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Cleaning rust from high tensile steel

April 2, 2009

Q. Hi, my name is Frank and I am involved in the restoration of vintage cars.

I have been using a very diluted retarded phosphoric acid based solution as a bath for removing rust and scale from steel, as a pre-treatment for painting.

I have been using this phosphoric acid based bath for a while with great success and generally find that I can leave steel automotive bits and pieces, etc., in the bath for a few days and any rust is removed without visually affecting the metal. Metal usually comes out nice and bright and clean.

On this occasion I put some bare steel rods (not zinc plated) 40 cm long and 1.5 cm in diameter, into the bath. I have subsequently found out they are a high tensile steel probably grade 10.8 (well, so I am told by someone who tested them for me).

After leaving the rods in the bath for about 4 - 6 hours I checked on them they were bubbling away. By this I mean there were hundreds and thousands of very tiny bubbles coming from the steel rods.
I pulled them out and they were coated with some sort of black substance which partially rubbed come off on my fingers. It tried to wash the black off with water, however the rods still retained this tainted black color and they had a funny sort of smell to them.

I finally got the rods shinny clean again by using a scotch brite scouring disk mounted on an angle grinder ⇦ this on eBay or Amazon [affil links] .

I would still like to paint and use these rods, however, my concern is that because they are high tensile steel that they may have been affected by the phosphoric acid bath that I put them in.

I am particularly concerned that this may have weakened the steel or may have led to some sort of embrittlement.

Would the high tensile steel rods have been affected in any way by the phosphoric acid bath?

Do the high tensile steel rods now have hydrogen embrittlement and will I have to get them de-embrittled by having them placed in an appropriate oven?

Your assistance would be appreciated.

Frank

Frank Ventura

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April 9, 2009

Q. These rods are part of the cars suspension and are known as radius rods / strut rods.

I believe the rods may be 10.9 I asked an engineer to look at them for me and can only rely on what I have been told. Mind you he only had a quick look at them and he did not perform any type of test on them.

Although, I must say I have seen many similar rods that have come from other vintage cars that are bent. Some people actually hammer them straight. So this makes me wonder whether they are actually high tensile or just mild steel.

What test or tests could be performed on these to determine what type of steel they are made from?

After I posted my question, someone else advised me that hydrogen embrittlement only occurs if the metal is electroplated. Is this correct?

How can I tell if they have hydrogen embrittlement? Is there any sort of test?

I also have another set of high tensile rods which I did not place into the bath because I saw what happened to the first lot.
I gave these rods a thin coat of a rust converter ⇦ this on eBay or Amazon [affil links] called "phoscoate". This converter is phosphoric acid based and is only applied very thinly and allowed to dry overnight.
Would this also cause hydrogen embrittlement?

Your assistance would be appreciated.

Frank Ventura
- Melbourne, VIC, AUSTRALIA

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April 17, 2009

Hi Jeffrey,

Firstly thank you for the information and advice provided to date.

I have been doing some research on the affects of rust converter ⇦ this on eBay or Amazon [affil links] s on high tensile / stress steels. During this I came across a test done by the United States Navy, who immersed two different types of high tensile steels in different rust converter ⇦ this on eBay or Amazon [affil links] s for 24 hrs and then subjected them to a number of tests. Makes for interesting reading.

Basically , the United States Navy conducted tests on the affects of two types of rust converter ⇦ this on eBay or Amazon [affil links] s on fasteners made from different types of steels including high tensile.

One of the rust converter ⇦ this on eBay or Amazon [affil links] s also contained phosphoric acid.

In their conclusions they found no evidence of "Environmentally assisted cracking resistance" in the tested fasteners when the rust converter ⇦ this on eBay or Amazon [affil links] s were applied.

The Navy document is available on the internet and is as titled as follows:
Paper No. 05206 Corrosion 2005

EFFECT OF COMMERCIAL rust converter ⇦ this on eBay or Amazon [affil links] COATINGS ON THE ENVIRONMENTALLY ASSISTED CRACKING RESISTANCE OF HY-100 STEELS AND HIGH STRENGTH ALLOY STEEL FASTENERS

Link : http://content.nace.org/Store/Downloads/05206.pdf

Regards,

Frank Ventura

Frank Ventura
- Dandenong, VIC, AUSTRALIA