Solving corrosion and oxidation of metals and ceramics problems
Corrosion and oxidation are responsible for hundreds of billions of dollars in losses to the US annually. Beyond the unsightly appearance of some rusted steel, the impact of corrosion on product performance, life, and safety can not be ignored. We have worked on identifying corrosion/oxidation mechanisms and protection strategies for years for the purpose of solving corrosion and oxidation of metals and ceramics problems. This includes developing new coatings for aluminum, magnesium, and carbon-carbon composites, as well as mitigating design strategies an models for ceramics, metals, and carbon. Whether it is corrosion of a stainless steel, aluminum, magnesium or even ceramics, there are both material, coating, and design solutions which can improve your product. A first step is to identify the source of the corrosion and its mechanism, e.g., galvanic corrosion, pitting corrosion, crevice corrosion, filiform corrosion, or stress corrosion cracking. Contact us for more information and to request consultation.
Pitting and crevice corrosion, a Time-lapse video illustrating a mechanism
Time-lapse video of a ferroxyl indicator test on carbon steel after: U. R. Evans, “The ferroxyl indicator in corrosion research, with special reference to the controversy regarding the cause of pitting,” Met. Ind., November 2, 1926, pp. 481–482, 507–508. The true elapsed time was 35′ 57″. Details of the experiment and its significance are described below.
Ulick R. Evans established the electrochemical theory of corrosion around 1925 and it holds to this day. Part of his discovery was based on local anodes (the part that corrodes) and cathodes (the source of the electrons.) Metal surfaces can have areas which are locally anodic to other areas. This can be caused by impurities, e.g. copper and iron in magnesium or smeared ferrous metals on stainless steel, or microstructural defects. Indeed, simply scratching a steel surface can expose more reactive metal vs. adjacent oxidized areas. The ferroxyl indicator, and aqueous solution of potassium ferricyanide, has several forms. With a nitric acid addition it is used as a very sensitive test for passivation of stainless steel. With phenolphthalein addition, it can be used to observe local anodic and cathodic action on a metal surface. Evans reported on this in 1926:
U. R. Evans, “The ferroxyl indicator in corrosion research, with special reference to the controversy regarding the cause of pitting,” Met. Ind., vol. November 2, pp. 481–482, 507–508, 1926.
The example shown above is a time lapse video of Evans’s experiment and post corrosion microscopic analysis of the resulting damage. This is referred to as an oxygen concentration cell, as the oxygen concentration gradient across the droplet provides the driving force for corrosion. This is a mechanism for both pitting corrosion and crevice corrosion. Experimental details are given at the end of this section. In the first few seconds of the video, light blue areas appear under the drop of test fluid, corresponding to the anodes (active sites) where iron is oxidizing to Fe2+. As time progresses, the center region takes on a blue tint while the periphery of the drop becomes pink. The potassium ferricyanide reacts with the Fe2+ ions, i.e., the initial corrosion product, to turn blue (the Prussian Blue reaction). The phenolphthalein is an acid/base indicator, clear in essentially neutral and acidic conditions and turning pink in basic conditions. Thus, basic conditions have developed at the periphery of the droplet and neutral or acidic conditions at the center. This is an expression of the two half cell reactions, with Fe —> Fe2+(aq) + 2e- occurring at the active sites and 2H20 + O2 —> 4e- + 4OH-(aq) at the cathode. Oxidation of the iron requires oxygen. At the start of the experiment, oxygen is homogeneously distributed in the water droplet. However, oxygen concentration dropps near the local anodes as the oxidation reaction is faster than the rate of diffusion of oxygen from the drop free surface. So, the OH- concentration is higher at the periphery of the drop leading to basic conditions and the pink color. The Fe2+ ions formed at the cathode hydrolyze forming the weak base iron hydroxide Fe(OH)2 and an excess of H+. The excess of H+ causes local acidic conditions at the anodic sites. Now, the reaction is occurring in sodium chloride solution, so sodium hydroxide is produced at the cathode and ferrous chloride is produced at the anode. These two chemicals react to form ferrous hydroxide. This gives rise to a clear ring around the white precipitate of ferrous hydroxide which exceeds the solubility limit. This shows up around 18 sec into the video. As time progresses to the end, the ferrous hydroxide further oxidizes to form rust, Fe2O3.
Turning now to the post test micrographs, extensive pitting corrosion was observed, with the pits corresponding to the local anodic sites. No pitting was observed at the edge of the droplet, which would be expected given that iron dissolves readily in acids but not bases. This is why rust inhibiting solutions used on iron and steel are commonly basic.
Stock solutions of potassium ferricyanide at 1 % mass in water and phenolphthalein at 1 % mass in ethyl alcohol were freshly prepared. The potassium ferricyanide is best prepared the day of use. 1 ml of potassium ferricyanide solution and 0.25 ml of phenolphthalein solution were added to 50 ml of standard saline solution. The carbon steel surface was activated by abrading with medium course emery paper and then by green Scotch Brite. A drop of the dilute test solution was applied to the active metal surface and the camera was started. Note that 2X the volume of potassium ferricyanide was employed vs. Evans’s starting recommendation to better define the blue colour for the photographs. The 30″ time lapse video was crated over 35’ 75”.