The active ingredient in passivating treatments is citric acid, and this is all you need for parts that can be disassembled and soaked. However, if you want it to remain active long enough to work properly on railings and installed fittings, you’ll need a thickener that does not react with nor is affected by acid. Here it is:

Gelled Citric Acid Passivating Solution

• 10-20 percent citric acid
• 1-2 percent by weight fumed silica

Dissolve the citric acid in warm water. Add the fumed silica (also called Cabosil M-5) and mix. A simple rule of thumb is 1/2 cup of fluffed fumed silica plus one cup of the citric acid solution. This will make 1 cup of gelled passivating mix.  For a thicker solution add more silica. For a sprayable solution that will cling and not bead up, add less.

Citric acid is available in grocery stores or online. Fumed silica is available through chandleries, fiberglass supply houses, and online. We bought a 15-pound bag 10 years ago, and we’ve been working our way through it ever since, thickening epoxy and cleaners.