Corrosion

Electrochemical Impedance Spectroscopy

EIS spectra for several polysaccharide films. The wider the semicircle is, the higher the inhibitive resistance of the film. The bare uncoated metal is represented by the asterisk symbol, Z-104 coating by Δ, Z-103 by ◊, Z-102 by ○ and Z-101 by □.

Experimental Nyquist plots for the best four performing polysaccharides films give Rp and RΩ values extracted based on extrapolated intercepts. The higher the radius of the semicircle, the higher the Rp value and the better protection the film provides. RΩ was found to be approximately 3.6Ω in all cases because experiments were conducted in the same NaCl solution. Conclusions regarding the efficacy of the films are limited because chemical structures for these polysaccharides are not yet available. An NMR investigation is under way. Better corrosion inhibition could be due to fewer defects in the films, a thicker film or higher hydrophobic character or even a conducting film (ennobling the metal).

SEM images

SEM photographs of bare and SPEEK-PNO2EPB coated steel wires after 24 hours. chronoamperometry in 0.7M NaCl water solution are shown. You can see how the coated wire is barely corroded when compared to the bare.

Two of the cheapest polymers on the market, ultrazine (-) and PVA (+) were also tried for anticorrosive properties. The graphs show a strong anticorrosive character, which along with the low cost and UV protection capabilities of ultrazine might prove to be a strong driving force towards commercialization. Coating of larger areas is possible with spray application of the coating solution. Scratch resistance of these polymers can be upgraded by incorporation of hard nanoparticles similar to zirconia.

Tafel plot comparison

Tafel plots for the best four inhibiting polysaccharides. They were obtained by scanning ±250 mV around the open circuit potential. The cell was left to reach steady state for 24 hours before the experiments were carried out. Scan rate was 0.166 mV/s, the EW was calculated to be 27.925g and the density of the corroding species was 7.8 g/ml. Working electrode area was 14.0 cm2. Films sprayed were ~50 nm on surrogate Si wafers. Better corrosion inhibition is denoted here by a shift in the inflection point (Ecorr) towards less negative potentials (to the right). Diamond represents the bare metal substrate, triangle is a Z-104 film, square is Z-103, circle is Z-102 and x is Z-101.

Polarization curves can help determine why a certain corrosion inhibitor functions. It can modify the anodic process, the cathodic process or both with an overall decreased rate of corrosion. Figure shows that both the anodic and cathodic reactions are reduced by the polysaccharide film inhibitor. The slopes of the linear portions of both corrosion regimes are steeper and at more elevated currents for the unprotected bare than for its coated counterparts. Also, the inhibitor film tends to shift the open corrosion potential towards less negative values with reference to the blank. These factors suggest that the polymer films studied act as anodic type corrosion inhibitor.

Linear Scan Voltammetry

0.7M NaCl. Water and organic solvent soluble PEMUs tested before and after 24 hrs of CA (polished vs. passivated).

The corrosion protective behaviour of our PEMU films could be partly attributed to their close affinity (charge bonds) to the steel substrate. Traditional paint coats cover similar substrates much more loosely, and even if thicker, thy create air pockets in between the substrate and themselves. These non-filled pockets are a pitting haven and allow for a quick start in the corrosion process. This kind of ‘inside out’ deficiency in the protective qualities of traditional paints is well combated by our polyelectrolyte films. They adhere closely to the substrate and allow for little permeability to water and other ions. When water is present its chemical properties are changed by its bound nature to the polyelectrolyte ion pairs and the difficulty to congregate in ‘pools’. Dust particles and other defects encountered during film formation were patched over by the depositing film.

Comments