How can I test for rust (iron oxide) at home?

The galvanized steel is galvanized for a reason: to keep it from rusting. If the zinc is already gone from the galvanized steel that there has been some serious chemistry going on in your wall. Either that, or the steel wasn't really galvanized, or the red stuff is not really rust.

You can get a test kit like this one: http://www.marinedepot.com/Seachem_Multitest_Iron_...

Get some de-ionized water (or distilled water) from the grocery story and test it for iron using the kit. This is called a "blank". Then place some of the "red stuff" in the de-ionized water and stir for a while, and then let it sit for up to 30 minutes so that the top of the water is clear of solids, and then draw a sample from the top and test it for iron.

Rust is not very soluble in water, but all you need is a trace of iron to show up in this test. If the test of the "red stuff" in water indicates more iron than the blank, then you've got some rust.

============ Follow up ==============

With all due respect to John and Doc you're not really going to have some hydrochloric acid or sodium thiocyanate lying about, or even likely to buy on the internet. If you haven't done this stuff since high school chemistry class, you're better off with a commercial test kit.

If you have a sample and access to two chemicals, you can qualitatively determine if the sample contains iron, which pretty much determines that what you have is rust. First you need to solubilize the presumptive rust, by dissolving it in an acid such as HCl (hydrochloric acid). It should be soluble even in dilute acid. After you've filtered off any insoluble material (you can use a coffee filter), you'd add a few drops of a solution of sodium thiocyanate (any thiocyanate that is water soluble will work) to the filtrate (the material you filtered). Just use a few drops of that as well. If rust was present, your pale yellow filtrate, which contains ferric chloride, will react with the colorless thiocyanate solution and produce an intense red color due to ferric thiocyanate.

One caveat: do this out of doors and don't breathe any vapors that might be produced in the reaction. There is a possibility of cyanide formation (as HCN). However if you use small quantities (a few drops of both filtrate and thiocyanate) and flush everything down the drain with lots of water after you're done, you should be perfectly safe. The chemicals you need are available on line for < $10 each.

Testing for rust is a little complicated in the home but testing for iron is simpler.

If you scatter a tiny sample of the dust you have scraped off into a blue flame (eg a gas cooker or calor gas camping stove) then an Iron compound will give tiny yellow scintillations (like a sparkler firework).

To test for Iron you could also dab the wall with a little dilute Hydrochloric Acid on a tissue until you have a brown spot on the tissue. If rust is present you may get yellow spots in the middle of the brown spot but this is far from reliable as a test.

At Home Iron Test