How can stainless steel corrode?

A. Hi Liljana. Stainless steel is also sometimes called corrosion resistant steel. It's more resistant to corrosion than plain iron or steel, but it's not corrosion-proof. Acids can rapidly corrode it, and salty air can corrode it ... even plain water can sometimes corrode it.

Please tell us what grade you are in and what you know about stainless steel, and then we may be able to help more, but the situation must be explained differently for elementary school students than for college students, as I'm sure you can appreciate.

Also, please include what you mean by corrosion. It means different things to different people.

A. All "stainless" steels can corrode under the wrong conditions. Many companies, in fact, do not call them stainless steels, but "corrosion resistant" steel. Remember that most of the metal is still iron and can corrode or rust depending on how much stress it is exposed to. One of the worst things you can do to stainless steel is expose it to chlorides. The better grades can withstand a lot of abuse, but most grades can eventually fail when exposed to chlorides in bad atmospheres.

Some grades of "stainless" steel have very poor resistance to corrosion in fact. They are marginally better than carbon steel, but when correctly hardened and passivated or electropolished can give fairly good performance. The grade of steel must reflect the exposure conditions and performance required.

A. Liljana,

Stainless can be corroded, i,e., attacked, depleted and dissolved in the presence of some acids ... but even seawater, which contains salt, as you know, can attack some of the various grades of stainless especially around the welds.

Why the question? What type of stainless are you talking about? Or is this just idle curiosity?

A. Regarding the corrosion of stainless steel, I would say stainless steel exhibits good corrosion qualities because of the chromium layer formed on it and this chromium which also contributes to one of its major constituents when exposed to brine, or calcium chloride is corroded (rusting can also be the initial outcome). Acidic attack which can include Hydrofluoric acid, Hydrochloric acid, Phosphoric acid & sulfuric acid can also do the job. Please don't hesitate to ask -- I know a lot about all this.

A. Probably the better part of the semester, Jacob, although it actually depends on what type of stainless steel, what concentration of acid (you probably don't have access to highly concentrated acid in high school), what temperature, and how much acid you have. "School strength" hydrochloric acid is probably much more corrosive than sulfuric acid.

The stuff you see in movies, where a drop of acid burns a rapid deep hole through everything it touches, is, well, the stuff of movies. In real life an acid dissolves metal via the metal ions taking the place of the hydrogen in the acid, so acid can only actually dissolve a very little metal before all the hydrogen it gone. So a drop of the strongest acid can only make a nice dent or pit in a solid block of metal before all of it's hydrogen has been consumed; it can't burn through even 1 cm of stainless steel, let alone 10 cm.

To dissolve 10 cm of metal, you need a lot of acid.

A. Dear Jacob,
Since you are conducting an experiment you would know anyway. let us know the results: which acid was better. While doing the experiment you should note down the parameters: the type of metal, initial thickness (you could weigh to three significant digits, thorough cleaning before pickling, (weigh it after you have cleaned), type of acid, the strength of acid, temperature of the acid (room temp if you are not heating), time of pickling. weigh it after this. see the difference and you have an answer. The parameters you should keep the same are: type of metal, cleaning procedure, use the same balance preferably, same strength of acid, same temperature. Also same grade: if you are using AR HCl then use AR H2SO4. Good luck

A. Hi Keir.

It would certainly not "eat right through"; it's probably more likely to very slowly dissolve stainless over a period of many years than to form a layer of tarnish on it, because citric acid is a chelating agent.

Even though stainless steel may not be 100.00% resistant like some plastics would be, electropolished type 316SS is widely used in food and pharmaceutical factories because it is so smooth that it can be cleaned very well to prevent cross contamination (which can be a bigger problem than very slow corrosion).

I don't have experience in orange juice factories, so I can't answer really knowledgeably, but based on more general experience I would expect that if the tanks are used for nothing else, they would probably be plastic or lined with plastic, but the valves and piping, etc. would probably be stainless steel.

Around the house, stainless steel "juicers" would be common because plastic isn't strong enough. And stainless is commonly used for measuring cups, spoons, mixing bowls, etc., that might be used for orange juice.

Regards,

A. Hi cousin James. If you search the site for "passivate stainless citric acid ", or similar phrasing, you'll find a bunch of threads addressing the subject of beneficially applying citric acid to stainless steel. Alternately, please post a question with full details of your situation. Thanks!

Regards,

A. James,
citric acid is known to leave the chromium and chromium oxide in stainless steel alone. The surface of your steel turning black probably has nothing to do with chromium.

What grade of stainless steel was this, what was your application process (immersion bath? dwell time, temperature, etc.), and was this a straight citric acid solution or a citric acid based product that probably has other things in it?

It turned black like coal, or merely a darker gray color?