of a polymer coating that binds alginates and multivalent cations. This coating utilized cellulose nanofibers (CNF) of 1% to create pathways for the corrosion inhibitors sodium alginate (Alg) and calcium nitrite (CN). These pathways allowed the release of Alg and CN to damaged areas for corrosion protection. Electrochemical impedance of scratches to the coatings was monitored in corrosive solutions to evaluate the self-healing properties of the polymer coatings. The positions of the Alg and CN layers in the coatings were instrumental in the effectiveness of corrosion protection to the damaged areas. As a result, the CN layer was the first to spread to the damaged area, and the Alg layer followed. The combination formed a coating of 8% CN layer and 5% Alg layer with CNF that delivered excellent self-healing properties. The corrosion inhibitors first formed a thin film via nitride ions, which was followed by a well-adhered thick film composed of alginates and calcium ions.

Introduction

Carbon steels are used for many applications that include shipbuilding, vehicle bodies, pipelines, storage tanks, wires, springs, and domestic appliances. The mechanical properties of carbon steels depend mainly on the concentration of carbon, but they are not resistant to corrosive elements in the environment such as chloride salt and dilute acid. Corrosion-protective coatings must be applied to the surface of carbon steels to prevent metal degradation and to maintain durability. These coatings provide barriers to avoid direct contact with corrosive environments, and they also suppress the anodic and cathodic reactions to damage from chemical attack. An ordinary coating such as plain epoxy could not provide corresponding protection. When damage occurs, self-healing properties are required from a coating layer if it is to provide anticorrosive effects [1]. Adding active chemical agents to a coating generates a protective film on the damaged surface of a substrate.

Various corrosion-inhibiting agents have been reported in research into coatings that could self-heal the surface of a substrate when damaged or scratched. There are two types of corrosion inhibitors as healing agents: organic and inorganic. The most common types of organic corrosion inhibitors are long-chain, polyelectrolyte complexes, hydrogen-nitrogen compounds such as various amines and ammonia derivatives, sodium benzoate mercaptans, sodium molybdate, and esters [2]. Meanwhile, chromates, arsenic, calcium carbonates, nitrites, molybdates, phosphates, zinc, and polyphosphate are the most common types of inorganic corrosion inhibitors [2]. Some inorganic corrosion inhibitors such as chromate-based chemical conversion have been banned due to the issues that include toxicity and environmental danger. Some researchers have reported the use of so-called green and eco-friendly inhibitors that provide alternatives to those negative issues, and these are derived either from plants or living creature in the form of amino acid [3], [4], [5]. The plant-based corrosion inhibitors are referred to as phytochemicals, and are extracted from various parts of plants such as seeds, stems, roots, oils, leaves, flowers, and fruits. The phytochemicals in plants are classified as either primary (chlorophyll, proteins, carbohydrate) or secondary (alkaloids, flavonoids, phenolic). The mechanisms for green corrosion inhibitors include chemisorption and physisorption, and a combination of the two [3]. Corresponding corrosion inhibitors have been extracted from Curcuma longa, Spilanthes acmella [6], neem leaf [7], Eucalyptus leaf [8], and Citrus sinensis [9]. Smart corrosion protective coatings have demonstrated the ability to respond to environmental changes and heal defects in a coating in order to prevent further corrosion [10]. Various anticorrosive agents have been tested as corrosion inhibitors, and this has included techniques for packaging and release mechanisms [11].

Alginate is a biomaterial that forms a hydrogel to promote wound healing, drug delivery, and tissue engineering, and alginate is applied particularly in corrosion-protective coatings [3], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Alginates are broadly applied due to factors such as low cost, low toxicity, and biocompatibility. Microcapsules fabricated from sodium alginate have been applied as containers for various resins and corrosion inhibitors where an alginate wall is helpful in controlling the rate of polycondensation, reducing toxicity, and providing good thermal stability [11], [13], [17], [18], [23]. The use of alginate as a corrosion protective film solved the problem of adhesion on a metal surface, and using silane as a coupling agent enhanced the adhesion between organic and non-organic substances [24], [25], [26]. Also, when the natural organic silk fibroin was combined with sodium alginate, adhesion on the metal surface was as much as three times more powerful [27]. The combination of sodium alginate and sodium phosphate acted as a synergetic corrosion inhibitor, which improved the corrosion inhibition efficiency by as much as 98.8% [28]. Combining two natural polysaccharides, sodium alginate and chondroitin sulfate, enhanced the corrosion inhibition efficiency by 95.18% [29]. Also, the addition of sodium alginate conferred the excellent properties of binding and gelling when combined with corrosion inhibitors derived from multivalent cations. Calcium nitrite is composed of divalent cations and oxidation agents, and is widely used as a corrosion inhibitor. Another problem was achieving an effective release of alginate from polymer coatings, because of the large size of alginate molecules. Self-healing coating process for corrosion protection is functionalities based on encapsulation of corrosion inhibitors [30]. The corrosion inhibitors were included into coating system by either direct mixing or load into micro/nanocontainer and then mixed with polymer coating. The advantage of using micro/nanocontainer are hinder the osmotic blistering effect, inhibitor deactivation, and uncontrollable leaching in the polymer coating [31], [32], [33], [34], [35]. Commonly, the release of inhibitor from its container as damage occurred was triggered by pH changing [36], [37], [38], in which the corrosion in the scratched surface always followed by pH changing of a local damage. In our previous study, however, we found that corrosion inhibitor loaded onto cellulose nanofibers was effectively released to defective areas with desorption and adsorption mechanism as pH change [39], [40], [41].

In the present study, polymer coatings of corrosion inhibitors sodium alginate and calcium nitrite loaded onto cellulose nanofibers were coated onto carbon steels. Then, the electrochemical impedance of scratches into the various polymer coatings was monitored in corrosive solutions to evaluate the self-healing properties of the polymer coatings. The change in corrosion potential confirmed the release behavior of each corrosion inhibitor. The process of self-healing was discussed based on observation of the films formed in the scratched portions and on the electrochemical measurements of each specimen.