The Atmospheric Corrosion of Trivalent Cr Metal / Cr Oxide Coatings for Packaging Steels
Authors: Bluett Ellen , Wint Natalie , McMurray Hamilton Neil , de Vooys Arnoud 
Published: July 2022

The superior mechanical properties associated with steel mean that it is heavily utilized in many industries including automotive, and construction (1). Although steel offers desirable mechanical properties such as toughness and tensile strength, it is known to corrode, this having cost implications and raising safety concerns, for instance within the food packaging industry. Thus, steel is often used in conjunction with a coating as a preventative measure for corrosion.

Within the food packaging industry, steel is often coated with a thin (~20 nm thick) chromium metal-chromium oxide coating, the product being referred to as electro chromium coated steel (ECCS), which is produced during a continuous electrodeposition process whereby a low carbon steel strip passes through baths of chromic acid electrolyte containing hexavalent chromium, Cr(VI). The final coating consists of two distinct layers; a 20-110 mg.m-2 chromium underlayer and a 2-20 mg.m-2 native (hydr)oxide outer layer (2). REACH, a regulation associated with the restriction of chemicals that are harmful to human health and the environment, have made a legislation that has imposed a restriction on the use of Cr(VI) due to its carcinogenic nature (3). This impact has resulted in the drive of the steel packaging industry to find a suitable, non-toxic replacement for ECCS.

Coatings made with Trivalent Chromium Coatings Technology (TCCT®) are currently being developed as a potential replacement for ECCS. TCCT® production has developed to consist of a two-electrolyte process involving an aqueous solution of Cr(III) sulphate, sodium sulphate, sodium formate and sulphuric acid leading to the formation/application of a bilayer coating. Various issues including coating inhomogeneity have previously been reported (4).

Due to the low coating weights involved, packaging products such as ECCS and TCCT® are also used in conjunction with an organic coating. Thus, they could be susceptible to various modes of organic coating failure including corrosion-driven cathodic delamination, whereby a coating becomes physically separated from the underlying substrate, a key failure process that affects organically coated metal products in high humidity, such as those experienced during storage in the packaging industry. Cathodic delamination occurs when an electrolyte comes into contact with the metallic substrate at a flaw in the organic coating at which anodic metal dissolution occurs. This is coupled to the cathodic oxygen reduction reaction which takes place at the delamination front. The hydroxide ions produced during the cathodic reduction of oxygen, or a reaction intermediate, disturb the bonds between the substrate and coating leading to adhesion failure and delamination of the coating (5).

To ensure that new products such as TCCT® are successfully incorporated into the market it is important that they are comparable in performance to existing products. For example, TCCT® must provide sufficient delamination resistance. The scanning kelvin probe (SKP) is a technique which measures corrosion potential under a thin electrolyte film and organic coatings, which can be used to create a surface potential map (6).

This study makes use of the SKP to investigate the rate of cathodic delamination as a function of TCCT® Cr(III) oxide (varying from 1.8 mg.m-2 to 23.35 mg.m-2). The results will help to determine the minimum satisfactory chromium oxide coating weight needed for commercial TCCT®.

It was found that higher Cr(III) oxide coating weights had a positive effect on inhibiting cathodic delamination, confirming the mechanism. The results were explained using complementary electrochemical data including OCP measurements as well as potentiodynamic polarisation which showed that higher TCCT® Cr(III) oxide coating weights were acting to supress the cathodic reaction.

The results also highlighted that a TCCT® Cr(III) oxide coating weight of around 7 mg.m-2 is sufficient with regards to resisting cathodic delamination completely. This work has determined that the Cr(III) oxide coating weight value is very important with regards to improving coating coverage and resisting cathodic delamination.

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