6 Assignment of ratings6.1 Assignment of protection rating (R p )The numerical rating system is based on the area of the substrate that exhibits corrosion, by the following formula:R Ap= − 3 2 ( log ) (1 )where A is the percentage of the total area which shows base metal corrosion. R p is rounded off to the nearestwhole number, leading to the tabulation given in Table 1 .NOTE 1 In some cases, calculating the exact area may be difficult, especially on deeply worked specimens such as thread,bores, etc. In these cases, it may be necessary for the inspector to estimate the area as closely as possible.Strict application of equation (1 ) would lead to ratings greater than 1 0 for specimens with extremely small defectiveareas. Therefore, equation (1 ) is only defined for areas where A . 0,046 41 6 %. A rating of 1 0, accordingly, isarbitrarily assigned to a surface showing no corrosion of the base metal.