MAGNETITE COATING
Why is a magnetite surface desirable?
The hard durable crystalline structure of magnetite is the reason why this coating is so desirable. This coating imparts the following benefits on boiler internals:
Highly resistant to corrosion – stable surface
Non adherent to scaling elements
Excellent heat transfer
Magnetite is iron in its’ most reduced form, FeO2. These exceptional surface characteristics are a direct result of the crystalline structure of magnetite. The picture of the natural magnetite nugget clearly shows this crystalline structure. Check out the magnetite crystal at the Museum of Natural History in Chicago – it’s the size of a football and looks like black obsidian.
What is required to establish a magnetite coating?
Basically what is required is a:
Clean, deposit free surface
Chemically reducing environment
This deposit free surface includes both hardness and iron oxide deposits - rust. To remove the iron oxide deposits, which we typically see, we first need to minimize the iron going into the boiler. This will allow the iron dispersant to more easily remove the existing iron deposits.
Once a clean surface has been established the catalyzed sulfite or other oxygen scavenger provides a chemically reducing environment to remove the oxygen from the iron surface resulting in FeO2, a magnetite coating.
What is the difference between rust deposits and magnetite?
The only difference is the number of oxygen atoms. Magnetite, FeO2, has only two oxygen’s, while ferrous iron, FeO3 has three oxygen’s and finally ferric iron, FeO4, has four oxygen’s. Both ferrous and ferric iron are considered rust and unsuitable for boiler application for the following reasons:
High corrosion rates
Poor heat transfer – worse than minerals deposits
The number of oxygen atoms and particularly their removal to reduce the oxidation state of the iron– down to magnetite – is key to understanding the chemical strategy.
What parameters are required for a successful program:
Basically your feedwater quality needs to be that for a 750 psi boiler.
zero hardness in boiler feedwater < 1 ppm
minimize iron going to boiler < 10 ppb
use an effective iron dispersant to remove existing iron oxide deposits
remove dissolved oxygen before the boiler using a catalyzed product
maintain minimum 25 ppm sulfite residual or other reducing material
How long does it take to establish a magnetite surface?
The key here is how fast a deposit free surface can be achieved. Once we have a clean surface in a matter of six months time a magnetite coating can be formed. However realistically it can takes two to three years to achieve a clean surface if the boiler is significantly fouled.
How can we tell the program is working?
We can determine if the program is working correctly when iron levels are at a positive mass balance.
A mass balance analysis is a simple analytical technique where you compare actual levels to theoretical levels. For example if we had 1 ppb of iron in the feedwater and we cycled up the boiler 10 times theoretically we should have 10 ppb of iron in the boiler. If our actual iron level tested out at 10 ppb then all the iron has been accounted for – a neutral mass balance. If our iron readings were only 5 ppb then we have not accounted for all the iron. Where is the rest of the iron? It most likely dropped out onto the boiler tubes. This lower than expected reading is a negative mass balance.
However when we feed the iron dispersant our iron reading would normally be 15 or 20 ppb – a positive mass balance indicating we have more iron than theoretical levels would show. This excess iron is coming from the deposits which the dispersant is removing. This positive mass balance indicates the program is working.
Another indicator is during annual inspection. Normally there is 10 to 30 lbs of rust chips in the mud drum. We know these deposits contain iron by the magnetic rust ball they form pictured above. These iron chips result from the iron dispersant removing existing deposits. These rust chips originated as condensate piping, corroded over time and transported back to the boiler where they dropped out as an iron deposit.
What condition is my boiler currently in?
What we typically see during our inspections are boilers with a rusty appearance on the internal surfaces. If you wipe your finger across the surface it is covered by fine rust. This rust originates as corrosion of the condensate system. As the piping corrodes the soluble iron, ferrous, is transported back to the boiler. Without an iron dispersant the iron will deposit on the boiler tubes. Iron oxide has a greater insulation than typical scale. Without an iron dispersant, ferrous iron will be continually deposited on the tubes and a magnetite coating will never form.
What can I do now to begin establishing a magnetite surface.
Reduce iron going into boiler.
By properly inhibiting the condensate system, corrosion levels are reduced resulting in lower iron levels returning to the boiler. This lower iron load allows the iron dispersant to function more effectively.Use an iron dispersant.
An effective iron dispersant is key in establishing a magnetite surface. The dispersant performs two functions. First by keeping iron in a soluble state deposition in the boiler is minimized. Secondly existing iron deposits are gradually removed providing a clean surface for magnetite formation.Check oxygen scavenger feedpoint.
Once dissolved oxygen reaches the boiler it is too late for removal. Dissolved oxygen removal must occur before the boiler. Ideally the oxygen scavenger should be introduced into the deaerator. This will allow the oxygen scavenger the greatest amount of time to remove the oxygen. This is the most critical injection point in all water treatment.Maintain reducing environment in boiler.
Finally we want to maintain a reducing environment in the boilerwater. This can be accomplished by maintaining a sulfite residual of 25 to 35 ppm or an erythorbate level of .5 ppm. Here again this reducing condition promotes the magnetite coating.
To summarize the operating parameters required to establish a magnetite coating are:
reduce feedwater iron levels to below 10 ppb
reduce feedwater oxygen levels to below 8 ppb
dose iron dispersant to achieve 30 cycles of concentration of iron
maintain catalyzed sulfite residual at 25 to 35 ppm
Case Study
A major university in northeastern Illinois was not satisfied with the boiler internals. Basically the boilers looked rusty. There was no black magnetite coating to be found – only a reddish brown rusty look to the tubes. The former vendor tested for iron in the condensate but not in the boiler so we have no idea as to the effectiveness of the dispersant program or even if there was one.
The initial changes we made where to increase the sampling points on the condensate system so we can better determine condensate distribution. We began testing for soluble iron in the boilers to ascertain the effectiveness of our iron dispersant. Most importantly we changed the injection point of the oxygen scavenger directly into the deaerator.
After these changes our typical tests results where:
feedwater iron levels averaging 8 ppb
feedwater oxygen levels averaging 6 ppb
average cycles of concentration of iron in boilerwater – 33
average sulfite residual – 30 ppm
Within one year of operating Boiler# 4 inspection showed a magnetite coating formed on the boiler internals. The photos speak for themselves. During the last inspection you could run your finger across the metal surface and it would remain clean – no rust.