Inhibition of Pitting in Ferrous Materials by Carbonate as a Function of Temperature and Alkalinity

The ability of bicarbonate and carbonate salts to inhibit pitting in ferrous materials was studied in aerated alkaline solutions at 60°C. These conditions were used in order that the systems would be representative of typical aqueous metal cleaning systems. Even though ferrous materials generally have been regarded as exhibiting a low susceptibility to corrosion at pH values > 10, it was shown that for C4140 (UNS G41400), increasing the temperature to 60°C at pH 10 increased the overall corrosion rate (as measured by linear polarization) by about 20 times to 100 times compared with the rate at room temperature. Addition of potassium bicarbonate/potassium carbonate (KHCO₃/K₂CO₃) to the solution significantly reduced the corrosion rate. The influence of temperature at pH values of 11 to 13 on overall corrosion rate was very minor. To investigate the influence of alloy type, electrochemical experiments were performed at pH 11 at 60°C on gray cast iron type G-2 class 40 (UNS F12801), C4140 carbon steel, C1008 carbon steel (UNS G10080), and unalloyed iron. In cyclic polarization studies, pitting was observed in C4140 and in the cast iron solutions at pH 11 when only sodium hydroxide (NaOH) was present. Inclusion of KHCO₃ and K₂CO₃ inhibited the pitting. The threshold concentration to inhibit pitting in cast iron was 2 orders of magnitude higher than in the case of C4140. C1008 and iron showed no visible tendency to pit even when no carbonate was present. The tendency to exhibit localized corrosion was therefore more prevalent in materials with a higher amount of carbon, although scanning electron microscopy (SEM) indicated that features in the initial surfaces also may have contributed to relative corrosion susceptibilities. Analyses via x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) indicated the probable presence of ferrous carbonate (FeCO₃) in the form of amorphous particulates on sample surfaces. Additional crystalline substances were found that were probably potassium and NaOH salts.