Passivating carbon steel / when to / how to

A. Your customer may be confusing "passivating" with "passive". Many times a steel part needs to be activated for plating or finishing because it has become passive. Passivating for stainless means to remove the iron from the surface and create a nickel and chrome- rich surface layer that is not as subject to rust (only iron "rusts"). Let your customer know that a steel part will always have a propensity to rust - even if it has become passive, because the iron is always exposed to the atmosphere. Hope this helps.

A. Herb:

You cannot "passivate" carbon steel. Passivation is removing the free iron and other exogenous materials from the surface so that a good passive layer can be formed. In the case of carbon steel, this is not possible. After cleaning with a good cleaner, there are a variety of coatings that can be used for carbon steel to provide a corrosion resistant layer, but passivation is not the correct process.

A. Passivating carbon steel. If carbon steel could not be passivated then there would be no boilers made of carbon steel. Boiler steel passivation occurs by the Schikorr reaction at about 300 °C or 30 bar pressure. A protective "passivating" magnetite layer, a few microns thick forms by a reaction between clean, pickled steel and water. The reaction lowers the pH of the boiler water. In practice, newly pickled boilers are subjected to controlled passivation using very small quantities of chemicals to control pH and prevent re-dissolution of new magnetite. The thickness of this magnetite layer will continue to grow at a decreasing rate until it hinders heat transfer. The boiler is then again chemically cleaned.

The chemical cleaning itself, by pickling, strips oxide layers from the boiler surface, leaving the steel active and prone to immediate oxidation (rusting).

To stop this "flash-rust" disturbing the Schikorr reaction, the pickled boiler receives an initial "temporary passivation". This involves dissolution of the flash rust in citric acid, followed by raising the pH to the range 8.8-9.5 using ammonia, which keeps sequestered iron in solution. Once the pH is correct, an oxidizing agent, for example sodium nitrite is introduced. This reacts with the iron in the steel to produce a very thin (40-100 Angstrom) layer of Fe2O3, which, when formed under these circumstances, inhibits flow of ions through it, thereby passivating the steel. This passivity is easily destroyed and is therefore temporary in nature (unlike passive oxides formed on stainless steel).

For carbon steel items to be used for everyday use, coatings are indeed usually applied. Some coatings involve chemical reactions in which iron from the steel plays a part (such as zinc phosphating), while other, usually more durable coatings physically attach themselves to the steel, for example paint. It is common that physically adherent coatings are applied to steel that has been temporarily passivated by, for example the formation of iron phosphate. Such phosphates are formed by dipping pickled steel into hot dilute phosphoric acid solutions followed by exposing them to air to complete the process. An example of such a process is phosphating and painting of car body parts.

For tools to be used in clean-room environments preference should be given to quality stainless steel implements.

A. I think Trevor has confused conversion coatings with passivation. To begin with, carbon steel boilers don't rust because the pH of the water is kept above the oxidizing point of steel which is about 8.2 - 8.3. The magnetite coating is a chemically induced conversion coating (also called black oxide), that if left in a pH environment below 8.2 will start to rust. If proper boiler chemistries and and periodic blowdown schedules are followed, there should be no buildup requiring chemical stripping of the boiler tubes. I have seen boilers that have been online for 10 years and more without needing a chemical strip.

Parts that receive a phosphate treatment likewise are not immune to rust and cannot be considered passivated. This is another example of a conversion coating.

To close, passivation is accepted as being a theoretically permanent condition. Passive means to not be active enough for the next finishing step.

A. Passivation is subjected to lot of conditions. Say when you talk about a boiler, carbon steel tubes in generating section will have a reduced oxide in a state of magnetite. it remains intact only when you treat the water and chemically condition the same. Demineralised water, free from oxygen and chloride, pH elevated to 9.0 to 9.5, free from caustic, etc.; it is non impervious to ionic intrusion.
Whereas if it is gamma ferric oxide, it will remain intact and it's P.B ratio is above 1.0 it is impervious due to structural integrity, and natural oxide growth rate during regular operation is retarded and beneficial in all respects.

A. Passivation is a state wherein we protect it from re-oxidation.
A chemically active surface is more prone to attack due to its active state after immediate chemical cleaning. It is important to make the metal passive to prevent the re-oxidation and thereby prevent further corrosion
Normally the medium is switched to an alkaline state after an acidic reaction with an oxidising agent to get the passivation layer on to the surface.

A. Carbon Steel can be passivated using .25% NaH2PO4, .25% Na2PO4 and .5% NaNO2 @ 150 degrees. Makes a passive phosphate layer.

A. Hi Liz. It is certainly possible to treat rusty steel with phosphoric acid to convert the fluffy red rust to the more stable black rust. Whether this should be called 'passivation' is a semantics argument over the use of that word rather than a technical issue. Some people consider the word 'passivation' to have a very specific meaning that applies only to the processes used on stainless steel.

Regards,

A. Hi M. As this page notes, "passivation" has a good definition when it is applied to stainless steel; ASTM & AMS and other specs provide the careful definitions that you seek. But the word is also used in slang fashion for many other situations and there is no way anyone can limit slang; that is just the way the world is :-)

We can say that a "tank" is an armored fighting vehicle and write careful definitions for how thick the armor must be and what specifications it must meet before it can properly be called "a tank" according to those specifications ... but that won't stop people from calling their aquariums, hot water heaters, propane cylinders, and gasoline reservoirs "tanks". And even if we add all those terms & more to the dictionary, it won't stop teenagers from calling their parents' old station wagon "a tank". Thus it probably will remain with 'passivation' :-)

Regards,

A. M.,
Passive is actually a chemist's term, it is the opposite of active. Active refers to atoms or molecules that "want" to react with something. Taking metals as a pertinent example, metal as we think of it has an electron valence of zero, which is to say the number of electrons in orbit is equal to the number of protons in the nucleus. However most atoms, including metals, "prefer" to have a different number of electrons, in the case of metals they want to give some away and become a positive ion in solution or become bonded to the atom that it gave its electrons to. Surface oxidation is the most common route to this, for instance if you cut a block of aluminum in half you can see the slight color change across the fresh surface as oxygen gas swoops in and quickly forms a layer of aluminum oxide. The new oxide layer is "passive" because it is happy and no longer want to react to anything, and as a bonus it seals off the zero valence aluminum underneath from being able to react with anything.

This is all fine and good for the majority of nonferrous metals, but when iron forms iron oxide it is not a nice and pretty layer, it turns to rust and falls off, exposing new iron to new oxidation. Common methods to combat this are coatings of another material, such as paint, powder coat, zinc (galvanizing), a chromate conversion coating, etc. There are also stainless steels, which use chromium to form a chromium oxide layer over the iron, protecting it the same way the aluminum oxide prevents further reaction in the underlying aluminum metal.

Anything that renders your surface less likely to chemically react with materials in the environment can legitimately be called "passivating".

A. Hi Hasan. I am not a chemist, and hope to be corrected if I'm seeing it wrong, but your question is partially technical and partially semantics. According to Wikipedia, sequestration and chelation are much the same thing, but others might use "sequestration" in a looser sense where the iron is tied up or complexed in ways that may not meet the actual definition of chelation -- for example, iron can be complexed with cyanide ("Prussian blue") and it is probably proper to say that the iron is sequestered.

I think any condition which keeps more iron in solution than the pH and alkalinity alone would dictate probably represents sequestration.

Flash rust is red, so I'd say it is Fe2O3. I believe the idea in pretreatment is to remove the rust and then get on with the treatment process immediately rather than letting flash rust form.

Regards,

A. Indeed, "sequestered" isn't really the preferred term by chemists but I'm pretty sure as a colloquial it means the same thing as "chelated". Wikipedia seems to agree.

There are a few different varieties of iron oxide, depending on the oxidation state of the iron atoms, or the ratio of iron atoms to oxygen atoms present. I always forget which is which, but black iron oxide tends to be pretty stable while red iron oxide is the rust that eventually leads to progressive deterioration of the metal.

I have heard of "temporary passivation" of carbon steel in regards to boilers -- I'm not overly familiar with the process but in general any coating that prevents oxygen molecules from contacting zero valence iron is getting you what you need, for as long at it's intact.

A. Hi hasan eghbali,

The difference between flash rust and temporary passivation is simply that the temporary layer is gamma Fe2O3 which is gamma hematite, but the flash rust is normal hematite (I think is alphaa hematite). the Gamma Hematite is produced at higher pH and enriched oxygen content (presence of strong oxidizing agent).

Hi Dhruba. I don't think anyone claimed that carbon steel "does not need" corrosion-proofing. I think what they claimed is that carbon steel does not use or benefit from the processes that are used to passivate stainless steel, and that some people reserve their use of the word 'passivation' to refer to those carefully defined and specified processes for stainless steel.

Sorry, I am not familiar with the protocol for preparing boiler tubes for service beyond what you read on this page.

Regards,

A. Aaron,
I may be missing some context to your question. What metal and for what purpose do you need oxidation? Gaseous oxygen, or just plain air since there's plenty of gaseous oxygen in there, is splendid for converting active zero valence metal atoms into metal oxides. It does vary a little depending on the metal, for instance if you are talking about iron it helps to have some moisture involved. If you are talking about converting zero valence metal atoms into metal ions via oxidation, you will need an aqueous solution containing an oxidizing agent, which can be but is not limited to acids.

A. The way it is professionally done is with hydrochloric acid that is strongly inhibited. This is the same as what metal finishers call 'pickling.' The acid attacks the metal underneath, lifting the rust, and the inhibitor coats the metal, largely preventing further attack by the acid.

A. Moataz,
As per the usual here, please read the thread above for comments that may have already answered your question.

The answer to "is it enough" is always "it depends". If your item is going to live in a cool and dry environment, hardly any protection is needed at all. If your item is going to live at the water's edge along an ocean coastline, you need a lot of protection.

Possibly you will want to run some kind of performance test, like an ASTM B117 salt spray.

A. Carbon Steel can be passive. Iron in its natural stage is almost never stable, it needs to bond with oxygen and form Iron oxide to be passive. Same happens with Stainless steel, difference is you form Chromium oxide (mrs stable) instead of Iron oxide (less stable) Pickling or descaling is not passivation, but the process to remove foreign matter from the metal surface. Once the metal surface is free of scales, then passivation can take place. For Stainless steel it is promoted with highly oxidative Acids In carbon steel is promoted with other oxidizers, being Sodium Nitrite the most common.

A. Hello Alfredo!

Can you take off this piping for treating? Which size is it? Is it pressurized?

If you have running DI water through this pipe, you should have the pipe coated from the inside with some coating to maintain your pipe without corrosion and your water without minerals! Really, steel passivating is not very effective for this use as DI water will corrode your pipe again. If it were stainless, you could pickle and re-passivate, but carbon steel can't be permanently passivated as stainless.

If you can't take your pipe out and can't replace it, I would suggest chemical cleaning and soft protection (hydrochloric acid, rinse, then some steel protection as sodium nitrite and rinse with DI water), then paint your pipe from the outside. But this won't last, as carbon steel is not the material you want to work with DI water.

Best of luck amigo!