chemistry of corroding machine screws

[chamois]

I've got a box of pozidrive machine screws and self-tappers in my attic that I've had for a good few years. The screws were all made by a company called GKN and were bright in colour when I bought them ie about the same colour as the bright zinc plated woodscrews you can buy in shops. The different sizes of screws sit in separate compartments and the screws are in contact with each other

I've recently noticed that many of the screws have become a dull, powdery-looking grey colour, but curiously only parts of the screws are affected. On some the thread has become grey, but the tip of the thread has remained bright. On others, part of the head of the screw has changed colour.

I don't think these screws are BZP types as I've never seen woodscrews corroding like this. Because of this, I've got no idea what metal or alloy these screws could be made of. The only other thing I can add is that the pozidrive screwsriver sits in the box amongst the screws and is obviously made of an appropriate steel alloy.

Could anyone explain to me the process that's occurring here please?

[Mortartube]

Just a guess here. Perhaps if there is a bit of damp in the attic, elctroloysis is taking part on a very small scale between the various screws and alloys present in the box.

[Bev2007]

Hi Chamois, likely to be some form of galvanic process. The screw would typcially be cadmium or zinc plated, and the thickness of the coating depends on the application. Screws for external use will have a thicker coating for corrosion protection when in-situ, interior screw have a very thin coating to protect them in the box as people dont like buying rusty screws!
Even if you dont have an external source of damp, maybe your screwdriver has a wooden handle - this would be an adequate source of moisture to start the process.

I'm sending this link to my husband who is a corrosion engineer, he may have something further to add!

[Bev2007]

OK, Mr Bev says:

....also, more stressed areas are more anodic and corrode faster. where the thread has been formed more enegy is placed on the metal and the threads will want to corrode faster E.g. look at a spring on an old motor cycle they always corrode where the spring is more stressed on the inside face....

[Teddio]

The screws are likely to mild steel coated with zinc or cadmium. If there's moisture, oxygen and carbon dioxide present, which there almost certainly is in your attic, an electrolytic process will take place. Anodic regions on the screws will corrode and look powdery. In these regions the metal atoms lose electrons and become positive metal ions. The powdery material will be a metal oxide, hydroxide or even a carbonate.

[Bev2007]

....also, the threads probably have a thinner coating of zinc from the manufacturing process. As such, the thread area may have a thinner galvanic coating and the galvanic cell between the parent metal (screw) and plating is faster....

Zinc when corroding will go a powdery grey colour from the zinc carbonate film formed on the screw

If screw driver has a wooden handle that was damp the compartment with the screws may have a higher humidity and corrode faster

[chamois]

Phew, what a response! Thank you all very much indeed for your answers.

I should have made clear that box of screws is actually a plastic case with a lift up clear plastic lid. It is a complete set of screws ranging from No 6 x 3/8 to No 10 x 3/4 and each size of screw sit in their own separate well-like compartment. The pozidrive screwdriver has a rigid plastic handle.

I�ve now found a small booklet that came with the set. The screws are described as �Bright Zinc Plated hardened steel, Supadriv, Pan-Head, Type AB, Self-Tapping Screws�

Could you please clarify why I'm likely to have moisture, oxygen and carbon dioxide present in the attic? Are we talking here of the normal quantities that are present in air in all attics? If not, how and why does the carbon dioxide and oxygen end up in my attic? Would my gas central heating make matters worse for my screws?

Thanks again

[Teddio]

Ordinary air contains nitrogen, oxygen, argon, carbon dioxide and water vapour ( and trace amounts of other noble gases). An attic is subject to extreme changes in temperature. When the temperature of a mass of air falls, the relative humidity of the air increases, eventually reaching 100%, the dew point. At temperatures at or below the dew point water vapour will condense out of the air and wet the metal screws. Electrochemical corrosion needs a liquid medium for the teansport of ions.

[chamois]

Thanks Teddio. Everything is much clearer now.

[sidkid]

The company GKN is (or was) Guest, Keen and Nettlefolds.....I believe based in the Midlands

[chamois]

Yes, you're right sidkid. They were based at Smethwick, near Warley.

I remember the time when GKN had virtually the monopoly on engineering fastenings in the UK. They were highly regarded the world over.