Frying Pan Pitting Corrosion and How to Prevent It

To RiverLight Corporation

Our company’s view regarding the aforementioned case about which you were contacted is as follows.

(1) The mechanism that causes pitting corrosion

Pitting corrosion occurs due to localized damage to the protective coating over the surface of the metal.
…In the areas where pitting occurs, the concentration of positive iron (II) ions increases. (Because the diffusion rate of positive iron (II) ions is slow.) And negative ions (negative chlorine ions, etc.) within liquids migrate in order to preserve electrical neutrality within pitted areas. Also, negative chlorine ions increase in concentration within pitted areas. Because negative chlorine ions promote active dissolution of iron and make it difficult to repair coating, the speed of pitting corrosion increases even further. As such, pitting corrosion could be considered an autocatalytic reaction. Pitting corrosion usually begins in locations where the protective coating is missing, and in terms of environment, the following could be considered prerequisites.
・In a neutral to alkaline environment, the existence of halogen ions (sodium chloride, etc.) that destroy the protective coating
・A strongly alkaline environment (pH value of 13 or higher)
Therefore, the ease by which pitting corrosion can occur and progress is determined by the nature of the metal’s surface and its environment, and it is not attributable to the composition of the metal.

(2) The causes of pitting corrosion on frying pans and preventative measures

In the case of the frying pans manufactured by your company, if salt is used in cooking when a nitride-treated coating or other protective coating has been generated over the surface of the frying pan, it creates the exact environment described above in which there is a high risk of pitting corrosion occurring.
・Handling that damages the frying pan while cooking (such as violently striking or scraping the frying pan with a cooking utensil).
・Leaving food that has been cooked with salt in the frying pan for long periods of time after it has finished cooking.

In order to prevent pitting corrosion, we recommend caring for frying pans in the following manner.

1. Wash the frying pan well using water after cooking.
2. Dry the pan well after washing it.
3. Apply cooking oil.