corrosion rate of steel in clay soil 1

Corrosion underground is very complicated and unpredictable process. It depends not only on the chemistry of the soil and water, but also on the bio-processes that may be occurring. In clay soils the bio-corrosion is slowed down very much unless close to the surface. The corrosion will depends also on the thickness of the SS plates, the recharge rate of the area, any surface waters around (like rivers, playas, arroyos, etc.). The part below the water table for sure will corrode faster than the part above the water table. I know of SS 304 groundwater screen totally immerced in water that are all right for the first 15 years of their life in clay soil (usually pH>7, and high sulfate and other anions content)
In short, equation does not exist. However, experienced hydrologists can give you an opinion that's worth more than equation.