Internal corrosion and control in Boiler

     In spite of external and internal  treatment  the concentration of impurities in the boiler water goes on increasing continuously as the generation of steam continues. In order to keep the concentration of boiler water below a set figure, a part of the boiler water is blown off to waste and fresh feed water is filled in to replace it. The blow-down ( or blow-off) may be done periodically or preferably, continuously. But recommended at least crack open in continuous form . During the blow-down process, a part of concentrated water containing dissolved impurities and suspended sludge is removed and the remaining saline get diluted with the fresh feed water make-up.

 

§               CORROSION

 

§   Corrosion of boiler tubes, plates, economizers and pipe lines can be majorly attributed to one or more of the following factors:

§  Presence of free acids in water.               

§   Acids generated as a results of hydrolysis of some salts in water.

§   Acids formed by the hydrolysis of fatty lubricating oils.

§   Presence of dissolved gases such as O2,CO2,H2S etc., in water.

§   Presence of salts like MgCl2 which directly attack the boiler metal:

   MgCl2 + Fe+ 2H2O -------------------- Mg(OH)2 + FeCl2 +H2

§  Presence of salts like MnS2 which may generate H4SO4 due to oxidation and hydrolysis.

§  Formation of galvanic cells.

 

§  Dissolved oxygen

Dissolved oxygen is the chief source of corrosion in boiler and ancillary equipment. It is highly desirable to removes the dissolved oxygen from boiler water by treatment of physical and chemical treatment. The concentration of dissolved oxygen in boiler water should be 0.005 ppm  for low pressure boiler and high pressure boiler should be 0.001 ppm .

 This dissolved oxygen enter in boiler water through make up water and also condensate (make up water) . This mixed dissolved oxygen convert the internal metals in oxide forms by means that whenever it heated oxygen liberated in free form .This free oxygen react with internal metals and convert it in oxides which is initiation of corrosion .

§  Removal of dissolved oxygen

 

According to law of Dolton and law of Henry   

The solubility of gas is directly proportional to pressure and inversely proportional to temperature .According to law of Dolton and Henry designed   deaerator to remove oxygen by giving mechanical treatment.

 In this treatment (Mechanical treatment )dissolved oxygen removed by injecting hot water spray into vacuum chamber heated externally by steam .for this deaerator design in according to removal of maximum best level.

 

Further for complete removal of dissolved oxygen chemical treatment is given accordingly   to their design, pressure and temperature basis.

  Sodium sulphite is used in low pressure boiler which is suitable up to 650 PSI Boiler and its convert into sodium sulphate.it is very effective in low pressure boiler but it is not possible in high pressure boiler because of increasing dissolved salts and concentrate produce foaming in boiler . Another thing is sodium sulphate decomposes into SO3 and H2S .Due to high concentration priming and carry over might be takes places.

 

 

Na2SO3 + ½ O2 ------------------------- Na2SO4 (effective in low pressure)

 

 

 

For high pressure boiler recently adopted very effective for removal of dissolved oxygen is  Hydrazine Hydrate (40% ) aqueous solution is uses . because its higher concentration is highly inflammable having boiling point 113.5 0 c temperature .In  this combination or treatment there no other compound is produced .There liberates nitrogen gas and H20 .Nitrogen gas is already inert gas .

 

N2H4 + O2 ------------------H2O +N2

 

Molecular weight of both Hydrazine Hydrate and dissolved oxygen is same (32) Due to this (1.0 PPM) dissolved oxygen required 1.0 ppm hydrazine hydrate.

 

Excess amount of hydrazine hydrate should also avoid ,it also decomposed  and converted into ammonia which is also corrosive and attack on some alloys  like copper alloys in condenser tubes.

 

3N2H4 ------------------------    4NH3  +  N2

 

 

Dissolved  Mineral Acids

 

In general most of the natural waters are alkaline excepting   mine area and some polluted area from acidic industrial in which wet oxidation of sulphides   minerals occur.

   

Some inorganic   salts like magnesium chlorides and calcium chloride are also corrosive agents. Magnesium chlorides (Mgcl3) hydrolyzed completely and convert into hydrochloric acid at 200 temperature.

 

MgCL2 + 2H2O  ------------------------------------------   Mg (OH)2 +2HCL

 

Calcium chlorides also producing mineral acids but lesser than Magnesium chloride   so must be avoid presence of magnesium chlorides. This hydrochloric acid attack on boiler metal and detoriated  metal quality.

 

   Fe + 2HCL  --------------------      FeCL2  + H2

 

FeCL2 + 2H2O --------------------------   Fe (OH) 2 + 2HCL

 

In boiler if magnesium chloride is present it chain of HCL  formation gets again and again  and metal get corrosion very easily .It can be properly neutralized by alkali adding and can be avoid  corrosion from mineral acids .

 

§  Dissolved carbon dioxide

In water carbon dioxide and bicarbonates are present. It further troubles in boiler steam and further it convert into carbonic acid .which cause pitting on internal part of local area.

In deaerator dissolved carbon dioxide also can be removed by Mechanical   treatment . And co2 removed by treatment of soda stone.

CaCO3 + Co2+ H2O   -------------------------  Ca(HCO3 )2  (In this process temporary hardness takes places)

 

For removing of carbon dioxide present in boiler water neutralized   by adding liquor ammonia which it converted in ammonium carbonate .

However if excess ammonia added to feed water there is possibility of it will enter into condensate and attack on condenser copper tubes. Hence  it should be use in safe mode as 10 mg recommended.

 

§  Formation of galvanic cells

Corrosion   can also occur because of galvanic cell formation between iron and other metals present in the alloys used in boiler fittings. This may also lead to pitting corrosion. This can be prevented by suspending zinc plates which act as sacrificial anodes

 

§    Caustic embrittlement.

 

 Caustic embrittlement is a form of corrosion caused by a high concentration of sodium hydroxide in the boiler water. It is characterized by the formation of irregular inter granular cracks on the boiler metal, particularly at places of high local stress, such as riveted seams, bends and joints. It is caused by the high concentration of NaOH which is capable of reacting with steels stressed beyond their yield point. It is most likely to occur in boilers operating at high pressures, where NaOH is produced in the boiler by the hydrolysis of Na2CO3 as follows:

              Na2CO3+H2O ----------------------------- 2NaOH + CO2

The extent of the hydrolysis increases with temperature and may reach even90% of the carbonate present. The rate and extent of corrosion by caustic embrittlement increases with the concentration of NaOH and temperature and hence with increasing operating pressure.

       NaOH reacts with iron forming magnetic oxide and hydrogen.

          3Fe+4OH ------------------------------Fe3O4+ 4H

        Under normal conditions  in unstressed metal a fairly continuous film of oxide is produced. When the metal is stressed beyond its yield point, the oxide coating cracks and chemical attack continues into the metal mainly along grain boundaries. This may be due partly to the energy stored there and partly to the increased E.M.F produced as a result of the stress. The products of the reaction viz., Fe3O4 and hydrogen also tend to favour penetration along grain boundaries. The attack is considerable under these conditions because of the large area of the metal exposed.

      It is observed that boiler waters containing sodium sulphate or sodium phosphate inhibit the caustic embrittlement either by crystallizing out and plugging the capillaries and crevices with the solid salts ( and prevent the infiltration of NaOH) before a dangerously high concentration of NaOH has been produced or that these salts act as buffer solutions and lower the E.M.F. to such an extent that the erosion cannot occur.

   Generally, the concentrations of Na2 SO4: NaOH are maintained at 1: 1,2:1 o 3:1 for operating pressures of 10,20 and >20 atmosphere respectively to check caustic embrittlement boilers. However, the requirements for water conditioning and prevention of caustic embrittlement may clash with each other , because a very high concentration of Na2SO4 may lead to the formation of CaSO4 scales. Under these conditions, Na3PO4 should be used . Na3PO4 is an effective conditioning agent and also is over 300 times as effective as Na2SO4 in suppressing embrittlement.

        Caustic embrittlement may also be prevented by adding lignin or tannins which help in blocking the infiltration of NaOH through the hair-cracks. Addition of NaNO3 neutralization of excess alkali are also used to prevent embrittlement.