Edinburgh Napier University

  New-generation protective coatings should respond to mechanical or chemical damage with active corrosion inhibition, preserving coating integrity and adhesion to the substrate. Due to the implementation of legislation imposed by REACH (Restriction, Evaluation, Authorisation & restriction of Chemicals), restrictions have been placed on the use of chromium (VI) compounds, which were the main components of self-healing anti-corrosion coatings. Having similar properties, e.g. wear and corrosion resistance, nickel-phosphorous coatings might be one possible replacement for coatings containing Cr6+ .

The novel work carried out during this research programme has focused on developing a process to include corrosion inhibitors within nickel-phosphorus coatings using the electroless co-deposition technique. Gelatine and sodium alginate microgels were investigated for their feasibility at this task. It was found that gelatine microgels of average diameter 2–6 μm, heat-treated at 150 °C for 15 hours exhibited the desired properties to survive the co-deposition process at 89 °C in acidic electroless nickel solutions operating at 4.9 pH.

Depositions on ISO 3574 Type CR1 steel were performed to determine the impact on the coating properties due to the inclusion of gelatine microgels. The microgels were produced without the inclusion of active corrosion inhibitors at this time. Various deposition parameters were investigated, and it was determined that the addition of small quantities, below 0.05 g/L, provided the best depositions. Above 0.1 g/L, the microgels would block the reaction sites required for the electroless deposition process which led to poor quality coatings and a reduction in deposition rate from 20 μm to 1.5 μm/hr.

The resulting novel coatings were investigated using a series of empirical and quantitative tests. The results of this study show that the coatings with gelatine microgels show increased phosphorus content, and a reduction in the surface hardness and abrasion resistance of the coatings. The corrosion studies show that the inclusion of gelatine microgels impacts the corrosion resistance of the coatings even when no additional corrosion inhibitors are included.