A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H₂SO₄ solution

Abstract

To investigate the role of polyaniline (PANI) in the corrosion protection of stainless steel (SS) in oxygen-deficient acidic solution, a separate doped PANI film electrode on a glass substrate was prepared and the test solution (1 M H₂SO₄) was purged with high-purity N₂ until dissolved oxygen level decreased more than two orders of magnitude. In this deaerated 1 M H₂SO₄ solution, the galvanic coupling interaction between the separate PANI film electrode and 410 SS was studied. Results reveal that the separate PANI film can passivate the 410 SS steadily for a long period of time. A variety of experimental methods including potentiodynamic measurement, potentiostatic (current–time) examination and X-ray photoelectron spectroscopy (XPS) are used to explore the mechanism by which the separate PANI film passivated the galvanic coupling SS in the deaerated sulfuric solution. These studies show that passivation is achieved because PANI film provides a large critical current at the early stage of coupling and a persistent passive current by its electrochemical dedoping/re-doping equilibrium activity with the acidic environment at the subsequent stage of coupling.

Introduction

Polyaniline (PANI) is one of the most intensively investigated of the intrinsically conducing polymers (ICPs) [1], [2], [3]. Since DeBerry [4] first found the use of electrodeposited polyaniline (PANI) films for corrosion inhibition on 410 steel, there have been numerous studies on the corrosion inhibition properties of PANI [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] and other intrinsically conducing polymers [22], [23], [24], [25]. ICPs are usually coated on metal surface as electrodeposited coating [7], [8], [9], [10], [23], [24], [25], solution-cast polymer coating [11], [12], [13], [14], [15] or additives of resins coating [16], [17], [18], [19], [20], [21], [22]. Owing to the wide variations in experimental procedures used (coating type, substrate preparation corrosion environments, test method), different mechanisms have been proposed, such as corrosion inhibitor [5], [6], barrier [7], [16], anodic protection [8], [9], [10], [11], [12], [13], [17], [18], [19], [20], inhibition of iron diffusion rates [14], [21] and shift of electrochemical interface [9], [16], among which the anodic protection is the most prevailing one. The anodic protection is believed to occur by shifting the corrosion potential to the passive region for the metal of interest in the electrolyte of interest. The proposed reaction between the oxidized form of the polymer (ICP^m+) and the metal (M) is:

$$\frac{1}{n}\text{M}+\frac{1}{m}\text{IC}{\text{P}}^{m+}+\frac{y}{n}{\text{H}}_2\text{O}\to \frac{1}{n}\text{M}{(\text{OH})}_y^{(n-y)}+\frac{1}{m}\text{IC}{\text{P}}^0+\frac{y}{m}{\text{H}}^{+}$$

The ICP can be re-oxidized by atmospheric or dissolved oxygen:

$$\frac{m}{4}{\text{O}}_2+\frac{m}{2}{\text{H}}_2\text{O}+\text{IC}{\text{P}}^0\to \text{IC}{\text{P}}^{m+}+m\text{O}{\text{H}}^{-}$$

Thus, oxygen is considered as the mediation and driving force of the anodic protection. However, Martin and Gašparac [26] reported that the deposited PANI film could passivate the underlying stainless steel surface in N₂-purged sulfuric acid solution. Moreover, in our earlier work [27], we also discovered that a separate PANI film could passivate the galvanic coupling stainless steel in N₂-deaerated Na₂SO₄ (pH 1) solution. These intriguing findings promote us to investigate whether PANI film can passivate stainless steel in a more highly corrosive N₂-deaerated solution and explore the mechanism by which PANI provides anticorrosion properties in a deaerated acid solution. Furthermore, in view of the field of application of PANI (anodic protection is usually applied in strongly corrosive solution, which is often deaerated or oxygen-deficient, and the interface between metal and the polymer coating is also oxygen-deficient), it also promotes us to explore the role of PANI in corrosion inhibition in deaerated acid solution.

In the present study, 1 M H₂SO₄ and 410 SS were used due to a marked difference among the passive current of SS (about 10⁻⁵ A/cm²), the reduction current of dissolved oxygen in deaerated solution (<10⁻⁶ A/cm²), and the reduction current offered by PANI. Once the SS is passivated by the PANI when they are coupled together, it is easy to distinguish the contribution between the PANI and dissolved oxygen left in the solution. As a result, a pure polymer protection mechanism without the contribution of oxygen can be present.

Furthermore, in this paper, an experiment of galvanic coupling between a separate PANI film electrode and 410 SS was conducted in the N₂-deaerated 1 M H₂SO₄ solution. The advantage of preparing a separate PANI film electrode on a glass substrate is that it provides a method to study how PANI works independently. The advantage of coupling PANI film with stainless steel is that it presents a pure polymer protection mechanism except the barrier effect.