Polymer-derived ceramic coatings for corrosion protection

Abstract

Corrosion is common in industrial sectors where structures and machinery are made of metals and alloys. SS304 is a widely used stainless steel grade in the world and is generally used in applications that require strength and good corrosion resistance such as, in subsea pipelines, desalination plants, and in steel canisters, etc. SS304 mainly suffers from localized corrosion (pitting and crevice corrosion) in chloride-containing environments. The corrosion resistance of SS304 is facilitated by the presence of a thin passive film of chromium oxide on its surface. It is a well-known fact that pitting occurs when localized breakdown of the passive film occurs and it further leads to failure of components. All these causes massive economic loss in the form of replacing the structure or repairing the damaged section which leads to a decrease in production efficiency or plant shutdown for a few days. The main objective of this thesis is enhancement of corrosion resistance of SS304 by depositing ceramic layers using polymer-derived route. Polymer-derived ceramics (PDCs) have gained good attention in the last two decades as they possess high thermochemical stability. In the present work, two types of ceramics (SiOC and SiCN) from polymer pyrolysis route were synthesized and coated on SS304. Corrosion protection performances of coated substrates were evaluated in 0.6 M NaCl solution. Firstly, ceramic coating parameters were optimized for the deposition of SiOC and SiCN ceramic layers on SS304 using different precursor concentrations and by optimizing multi-fold coated layers followed by examining surface morphology. Subsequently, an amorphous SiOC layer with a thickness of about ~1 μm was deposited after two-fold coatings on SS304 by dip-coating with polysiloxane solution followed by pyrolysis at 800 °C under argon environment. Corrosion resistance of SiOC-coated SS304 was performed using open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. Electrochemical results confirmed improved corrosion protection behavior of SiOC-coated SS304 over uncoated SS304. Afterwards, SiCN was synthesized by pyrolysis of a commercial polysilazane at 800 °C and a crack-free SiCN layer of ~3.2 μm on SS304 was coated using a multi-step dip coating/pyrolysis process. Electrochemical characterization revealed that SiCN-coated SS304 had a significantly lower corrosion current density (and thus a lower corrosion rate) compared to that of uncoated SS304 samples. Enhanced corrosion protection behavior of SiCN-coated SS304 was also confirmed by EIS measurements. These findings signify the potential of SiOC and SiCN ceramic coatings for the protection of stainless steel in a seawater environment.