Traditionally sulphuric acid but this is listed as an Explosives precursor nowadays so the slower citric acid is substituted

Sodium bisulphate should work, it’s sold as pH-down for swimming pools.

Hi I use wicks brick and patio cleaner £19 for 5 litres and works well. you can reuse it many times. It is an acid so wear protective equipment and wash of in plenty of water. hydrochloric acid 5 – 10 % concentration.

Sika Brick and Patio Acid Based Cleaner – 5L

David

Citric Acid + a biocide. CuP Alloys Cleaning Salts does the job, right grand.

You really don’t want to mess around with Sulphuric Acid etc, just use the correct stuff.

JA

Hi, yes brick/patio cleaner does work well, but you need one that contains hydrochloric acid, this is the one that I get https://www.toolstation.com/bostik-cementone-brick-patio-cleaner/p29118

Regards Nick.

Also known as Muriatic acid in the US

Argh, NO! You add conc. sulfuric to water, not the other way round, remember!

Yes, hydrochloric acid and its fumes are very corrosive. It also removes zinc from brass very quickly leaving a copper surface.  Sulphuric acid is much safer to use, about 5% w.w. solution.

I don’t know why sulphuric acid is classed as an explosive precursor.  You need concentrated sulphuric and nitric acid for that.  30% sulphuric acid is easily available here in France.  It is even stocked in some supermarkets!

Russell

On 1 January 2024 at 10:14 Russell Eberhardt Said:

…

I don’t know why sulphuric acid is classed as an explosive precursor…

It’s classed as an explosive pre-cursor because it is an explosive pre-cursor!

Mechanical Engineers tend to underestimate what other engineers are capable of.   We assume mechanical mechanisms are simpler and more reliable than electronics, steel made from recycled metal must be impure, electric cars don’t work, and many other half-true simplifications.  The truth is experts within their specialisation run rings round everybody else.  It’s what they know that matters, not what we believe!

A chemical engineer knows how to manipulate Sulphuric Acid, and much, much more.   Sadly, some chemists support terrorists (or freedom fighters depending on the politics) or make illegal Class A drugs for a living.  Misuse is a real problem, and I’d rather inconvenience a few hobbyists than make it easy for persons of ill-intent.

For pickling, a cheap fast acting clean mineral acid is best, and dilute Sulphuric ticks most of the boxes.  Industry use it in very large quantities, and for them the paper-work needed to buy it is trivial. Unfortunately, mad bombers have much the same profile as Model Engineers.  They order a small quantity (less than a ton) for delivery to a private address, and have no established commercial or technical purpose. Expect to be treated with suspicion.

Hydrochloric Acid is also cheap, fast acting, and readily available but it’s less suitable for pickling because it leaves corrosive Chloride ions in the pores of the metal. These undermine the effectiveness of Galvanising, Paint, and other anti-corrosion treatments.  Unlikely to show up on amateur work, but it noticeably reduces the corrosion resistance of steel structures; bridges that should last 80 years, only last 50 etc.

Citric Acid is slow acting and expensive, but it’s completely safe, doesn’t smell, and is readily available.  The cost doesn’t matter much in small workshop, because we rarely pickle large amounts of metal, nor do we need results in minutes.  Leaving Citric Acid overnight will do the job.  Citric Acid’s slow action is an advantage in some cases, for example it’s less likely to seriously de-Zinc Brass before anyone notices!

For ordinary sized model engineering objects, I use Citric. If I needed to pickle a large amount of metal quickly, I’d buy some Hydrochloric Acid.  It’s sold by most Builders Merchants and DIY stores as Brick or Patio Cleaner, and is also used in Swimming Pools.   Not needed to yet.  If I was a professional Boiler maker, where cost and speed are  important, I’d do the paperwork and buy Sulphuric.

Dave

PS I know Sulphuric is spelt Sulfuric, but I’m an old-fashioned Brit!

On 1 January 2024 at 14:32 Roderick Jenkins Said:

Regarding concentrated sulphuric acid, a few years ago it became popular to use the acid as a disfiguring weapon of revenge and “honour” punishment.  I believe this was the real driver to restrict easy availability and adding it to the explosive precursor list was a quick and convenient way to achieve this.

Rod

That seems a more likely explanation of the restriction rather than it being used for making explosives when sold at about 30%.

Russell

I went to check the concentration of Sulfuric acid that requires an EPP licence and found that both Hydrochloric and Phosphoric acid have been added to the EPP list effective 1st Oct 2023. The relevant concentrations are:

Sulfuric acid 15% W/W
Hydrochloric acid 10% W/W
Phosphoric acid 30% W/W

As only about 5% Sulfuric acid is required for pickling it is quite legal to buy 10 or 12% and dilute it 1:1 with water. A supplier may even be prepared to make it up to this strength for you.

Ammonium nitrate, Mercury compounds and Arsnic compounds have also been added to the restricted list.

Reference:

https://www.gov.uk/government/publications/supplying-explosives-precursors/supplying-explosives-precursors-and-poison#fulllist