Prevention of corrosion in steam boiler systems

A. Corrosion protection in boilers and cooling towers is accomplished by monitoring the conductivity of the water. You might want to find a supplier of the chemicals, such as Garrett-Callahan. They could probably advise you, and maybe get you set up with the proper equipment.

A. Tracy,

Contact the representatives of Nalco in Ontario. I am sure they can solve your problem.

Ling

A. Tracy, Steam boiler corrosion can be caused by numerous conditions some of which are:

I. BOILERS:
(1.) Dissolved Oxygen:--- Causing localized pitting (2.) Scale: Mineral or chemical deposits)--- Can result in under deposit corrosion
(3.) Deposition of dissimilar metals:--- Can cause galvanic corrosion (4.) Overfeed of Chelants and Phosphonates can also result in chemically induced corrosion in boiler

II. FEEDWATER:
(1.) The above can be associated with corrosion in feedwater systems also.
(2.) Low pH can also be a mechanism causing corrosion in feedwater systems. This is especially true if a high condensate return is combined with the use of an acidic oxygen scavenger such as sodium bisulfite. (3.)Erosion corrosion may be experienced in either the suction or discharge piping of the feedwater pump if cavitation is being experiences due to improper pump sizing or possible restriction in the piping.

III. CONDENSATE SYSTEMS:
(1.) The previous corrosion mechanisms can also be experienced in condensate return systems to some degree but the most prevalent type of corrosion is associated with low pH and/or oxygen.
(2.) The thermal breakdown of the alkalinity in the boiler results in the release of carbon dioxide. Since this is a gas it is transported with the steam into the system. Upon condensation, the carbon dioxide will form carbonic acid which will drop the pH of the unbuffered condensate. This acidic condensate will attack the return lines causing a characteristic trough at the bottom of the return line. The corrosion products will in turn be returned to the feedwater and can result in iron deposits in the feedwater tank as well as the boiler.
(3.) If the condensate return tanks are vented to the atmosphere, and don't have vacuum breakers, the cooling and pumping of the condensate can result in the addition of atmospheric oxygen into the condensate return system resulting potential for oxygen pitting corrosion.

These corrosion mechanisms can be addressed by a combination of mechanical and chemical means by an experienced chemical water treatment consultant.

A. Hello Mr. Saeid:

Kindly be advised that the Phosphate/Caustic treatment for high pressure steam boilers is delicate and needs some backing of additional products. You should maintain the P₂O₅/OH- ratio 1/3 at total concentration of 15 ppm. In addition, you have to apply Morpholine and Hydrazine (Steam conditioner & Oxygen scavenger) at a level 5-10 ppm. This is providing good protection and smooth performance of high pressure steam boilers.

Sincerely,

A. Saeid and Dr...

Actually for high pressure boilers especially at 120 bar, with a demineralized make-up water, coordinated pH/PO₄ treatment is mainly for caustic corrosion protection. Again, the PO4 to sodium (Na) ratio is NOT 3:1 but should be between 2.2 - 2.8 to 1, best at about 2.5 - 2.6 to 1. Running at higher would increase the caustic corrosion as at higher caustic ratio, you'll be at the free caustic region. The pH of the boiler water should also be at 9.1 - 9.8 with PO₄ level at about 6 - 12 ppm.

Deposition with demin. water is unlikely unless you have iron corrosion. This can be tackled using dispersants added into boiler.

As for oxygen control (obviously, elimination of oxygen would eliminate corrosion cell), dosage of oxygen scavengers such as hydrazine (suspected carcinogen) or hydroquinone, should be based on the residual dissolved oxygen level - below 5 ppb in the boiler make-up water is acceptable.

If steam is being condensed, at the point of condensation, the pH would be low due to the presence of carbonic acids (from the CO₂ in the steam). This would drive the pH down and will cause corrosion in the condensate lines and equipment. The best way is to use amines to neutralize the pH and further bring it up to at least above 8.8. The feed rate would be targeted to get pH of above 8.8 in the condensate water.

Boiler chemical treatment is tricky and can SERIOUSLY pose damages to boilers and subsequent equipment. You should get advice from chemical specialty company in this field, such as GE Water and Process Technologies (formerly known as Betz).

Regards.

A. There are some questions [and responses] about boiler corrosion. (Mc Connell 1998, Pace 1999, Hao 2000, Nasse 2000, Saide 2001, Netiv 2001, Gansad 2001). In the discussions no attention has been given to the corrosion resistance of the carbon steels. I did a lot of corrosion research on boiler corrosion and erosion. Of course, for an existing boiler you cannot change the carbon steel quality but in case of failures you can do something. Besides for existing boilers you can get a feeling of the corrosion sensitivity of the boiler if you analyze samples of the failed tubes.

Best regards,

A. There shouldn't be much concern of corrosion on steel if the surface is clean, low levels of dissolved oxygen and no critical excursion in causticity or acidity.

There is also a test called Deposit weight density test of boiler tubes, to see over time the weight of corrosion products and deposition per unit area on tubes. This is destructive to the specific tube but can save the plant before waiting for accidental tube ruptures (before tube metallurgical analysis can be done).

Regards,

A. Dear Mr. Saeid Shahmohammadi,

If you use a phosphate stabilizer to boiler feed water then you should not face a calcium phosphate precipitation. Such phosphate stabilizers are polymers with good dispersency performance on Iron oxides , Ca and Mg compounds as well as Silica.

Kind regards,

A. To minimize deposition in your high pressure boiler, please ensure to maintain the below recommended water characteristic of your boiler operating at 120 bar:

Feed water: Demin water with pH 8.5 to 9.5, total hardness should be ND (not detectable.

Boiler water: Na3PO4 (Sodium Phosphate) 3 to 10 mg/li or 5 to 15 mg/li as PO4- (phosphate), caustic alkalinity 5 mg/li as CaCO3, total dissolved solids 100 mg/li max., chloride 5 mg/li as Cl- max.

I hope this would help.

A. Dear Harsh In order to ensure reduction in the boiler corrosion, following aspects are very important
(a) Use of correct water, as specified by the OEM
(b) Use of correct grade of fuel and additives if any recommended.
(c) Ensuring correct PH value and salinity in side a boiler.
(d) Slow initial steam raising process, which is generally accelerated for timely steam requirements.
(e) Preservation of boiler Not in use is the most important and often neglected part.
(f) Correct amount of additives for dousing and the proper flushing through of the residual sludge will ensure greatly reduced pitting inside and proper cleaning of the tubes outside will ensure lesser carbon deposits, hotspots and thus reduction in pitting.
The economizer areas are generally more sensitive to acidic corrosion and this is more prominent in the initial steaming conditions when the flue gases are are lesser in quantity and they are at low temperatures by the time they come in contact with the economizer elements. This causes sulfur in the fuels and moisture to produce acid and the deposits cause the damage.

The bends and the weld seams are also more susceptible to corrosion as the depositions are higher in these areas and also the stress concentration is far more than in the straight tube part.
Since galvanic corrosion is more to do with initial selection of material for boiler erection, I would not dwell much on this.
Hope I have partly answered your question.

A. Dear mrs Sahar Taghi pour

REACTION:
N2H4+ O2 => N2+ 2H2O
DECOMPOSITION REACTION:
2N2H4+ HEAT + 2H2O => 4NH3+ O2
CONTROL LIMITS:
RESIDUAL N2H4 AT ECONOMIZER INLET Hydrazine

Hydrazine

ADVANTAGES:
1-DOESN'T CONTRIBUTE TO TDS
2-TRUE RESIDUAL TEST

DISADVANTAGES:
1-POOR REACTIVITY WITH LOW TEMPERATURE
2-EXPENSIVE COMPARED TO SULFITE
3-SUSPECT CARCINOGEN
4- REQUIRES SPECIAL HANDLING / FEED EQUIPMENT
5-DECOMPOSES TO NH3 WHICH CAN LEAD TO COPPER CORROSION

Carbohydrazide

Indirect reaction
(N2H3)2CO + H2O =>2N2H4+ CO2
2 N2H4+ 2 O2=>4H2O + 2 N2
Decomposition
(N2H3)2CO + H2O =>2NH3+ N2+ H2+ CO2
1 ppm Carbohydrazide liberates
15 ppb NH3
14 ppb CO2

The results indicated the suitability and efficiency of Carbohydrazide oxygen scavenger as an alternative to hydrazine in SWCC high pressure boiler provided the concentration of residual hydrazine (decomposition by-product of Carbohydrazide) is maintained at levels between 30-40 ppb in the feedwater.
- No harmful degradation by-products were found.
- Carbohydrazide was found to be a good oxygen scavenger at a concentration dose rate of0.7 ppm.
- Optimized dose rate resulted in maintaining the boiler chemistry with in design limits.

A. Dear Rakesh,

Always Check pH, Silica, Conductivity, Total Hardness before entering any types of water in Boiler & pH should be maintained by adding morpholine (as per pH 8.5-9.0)

Regards

A. Dear amit sawant,
Red feed water could be caused by too much oxygen in the water. Please check your de-aerator. Oxygen should be taken out in deaerator. Keep steam pressure 0.1 bar in the de-aerator.
Hopefully it helps you.

A. Hi Nimesh.
I look after a boiler and we add a protective chemical mix to ours.
The solution is brown in colour, having tannin as one of the chelating ingredients. When we analyse the water from the boiler, part of the assessment of the state of the water is the fact that the water is brown in colour, If it looks like murky tea, or here in the UK a pint of bitter, (warm beer) then the dosing is about right. So depending on the type of boiler dosing chemical this is not necessarily a problem. The scale that builds up in boilers tends to be brown anyway. So as long as the pH of the water is around 12, the TDS is about 2000 to 3000 ppm and the caustic alkalinity is around 800 then all is well. If you are still concerned, you can always get the inside of the boiler inspected by a qualified and registered expert to check for cracks, or thinning of the fire wall or tubes. He will be able to put your mind at rest.
Also as you say it is a new boiler, so it should be fine.
But be certain with an expert on site opinion.
Best regards