Impact of the in-situ phosphatization on the corrosion resistance of steel coated with fluorinated waterborne binders assessed by SKP and EIS

Highlights

• Fluorinated acrylic phosphatized latex synthesized by seeded emulsion polymerization

• Coatings provide weak barrier protection due to non-coalesced polymer particles

• Phosphatization of the metal interface seems to govern the corrosion protection

• Synergy of SKP and EIS is crucial for understanding corrosion at the metal/coating interface

Abstract

A thin stand-alone waterborne coating that combines hydrophobicity and a built-in ability to in-situ phosphatize low carbon steel substrates, avoiding flash rust in the interface is presented. Despite the hydrophobicity of the coating is enhanced by the presence of a fluorinated comonomer, the barrier protection is substantially decreased as compared with a pure acrylate coating. The bad performance is independent of the thickness and it is mostly attributed to the lack of coalescence of the copolymer particles containing fluorinated comonomer (detected by Scanning Electron Microscopy (SEM)). The best way to hinder this harmful effect is to trigger the formation of the in-situ phosphatization layer on the metallic surface; namely slowly drying the coating at 60 % of relative humidity (R.H.). The combination of Electrochemical Impedance Spectroscopy (EIS) and Scanning Kelvin Probe (SKP) has shown the beneficial effect of having a protected interface when it is exposed either to bulk electrolyte or to a droplet of 3.5 wt.% NaCl. A methodology using both techniques to evaluate coatings with an artificial defect has proved that the phosphatization is governing the corrosion protection for these coatings.

Introduction

Most of the anticorrosion coatings are based on solventborne systems, which are being slowly shifted towards waterborne coatings due to the stringent environmental regulations (VOC’s restriction) [1]. In order to prevent inherent problems of waterborne systems such as higher hydrophilicity of the films (e.g. due to the presence of salts or surfactants that remain in the final film), a thin standalone protective coating based on an acrylic waterborne latex stabilized by a polymerizable phosphate surfactant has been recently reported [2]. Moreover, it was shown that during the film application, the phosphate groups on the surface of the polymeric particles were able to react with the surface of the metal throughout the formation of an iron phosphate layer at the coating-substrate interface (Fig. 1) and hence, to provide great corrosion resistance even in harsh conditions [2].

The corrosion resistance of phosphate containing latexes was further improved by modifying the barrier properties of the coating. The introduction of crystalline nanodomains in the polymer particles decreased the permeability of the coating to agents that initiate the corrosion (e.g. chloride anions, dissolved oxygen in water, etc.) [3]. In this work, a different approach was used to increase the hydrophobicity of waterborne coatings containing phosphate moieties without introducing any nanocrystalline domains. This was achieved by including a fluorinated monomer, perfluorooctyl acrylate (POA), in the formulation.

Although the corrosion protection of the phosphate containing coatings was studied [2,3], the contribution of the phosphatized interface to the overall corrosion resistance was not discerned from the barrier protection. Therefore, a first insight about the impact of the in-situ phosphatization without the barrier protection provided by organic matrix is needed. One of the most popular techniques to address this issue is Electrochemical Impedance Spectroscopy (EIS). It is extensively used to study the metal/coating interface and the overall performance of organic coatings [[4], [5], [6], [7], [8], [9]]. The role of inhibitors in coating formulations and their impact in the metal/coating interface (passive or active) can be explored easily if the barrier capabilities of the coating are not optimized [[10], [11], [12]]. However, if the organic matrix shows excellent barrier protection, any passivation or protection effect of the metal surface (e.g. in-situ phosphatization [2,3]) will be hindered or overlapped with the barrier protection.

A complementary technique like Scanning Kelvin Probe (SKP), where a bulk electrolyte is not needed, can be useful for studying corrosion problems under thin and ultra-thin electrolyte layers [13,14] and at the underlying metal/coating interface [[15], [16], [17]]. A singular experiment with the SKP is the presence of a drop on the surface rather than using an ultra-thin electrolyte. It can allow to identify cathodic and anodic areas over a sample and speed up the corrosion process [[18], [19], [20], [21], [22]].

Therefore, SKP and EIS can be successfully applied to study the degradation of metal/ coating interface [23,24]. Hence, their combination seems to be a promising approach for studying the in-situ phosphatization effect, and particularly the SKP droplet test using artificial defects at localized scale aiming to reveal features of the metal/coating interface.

The goal of this work is to develop a hydrophobic waterborne binder to minimize water permeation and hence to enhance the corrosion protection by introducing a fluorinated acrylate comonomer in the coating composition. The properties of the film cast from the synthesized waterborne binder will be studied, focusing the discussion towards the impact in terms of corrosion protection at the metal/coating interface. Electrochemical impedance spectroscopy (EIS) and Scanning Kelvin Probe (SKP) have been used as non-destructive testing to explore the overall protection of the coating as well as the passivation of the interface: the evolution of coatings with time has been monitored on samples without and with the presence of an artificial defect able to reach the interface.