Corrosion mitigation in boiler plant (Part 2)

Excerpt: Boilers are large and expensive installations which can suffer enormously from damage caused by corrosion mainly and erosion, aggravated further by high temperatures

Corrosion Process

Dr. P.K. KAMANI

Corrosion mitigation in boiler plant (Part -2)


Dr. P. K. Kamani (Associate Professor) Department of Oil and Paint Technology Harcourt Butler Technological Institute, Kanpur - 208 002



ABSTRACT

Boilers are large and expensive installations which can suffer enormously from damage caused by corrosion mainly and erosion, aggravated further by high temperatures. The exact type of damage experienced varies from one part of a boiler to another and is influenced by the feed water, combustible fuel and boiler design. The most common causes of corrosion are dissolved gases (primarily oxygen and carbon dioxide), under- deposit attack, low pH, and attack of areas weakened by mechanical stress, leading to stress and fatigue cracking. The corrosion can be mitigated by taking some precaution: treating the feedwater to coating the metal surface.


Introduction

IN the first part of the subject we studied about the type of boiler and the causes of boiler trouble with special stress upon feed water. In this part we will find the way of control of corrosion in the boiler.

To prevent corrosion in boilers generally the water is conditioned i.e. the water is treated by chemicals, pH is changed or mechanical measures are done. Three types of water are generally encountered: raw water, condensate return from steam turbines and boiler water. The conditioning results in: freedom from deposits, absence of internal corrosion and free from carry over boiler water solids into steam.

Water treatment

There are various methods of water treatment to mitigate the corrosion problem, of which coagulation and filtration of suspended materials are the oldest techniques. In coagulation the chemicals (coagulating agents e.g. alum, iron sulphate etc.) are mixed rapidly in water which form a precipitate (floc) of small particles. The flocs are coagulation of small particles which are further brought together by gentle agitation to form large coagulation particles. These large particles settle down and removed. The coagulation is followed by chlorination to remove organic matter by destruction. Some time the particles are not removed by coagulation then the water is filtered by using charcoal filters.

After the coagulation and filtration process the hardness forming constituents e.g. calcium and magnesium salts, present in the water should also be removed as these are active corrosion centres (discussed in earlier parts). The ion exchange is one of the popular and effective method of removing these hardness constituents. To remove hardness constituents (Ca++ and Mg++ ) the water is treated with ion exchange resin. In this process the undesired ions of a given charge are absorbed from solution within an ion permeable absorbent. The hardness constituents are removed by sodium zeolite resin. This process utilizes materials that have the property of exchange of hardness ions of water. The hardness ions (Ca++ and Mg++ ) are replaced by Na+ ions present with zeolite. The reaction can be depicted as:

Sodium ions from resin and scale forming calcium and magnesium ions from water are exchanged.

Here, the active sodium zeolite can be regenerated by treating with strong salt solution (brine)

After the process, the water contains bicarbonate, sulphate and chloride. Only Ca++ and Mg++ are exchanged for Na++ ions. No reduction in dissolved solids and no reduction in bicarbonate alkalinity results. The bicarbonate alkalinity can be removed by soda-lime process. In this the hydrated lime is used to react with bicarbonate.

The calcium carbonate, being precipitate can be filtered easily. If the magnesium bicarbonate is present about two times the lime is used to remove hardness.

Above water softening process would contain more sodium bicarbonate in boiler water, and produce caustic soda which is highly corrosive in nature. Hence another modification of the process has been suggested known as “hot lime zeolite softening process”. In this process silica is reduced, as Mg is precipitated as Mg(OH)2 which acts as an absorbent for silica. The residual hardness is removed by sodium zeolite softener

Demineralization

In demineralization, both metal ions and salt anions are removed. The metal ions are removed with the hydrogen zeolite system and the anions are removed by a resin saturated with hydroxide ions. It is desirable at drum pressures over 1000psi to remove silica in boiler water as silica may deposit on the turbine surface. It involves two ion exchange resin, a cation exchange, and an anion exchange. The water is passed through a cation exchange resin:

The anions are removed by anion exchange resin, either weak or strong base. All the acid is removed by exchange of anion part of the acid (negative) by hydroxide.

Mixed bed exchange

Water is finally passed through a mixed bed comprising of cation and strong anion resin bed. At this stage nearly all salts are removed.

Boiler feed water is defined as Feedwater = Makeup Water + return condensate

Dissolved bicarbonates of calcium and magnesium breakdown under heat to give off carbon dioxide and form insoluble carbonate. We have discussed the severe attack of carbonate by way of depositing on the boiler tube in the first part. Similarly calcium sulphate, being less soluble, deposits with silica directly on the boiler tube surface in extremely hard scale form. It is therefore imperative to treat the feed water to minimise corrosion problems in boiler. The notorious corrosion role of dissolved oxygen and carbon dioxide were discussed in previous part. The best way to control corrosion is to keep the pH in the range of 8.5 to 9.2. The oxygen from the feed water must be removed even lower than 7-10ppb. A complete removal of oxygen requires the scavengers. The most common oxygen scavengers are sodium sulphite and hydrazine. The sodium sulphite reacts directly with oxygen as:

The reaction is catalysed by cobalt salts and takes place immediately between pH 5 and 8 at elevated temperature. Common sodium sulfite should contain 0.25% of cobalt salt. The only disadvantage is that some quantities of sodium sulphate are introduced in the system which may cause scaling problems in the systems. Hydrazine is other scavenger. Hydrazine is also known as corrosion inhibitor. Other reductive inhibitor is amines. Hydrazine (N2H4) being carcinogenic is avoided. Its reaction is given below

Nitrogen drives off oxygen out of solution i.e. hydrazine converts oxygen, a common corrosive agent to water. The related inhibitors of oxygen corrosion are hexamine, phenylenediamine and dimethylethanolamine and their derivatives. The amount of scavengers should be controlled as excess can produce sulphurous and ammonia gases which are highly corrosive. A comparison between sodium sulphite and hydrazine is given here below:

  1. Na2SO3 is safe and easy to handle whereas N2H4 causes skin irritation and must be used with care.
  2. Sodium sulphite reacts more rapidly than hydrazine.
  3. Sodium sulphite form sodium sulphate on reaction with oxygen which contribute solids to boiler water, whereas hydrazine does not.
  4. Hydrazine reduces ferric oxide to a protective magnetite as shown below, whereas sulphite does not.

The condensate systems which are the integral part of water boiler systems, are also affected by the corrosion. Normally the condensate systems are made up of iron and copper. Iron is more damaged than copper. The major corrosion reactions taking place with iron, in condensate are :

  1. Carbonic acid corrosion
  2. Oxygen corrosion
  3. Combination of carbonic acid and oxygen corrosion

Copper and copper alloys in condensate systems are also affected as, for example ammonia dissolve the protective layer of copper Cu2O. In presence of oxygen ammonia is more corrosive.

Antioxidants such as sulfite and ascorbic acid are sometimes used. Some corrosion inhibitors form a passivating coating on the surface by chemisorption. Benzotriazole is one such species used to protect copper. For lubrication, zinc dithiophosphates are common - they deposit sulfide on surfaces.

Benzotriazole inhibits corrosion of copper by forming an inert layer of this polymer on the metal's surface.

The suitability of any given chemical for a task in hand depends on many factors, including their operating temperature and pH.

Mitigating condensate corrosion

The following methods can be helpful in controlling the corrosion.

  1. Minimizing oxygen and carbon dioxide in the system by mechanical and chemical methods i.e. by deaeration and scavenging, as discussed earlier.
  2. Minimising air leakage. Neutralization of carbon dioxide

Neutralization of carbon dioxide

The neutralization of carbon dioxide is done by using chemicals which are volatile so that they can be removed with the steam. The chemicals used for this purpose are organic compounds e.g. amines (R-NH2 ). Examples are cyclohexylamine and morpholine, benzylamine and aminomethyl propanol (AMP) and diethylaminoethanol.

(Morpholine) (cyclohexylamine)

These compounds neutralize the acids by hydrolysing the condensed water to form a base.

The more the hydroxyl ion is produced the more acid is neutralized. These amines volatilize with steam and condense at the same temperature as water. Morpholine and cyclohexylamine can also be used together or separately to neutralize carbon dioxide but the pH should be maintained around 8.5 to 9.2.

Filming amines

This process is used to prevent corrosion when the concentration of carbon dioxide and oxygen is very high and their removal is not economical. In this process long chain organic molecules of 10-18 carbon atoms with one amine group at the end is used. These organic compounds prevent corrosion by forming a layer on the surface and preventing the contact of environment with the metal surface. For example octadecylamine which do not neutralize carbon dioxide but form a non wettable film on the metal surface by absorption on the surface and act as barrier coating between condensate and metal. The amine group is polar due to its being ionic, and absorb physically on the metal surface. A one molecule thick coating of amine (C18H38NH2 ) having oil like property does not allow oxygen to come into contact with the surface. The use of filming amines has been successful in low pressure boiler to prevent corrosion. The pH range is between 6.5 to 7.5. They have following advantages and disadvantages.

Advantages

  1. They are economical.
  2. They are effective against oxygen and carbonic acid.
  3. They are widely used chemicals.
  4. They form highly protective films by which they resist corrosion by restricting the ingress of carbon dioxide and oxygen.

Disadvantages

  1. The film can be destroyed by oily matter
  2. The film can be stripped by high velocity water.
  3. The film is degraded by solid contents

Hydrazine has been listed as a carcinogen therefore its use has been restricted. Some new compounds have been marketed since 1980, like carbohydrazide, hydroquinone and diethylhydroxylamine.

Internal treatment of boiler water

The formation of deposits (scales, sludges, corrosion products etc. ) should be prevented as these act as insulator causing loss of heat. The thicker the deposit, the greater the driving force requires for heat to penetrate to raise the temperature. There are several methods to control deposits. Some of the important methods are discussed here below. The following three methods are generally employed in industrial boilers:

  1. Conventional treatment methods
  2. Coordinated phosphate methods
  3. Chelant treatment

Conventional treatment methods

This is commonly used method which involves the addition of phosphate and caustic to boiler. The caustic is added to maintain a pH in the range of 10.5 to 11.2. This practice is used in high-pressure boilers. In this process, calcium and magnesium salts are precipitated in a sludge form. The reactions involved here are given below:

The products formed above are not very adherent and can easily be removed by blow down.

Coordinated phosphate methods

In this method, trisodium phosphate is added to maintain the level of free caustic. Trisodium phosphate is hydrolysed to produce hydroxide ions. Phosphate acts as buffer to minimise caustic cracking. The reaction is given below:

Chelant treatment

Chelants have been used successfully to control the formation of deposits. They form soluble complexes with metal cations. Chelants of calcium and magnesium are soluble. Some common chelating agents used are ethylenediamine tetraacetic acid (EDTA), nitro triacetic acid (NTA), etc. The chemical equation for chelation is similar to that for ion exchange. For instance, by adding the sodium salt EDTA to water

The formation of CaCO3 is eliminated, as shown in the above equation. Extreme care is needed to control this program. The following are the limitations in application of chelants:

  1. The feed water hardness should not exceed 2ppm.
  2. The ferric ion concentration in feed water and condensate must be low.
  3. Oxygen should not be trapped even in the shutdown period.

Polymer treatment

The polymers are being used in controlling the deposits in boilers of up to 1500lb/in2 . The treatment solubilises Ca++ , Mg++ , Al+++ and maintain silica in solution. It removes scale from boilers. The effectiveness of a polymer depends upon its molecular weight and concentration. For example , polyacrylic acid ( mol. weight = 20000) if added reduces scale formation of only 52% compared to polymeric acid (mol. wt. = 5000) which can reduce scale formation by 97%. Some typical polymers in practice are polycarboxylic acid, polymethacrylic acid, styrene and maleic acid.

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