Letter 9549

What is "black rust" on steel? 

+ I am a law student working for a law firm and we have a document from a client which refers to "black rust" on steel. I don't know much about rusting, but I didn't realize that rust could come in different colors. Does anyone know anything about "black rust" and what distinguishes it from any other colors of rust? I've searched everywhere I could think of on the web, but nothing I've tried seems to distinguish colors of rust. The firm is expecting me to be able to jump on the web and find an answer to this, so any help would be greatly appreciated. Daniel T [name deleted for privacy by editor] - Valparaiso, Indiana + That's a new one to me. I have heard the term "white rust" which refers to the corrosion products of zinc, cadmium, aluminum, etc; and the term 'red rust' which means real rust, i.e., the corrosion product of iron. Ted Mooney, P.E.  finishing.com Brick, New Jersey + There is a reference to BLACK RUST protective coating in the lower left-hand table of http://amgoz.freeservers.com/chain.htm so maybe those people can supply info. I've never heard of the term being applied to rust on steel to distinguish it from ordinary brown rust. Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

I don't think so, Bill. I believe that chain-maker was talking about a BLACK rust-protective-coating, not a BLACK RUST protective-coating.

If you disagree, we have an even bigger problem because someone is going to want to know about another color of rust listed on that page: a GOLD PASSIVATED RUST protective-coating.

Ted Mooney finishing.com Brick, New Jersey

A wild guess--They might be referring to the black "mill scale" that is an oxide on steel, when it is quite hot and exposed to air.

James Watts
- Navarre, Florida

Yes, Ted Mooney ... After reviewing the amgoz page of chain data, I agree with you. I previously interpreted the phrase "BLACK RUST" as being some trade or proprietary name for a coating, but the reference to gold rust on the same page certainly does cast doubt on such an interpretation. So still nobody knows what the original reference is to.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

The common chemical name for rust is Iron Oxide(s). It can actually be separated into different chemicals. The three most common are:

a) FeO = ferrous oxide

b) Fe₂O₃ = ferric oxide, hematite, and red iron oxide

c) Fe₃O₄ = ferrous ferric oxide, magnetite, and black iron oxide

They are widely used in the cosmetic and pharmaceutical industries as colorants. The shade ranges from a dull yellow through various oranges and reds to a deep black. Magnetite, or black iron oxide is also used to make recording media:audio and video tapes, floppy and hard disks, etc.

I'm not aware that "black rust" has any significantly different properties chemically than any of the other forms of Iron Oxides.

Bob Zonis
cosmetic pencil manufacturing - Bohemia, New York

Not much progress! Some context from the original document would be helpful. That might indicate whether we're looking for a processing byproduct or a weathering/corrosion product or a deliberately applied coating or whatever.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

I assume you have probably already attained the answer you were looking for, but if not here is some more information for you.

The "rust" converter concept does exist, but there is another possibility. In thermal processing of metal there is a process called blackening. This is a high temperature process used on (in my specific case) steel. The process promotes the "rusting" of the metal. The oxidizing of iron initially goes to FeO it then further oxides to Fe₂O₃ (your red rust). Another possibility is to hinder the formation of Fe₂O₃ and promote further oxidization to Fe₃O₄. This skips the layering effect. I mean red oxide (Fe₂O₃) which is then is oxided to Fe₃O₄ (black oxide). Instead you are attempting to have a larger % of FeO going to Fe₃O₄ than Fe₂O₃. Why is this important? Well Fe₃O₄ is more adherent and it has nice magnetic properties (very good for TV tubes). It also then inhibits red rust. So it is a "good" rust.

Regards,

Doreen Fulmer
- Lancaster, PA

The context in which you have encountered the term "black rust" would be helpful, but I have also encountered the term recently as it relates to the a black, marbleized discoloration on corrugated metal roof panels that have been galvanized with zinc. It appears that the panels were stored outside for a period of time, and some water got in between the stacked corrugated sheets. The prolonged exposure to the water (so the theory goes) created the discoloration as the water could not evaporate quickly. In my case, both the terms "black rust" and "white rust" have been used to describe this condition. It is unclear whether this discoloration is merely aesthetic or whether it has impaired the rust-inhibiting properties of the zinc galvanization. A prior letter in this chain referred to "white rust" as the result of oxidized zinc. Does anyone know whether this condition impairs the zinc coating or the underlying steel?

Robert Dunn
- Farmington, CT

Hi, Robert. "White rust" is the corrosion product of zinc, not the underlying steel. However, it is often associated with poor storage conditions. In use, galvanized coatings "see" carbon dioxide in the air and form a more stable corrosion product; but when stacked and stored wet, they suffer this "white rust" wet storage stain.

Regards,

Ted Mooney finishing.com Brick, New Jersey

The black rust could possibly be a coating, which is a form of rust. There is black rust which is good, and red/brown rust which is bad! The black rust is applied through a process of heat treatment (sort of a controlled rust)...its purpose is to reduce corrosion rate and its also a form of lubrication...its used in aviation parts as a break-in period.

San
- Norm, OK

If this is still an active enquiry, I would like to add my own experience.

My understanding of the term "Black Rust" is the corrosion products that form when iron or steel corrodes under conditions of limited oxygen availability. It is a sticky material that rapidly turns to the more conventional red/brown rust if it encounters a more oxygen rich environment.

Such black rust might be encountered on steel or cast iron under layers of aquatic growth or marine fouling, or on reinforcing steel in concrete immersed in sea-water. It may also be present on the surface of pipelines underneath disbonded but otherwise impermeable coatings. I suspect that black rust forms at the anode of macro corrosion cells in oxygen poor environments, and therefore does not encounter the cathode reaction products which under more normal conditions would allow the formation of conventional rust, Fe₂O₃.H₂0.

I am not sure of the composition of this black rust, but suspect that when it forms on steel immersed in sea-water it is a colloidal mixture of FeO (black) and FeCl₂ since it sometimes has a greenish hue to it.

Since corrosion of steel to form black rust does not produce a crystalline material, it will occupy voids in concrete or other available spaces in concrete without exerting expansive pressures on the surrounding material, and therefore has the ability to dissolve substantial amounts of reinforcing steel without the concrete cracking and spalling. This can result in gross underestimation of the severity of corrosion and the weakening of RC structures.

I have heard of failures in the UK involving prestressed concrete beams due to black rust, but I do not have any more details. I would also welcome more information about black rust.

Fred Salome
- Sydney, NSW, Australia

Ed. note: All inquiries remain "active", Fred, and the discussion remains interesting for other readers even if the original inquirer has moved on. Per google, 147 people read this page each month, spending over 6 minutes on it. Your reply is a good part of that -- thanks!

Dear Sir,

Black rust is the ferric tannate formed by the reaction of tannic acid with iron. All forms of rust are converted to the ferric salt and passivated. It is also the classical ink which lasts very long.

Bye,

Ashok Surana
- Chennai, India

I live in an area where road salts are used a lot and rust is a constant problem. I have done a bit of research on rust prevention and will answer your question about BLACK RUST.

Black Rust on steel or Iron is Fe₃O₄. The red rust you normally see is Fe₂O₃. Black Rust is protective in nature as it's molecules are not as large as Red Rust. Black rust will coat the iron/steel and prevent oxygen from reaching the underlying metal. Red Rust on the other hand puffs up because of it needs more room than in it's previous unoxidized state. This expansion exposes bare metal to oxygen and causes the Red Rust to spread.

There are many products on the market that will convert Fe₂O₃ (red) to Fe₃O₄ (black). The resulting Black Rust will protect the metal and can be painted but does not offer a smooth surface. It's not too practical for auto body but for the underbody, inside wheel wells, etc it's perfect. Black Rust conversion is also perfect for marine applications, steel fences, towers, or anything else outside that has rusted and does not require a smooth finish.

David Zimmerman
- London, ON

Hello, perhaps I would be able to answer this question since I majored in Chemistry. First off, as was already explained, rust as you probably are familiar with it is an iron oxide. The red/brown stuff is Fe₂O₃, or Iron III Oxide with a +3 charge on the iron. However, being a transition metal, iron may also exist with a +2 charge. Oxygen under ordinary conditions only exists with a -2 charge. This means that you can have FeO or Fe₂O₃. In certain mineral structures you can also have Fe₃O₄ (This is also an Iron III Oxide.) FeO and Fe₃O₄ are usually black. Unfortunately, this is not the whole story. Sometimes, molecules of the same chemical can be arranged in different ways or in different structures. Usually one of a few of these forms will predominate, however atoms arranged in different ways will effect light in different ways. For example, you are probably familiar with diamonds and graphite. Technically, they are both made of nothing but carbon atoms. But most people would rather wear a diamond on their finger than a chunk of graphite. Actually, Fe₂O₃ can exist in a black state, the structure known as Black Alaskan diamond. If I am not mistaken it also plays a role in giving color to other precious gems but since these gem structures involve more than a single type of molecule I will avoid further discussion. If you want to know if you have FeO or Fe₃O₄, you can try to put a flame to it. FeO should become rust like you know it if it is forced to react with more Oxygen. Fe₃O₄ on the other hand already has given up all the electrons it can and actually there is something else going on with it that I will not explain. Suffice it to say that 2 of the irons in Fe₃O₄ are +3 while one is +2, but due to something called resonance the charge is distributed.

Hope this answers your question.

Daniel Villageliu
- Iowa City, IA, United States

Black rust is caused by heat.

Nicholas Ross Clements
- Gainesville, Florida, USA

I hope this will help. I found it posted online on the MadSci Network: Chemistry site hope it helps.

"When iron rusts quickly and is kept wet, the first product is usually Fe₃O₄, the mineral with this formula is called Magnetite. It is magnetic, colored grey to brownish-black. When finely divided it is black."

Date: ++
Posted By: Charlie Crutchfield, Retired

Ginger Botzi
- Sooke, BC, Canada

Wait....if you keep the rust in water the whole time, it will turn into black rust, and if you put it in water, then take it out, its normal rust?????????

Samuel Freedman
- Jenkintown, Pennsylvania, United States

Hi - I am doing some R&D; but just for interest -
Circ 1700 Guns were made - then they discovered brown rust (red rust) protected the metals - as used on guns like the brown bess a thin coating usually polished and oiled protected the surface from further rusting -
Circ 1900 the gun trade discovered that if you boil in water the brown rust - you ended up with a nice black rust - known as blacking / blueing - hence blacking of steel -process is still used today .

Andy T
- Kent, UK

Fe₃O₄ is Black Magnetite
a black colored corrosion product

This came off of:
METHODS OF CONSERVING ARCHAEOLOGICAL MATERIAL FROM UNDERWATER SITES

The primary anodic reaction of electrochemical corrosion of iron is the production of ferrous ions. The secondary stage, the oxidation of the ferrous ion compounds to a ferric state, is modified in anaerobic environments. Intermediate oxidation products of ferrous hydroxide, such as hydrated magnetite and black magnetite, are formed (Potter 1956:236-237; Evans 1963:28-29, 75):


6Fe(OH)₂ + O₂ >> 4 H₂O + 2Fe₃O₄ . H₂O
(green hydrated magnetite)


Fe₃O₄ . H₂O >> H₂O + Fe₃O₄
(black magnetite)

Depending on the environment, the corrosion products can take on a variety states of division and hydration, as well as a variety of physical forms. It is common to find corroded iron from marine sites with an outer layer of hydrated ferric hydroxide (common rust), which has restricted the supply of oxygen to the ferrous hydroxide briefly formed at the surface of the metal. Laminated corrosion layers consisting of an inner layer of black magnetite, a thin layer of hydrated magnetite, and an outer layer of hydrated ferric hydroxide are formed:

Fe₃O₄ /2 Fe₃O₄ . H₂O or 2 Fe₂O₃ . H₂O

Jesse Meisterling
- East Hampton, CT, USA

Black rust forms during high temperatures in a reducing atmosphere.

Tristan pope
- Coventry, England

I am in Year 9 at School, and I Make Chainmail. We mailsmiths use lemon juice or a similar acid to turn galvanized iron or steel darker.
I don't know if this is any help.

Ben B
Student, Turramurra High School - Sydney, NSW, Australia

Fe₃O₄ is not black magnetite. Fe₃O₄ . H₂O is black magnetite. The difference is marked, and the two should not be confused.

In any event, Fe₃O₄ occurs in low moisture environments when iron is heated substantially. As far as I know, black rust does not occur naturally. Fe₂O₃ (the bad kind) occurs when iron hydrides are formed during the partial oxidization of iron.

Fe₂O₃ flakes off and exposes more of your metallic crystal iron/steel to be oxidized. Fe₃O₄ does not.

James Saunders
- Odessa, TX, United States

Daniel et al.,

Sorry that I did not see this posting earlier. There is a quite excellent article on anhydrous (5 types) and hydrated (10 types) of iron oxide authored by Mr. Douglas Godfrey. It appears (pages 33-37) in the February 1999 edition of Lubrication Engineering Magazine (a.k.a. The Journal of the Society of Tribologists and Lubrication Engineers).

Three of the anhydrous types are black. All other types are red-yellow-brown.

Charles Bice
- Saxonburg, PA

In the coil coating industry, black rust is the oxide that forms on zincalume coated steel. It is the product of wet storage similar to white rust formed on galvanized coating. It is aesthetically displeasing and if allowed to continue to be exposed to moisture will eventually result in perforation of the steel substrate (iron oxide will begin).
For more information visit galvinfo.com.

Gary Preisendorfer
- Kalama, Washington, USA

You refer to a what seems to be a coating on metal.
there is not enough information in your request to confirm that conclusion
perhaps the reference in the document does not mean a coating...
black rust is a component in manufacturing thermite:

from wikipedia:thermite entry:

Black or blue iron oxide (Fe₃O₄), produced by oxidizing iron in an oxygen-rich environment under high heat, is the most commonly used thermite oxidizing agent because it is inexpensive and easily produced.

Helios Higgins
- Lubbock, Texas

Well I am doing a project on rust and I must find out how many types of rust there is and what they are so can anyone help me?

Sarah Bruno
University of Chicago - Chicago, IL, USA

Red rust is the oxidation of iron with water

Blue rust is the oxidation that occurs after you burn iron(light a match and hold to some Steel Wool [link is to product info at Rockler], be careful I'm not responsible if you burn your hand off!)

Black rust occurs when you run electricity through steel wool hook a - and a + current up to it and after it sorta burns it leaves behind black rust

black rust is much more explosive

I don't know much about blue rust and red rust is a much better oxidizing agent which is why its used in thermite.

Lukas Schulz
- Pville, Iowa, U.S.

Sorry this is so late but I came across it as I was researching for a project. The "black rust" you mentioned may be caused by the solution that the steel came into contact with. Sulfur, sulfide, and sulfamic acid (used in construction to dissolve concrete) contain Sulfur, Hydrogen, Oxygen, and Nitrogen, all of which react with the steel to create a rust which is black in color. I am unsure of its official name, however I also know that it is common with Sulfamic acid and occurs at low pH. There are probably other instances where the "black rust" can occur, but this is the one I am familiar with.

John Reddings
- Sacramento, CA, USA

Black Rust, without discussing the chemistry, has been a common phenomenon with bridge painters for at least a few centuries. The common terms in this industry is; Red Rust (surface rust red/orange in color). Black Rust- commonly called Pack Rust (Black/ grey/yellow in color and stratifying- Layers). You can find this phenomenon on older bridges in the North Eastern US. Common terminology Ferric Oxide -red surface rust and Ferrous Hydroxide black stratifying rust. Contact NACE International; www.nace.org and ask for a CIP level 3 instructor for more detailed information.

John Brown PE
- Luanda, Angola

On a current archaeological dig in Ottawa, Ontario, many of the iron items dug have had a bright blue rust just like the toolbar & browser frame on this pc computer.

Ted Stewart
- Rideau Lakes, Ontario, Canada

Black rust isn't rust but a phosphric reaction so look that up.

William Nefr
- San Diego, CA, North America

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