If the hydrogen ion concentration (low ph) is increased, the rate of the corrosion would increase since there would be more H+ ions to receive electrons at cathode. The metal ion combines with the OH- ions to form atoms of ferrous hydroxide which dissolves in the water thus wasting the metal away. Therefore, electrochemical corrosion cells with cathodic and anodic areas will have a current flow through the electrolyte from anode to cathode and back through the metal from cathode to anode. During this process, material from anode is transferred to the electrolyte resulting in corrosion of the anode.
Forms of Electrochemical Corrosion
1. General Wasting Type Corrosion
General wasting type corrosion is a term expressing electrolyte of a more uniform nature rather than selective attack by pitting. It implies reduction in metal thickness over comparatively large areas in a fairly uniform manner. Here, the anodic surface constantly changes the position; hence attack occurs over a wide area. If dissolved oxygen is present, the hydrogen polarizing layer is destroyed by formation of water and even in the absence of dissolved oxygen; this form of corrosion can take place when water has pH values below 6.5.
Apart from the general wastage type of electrochemical corrosion, another form of corrosion that from pits on the metal surface can be termed under corrosion due to differential aeration, oxygen absorption or simply pitting type corrosion. There are types of pitting corrosion:
Air Bubble Pitting
Found in the roof of steam drum in the boiler. In the air bubble typing pitting, an electrolytic action is initiated between the oxygen reach surface under the bubble and the surrounding water areas that are less rich in oxygen. By experiment it is found that if a portion of a metal becomes partially inaccessible to oxygen, it becomes anodic and so, differential oxygen levels on a surface can give rise to active corrosion cell.
The ferric hydroxide as the corrosion product settles over the bubble, forming a semi-permeable membrane that permits free passage of ions but not oxygen. When oxygen gets exhausted, a reversal of galvanic currents occur, this causing the metal under the “cap” less noble and hence highly localized corrosion proceeds.
A hard cap of corrosion product occurs in hotter areas of generating surface; mostly found on the side of the fire row tubes. The hard, black scab is difficult to detect, remove and arrest once initiated.