Why would bleach rust a steel screw?

Answer 1

A bleach is a chemical that removes color or whitens, often via oxidation. Common chemical bleaches include household "chlorine bleach", a solution of approximately 3-6% sodium hypochlorite (NaOCl).

"Rusting", is an oxidization process.

To indicate the easiness or hardness of the oxidation/reduction process, people use so-called "Standard Reduction Potentials", to show how it is. The more negative the number is, the easier it is to be oxidized and the harder it is to be reduced (back to metal). Vice versa, the more positive the number is, the harder it is to be oxidized and the easier it is to be reduced.

For non-metal element, we also can assign the standard reduction potential such as:

2HClO(aq) + 2H+ + 2e− → Cl2(g) + 2H2O, +1.63V

Cl2(g) + 2e− → 2Cl−(aq) +1.36V

2H+(aq) + 2e− → H2(g), 0.00V

This indicate that bleach (+1.63V) is easy to be reduced, thus it would grab the electrons from something else. That is to say, it is a strong oxidizer to cause other metals to be oxidized. Even Cl2 formed as product still serves as a strong oxidizer (+1.36V). On the other hand, water, containing small amount of H+, is extremely weak oxidizer (with reduction potential of 0.00V).