[bjartmaregill]

Hello.

I'm doing a corrosion resistance test for stainless steel against sodium hypochlorite. (NaClO)
I really want to be able to show some chemistry of what happens...

Steel:
My type of stainless steel contains 18% Chrome, 8% Nickel, 0.07% Carbon and the rest Fe (Iron).
By oxidation by oxygen it forms thin chromeoxide layer, Cr₂O₃ that has higher corrosion resistance which protects the steel.

Solution:
When I add water to Sodium Hypochlorite I believe I get hypochlorite ion.

NaClO + H₂O  OCl^- + Na⁺ + OH^- + H⁺

As the pH gets lower it forms hypochlorite acid.

OCl^- + H⁺ + OH^-   HClO + OH^-

When I get the pH even lower it forms chlorine gas.

HClO⁺ H⁺ + Cl^-   Cl₂ + H₂O

Question 1:

• How does the above solution disrupt the chrome-oxide layer?
• How does it corrode the steel after the layer has gone?

Question 2:

• How corrosive is OCl^-
• How corrosive is HClO

(I've read that HClO is 100 times more corrosive/oxidant than OCl^-.)

[Borek]

NaClO + H₂O  OCl^- + Na⁺ + OH^- + H⁺

No need for water in this equation.

NaClO OCl^- + Na⁺

will do (but read on).

As the pH gets lower it forms hypochlorite acid.

OCl^- + H⁺ + OH^-  HClO + OH^-

Once again, you put too many substances in teh equation. OH^- is just a spectator, so the reaction is:

OCl^- + H⁺ HClO

However, the same equilibrium can be described by water ion autodissociation and OCl^- hydrolysis:

OCl^- + H₂O  HClO + OH^-

When I get the pH even lower it forms hypochlorite gas.

HClO⁺ H⁺ + Cl^-   Cl₂ + H₂O[/color].)

No such thing as a hypochlorite gas, what you have here (Cl₂) is chlorine.

[bjartmaregill]

Thanks for the answer Borek. I really appreciate it.

Okay, I don't really need the water to make the OCl^- but I need the hydrogen lying around to form the HClO. I just wanted to show where the Hydrogen came from.

I originally though that I needed the water to make this start.

And about the gas. I meant to say chlorine gas.

*Edit*.
What I want to do is calculate how the corrosion happens - And how much more corrosive the HClO is compared to OCl^-

Corrosion is used when metals loose electrons. So I'm trying to understand how to use the electronegativity e.t.c.
I've made two tables with all chemicals I'm using but I don't know how to make the calculations.

Can anyone help me with that one?

[Borek]

This is tricky.

Electronegativity is of no use here.

Formal redox potentials will be much more reliable, but when the surface is passivated by relatively inert oxide, we can be limited by how fast the charge can be transferred to the metal below. That means we are limited not by equilibrium (which can be calculated from the half cell potentials), but by kinetics. As far as I am aware kinetics needs experimental data.

That being said, I find it highly unlikely that such data doesn't exist. It is either in engineering handbooks, or in catalogs. You can try asking any producer of stainless steel about its compatibility with hypochlorite solutions.

[bjartmaregill]

This is tricky.

Electronegativity is of no use here.

Formal redox potentials will be much more reliable, but when the surface is passivated by relatively inert oxide, we can be limited by how fast the charge can be transferred to the metal below. That means we are limited not by equilibrium (which can be calculated from the half cell potentials), but by kinetics. As far as I am aware kinetics needs experimental data.

That being said, I find it highly unlikely that such data doesn't exist. It is either in engineering handbooks, or in catalogs. You can try asking any producer of stainless steel about its compatibility with hypochlorite solutions.

Okay, I see. So I can't really calculate which chemical compound is more corrosive. But can I show that these chemical compounds will corrode/take electrons from the chromeoxide-layer and the steel (Fe+Cr+Ni)?

i.e.

HClO:
1. HClO + Cr₂O₃   
2. HClO + (Fe + Cr + Ni) 

ClO^-:
3. ClO^- + Cr₂O₃   
4. ClO^- + (Fe + Cr + Ni)