The -OH in the lye attacks the oxygen in the anodized layer. It does combine somewhat with the bare aluminium underneath, but at a much, much slower pace. You can control the reaction amount by timing a 5-minute dip interval, then rinse and inspect. Another method is to use Easy-OFF oven-cleaner. Spray a thin layer on so that it even coats the part, let sit for 5-minutes, and rinse to inspect. The sprayed-on thin layer ensures that there's only so much reactant and when it's converted to water, the reaction stops and doesn't proceed any further.