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CHEMICAL CLEANING PROCESS
DURING THE NORMAL OPERATION OF THE BOILER THE
INTERNAL SURFACE OF THE PIPES IS COVERD BY A LAYER OF
OXIDES MAINLY COMPOSED OF MAGNETITE ( Fe3O4 )
ACCORDING TO THE CONTACT BETWEEN IRON ( Fe ) AND H2O
STEAM AT TEMP.= 200 400 C THE FOLLOWING EQAILIBRIUM
IS ESTABLISHED:-
3Fe + 4H2O = Fe3O4 + 4H2
A STABLE EQUILIBRIUM CREATS A HOMOGENEOUS LAYER
OF MAGNETITE PREVENTING THE IRON FROM GETTING
THROUGH THE SOLUTION.
THE NECESSITY OF PERIODICAL CHEMICAL CLEANING IN
ORDER TO GUARANTEE REGULAR OPERATION OF POWERS
STEAM BOILERS DUE TO THE EXPERIMENTAL OBSERVATION
OF MAGNETITE LAYER NOT ALWAYS UNIFORM AND
HOMOGENEOUS.
BOILERS DIRTY DEPOSIT LIMITS
STEAM DRUM BOILERS = 4.0 GR/M2
HYPER CRITICAL PRESSURE BOILERS = 2.5 GR/M2
DIAGRAM OF CHEMICAL CLEANING PHASES
FOR A STEAM GENERATOR CHEMICAL CLEANING, THE
BASIC TREATMENTS MAKE PROVISION FOR ALL OR SOME OF
SUBSEQUENT PHASES OCCURING AND SPACED OUT BY
DEMINERALIZATION WATER INTERMEDIANT RINSES
PREBOILING PHASE
AIMS AT REMOVING GREASE AND OIL SUBSTANCES AND AT
SOFTENING THE OXIDES FOR THE SUBSEQUENT ACID
ATTACK
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FIRST DECOPPERING PHASE
IT WILL BE NECESSARY IF THE CONTENT OF COPPER EXEEDS
5 % OF TOTAL DEPOSIT CONCENTRATION.
ACID PHASE
AIMS AT REMOVING THE OXIDES AND DEPOSIT THIS PHASE
CAN BE PERFORMED BY USING INORGANIC ACID (HCl, HNO3)
OR ORGANIC ACID ( CITRIC ACIDE , FORMIC ACID , ACETIC
ACID )
COMPLEXATION , NEUTRALIZATION , TEMPORARY
PASSIVATION AND COPPER REMOVAL PHASE
AIMS
AT COMPLEXATION OF IRON BY CITRIC ACID
AT NEUTRALIZATION OF SOLUTION
AT FORMATION OF A POWDER MAGNETITE FOR TEMPORARY
PASIVATION
AT REMOVAL OF COPPER RESIDUE
IN SERVICE PASSIVATION
DURING THIS PHASE A COMPACT AND CONTROLLEDMAGNETITE FILM WILL BE FORMED
IN A LITERAL SENSE SOME CORROSION MUST ALWAYS TAKE
PLACE IN AN OPERATING BOILER, THE UNIFORM SURFACE
OXIDATION OF STEEL BY DEARATED WATER TO FORM
MAGNETITE ( WHICH IS PROTECTIVE AND SELF INHIBITING )
IS DEVIDERABLE AND TUBING . THE OPTIMUM AVARAGE
THICKNESS OF MAGNETITE LAYER IS ABOUT 0.15 MM
(MILLIMETERS ).
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BASIC CHESTRY OF CLEANING PROCESS
ALKALINE PHASE
THIS STEP IS OPTIONAL AND ISUSED PRIMARILY FOR
SOLUBILIZING ALL TYPES OF ORGANIC MATERIAL BEFORE
ACID CLEANING . GENERALLY WHENEVER ORGANICS ARE
USED IN WATER TREATMENT OR OIL CONTAMINATION IS
EXPECTED AN AL;ALINE PRE-BOIL IS NECESSARY
ACID PHASE REACTIONS
CHEMICAL DISSOLUTION OF MAGNETITE
Fe3O4 + 8 H+ ------------ > 2 Fe +3 + Fe+2 + 4 H2O
THIS REACTION TAKES PLACE IN THE FIRST PHASE OF ACID
CLEANING. ALL METAL COMPOUNDS OR MIXED OXIDES ARE
DISSOLVED IN THE SAME WAY.
GALVANIC DISCHARGE OF FERRIC ION
Fe + 2 Fe+3 ------------- > 3 Fe+2
ELECTROCHEMICAL DISSOLUTION OF Fe3O4
Fe3O4 + Fe + 8 H+ --------- > 4 Fe+2 + 4 H2O
THIS REACTION IS INHIBITED BY CORROSION INHIBITOR
THAT PREVENTS H+ FROM DISCHARGING OR THE BARE
METAL WITH ORGANIC METAL COMBOUNDS
IRON DISSOLUTION FOR HYDROGEN CATHODIC DISCHARGE
2 H+ + Fe --------------> Fe+2 + H2
THIS REACTION OCCURS IF CORROSION INHIBITOR FAILS OR
GETS MALFUNCTION . IN THIS NEGATIVE CASE IT CAN BE
OBSERVED A SUOLOLEN INCREASE OF CORROSION RATE
AND HYDROGEN DEVELOPMENT.
IRON DISSOLUTION FOR OXYGEN CATHODIC DISCHARGE :-
1- O2 + 2Fe + 4H+
--------------- > 2Fe+2
+ 2H2O
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2- O2 + 2Fe+2 + 2H+ ----------- > 2Fe+3 + H2O
THIS IS THE REASON WHY THE CIRCULATION IS PERFORMED
IN THE ACID PHASE UNDER NITROGEN INJECTION.
Copper dissolution for Fe+3 AND O2 CATHODIC DISCHARGE:-
2Fe+3 + Cu ------- > 2 Fe+2 + Cu++
O2 + 2Cu + 4H+ -------- > 2Cu++ +2H2O
IRON DISSOLUTION FOR Cu++ CATHODIC DISCHARGE :-
Cu++ + Fe ---------- > Fe+2 + Cu
SECONDARY REACTION OF OXIDIZED COPPERREDEPOSITION.
THIS COPPER COVERS WITH A LAYER THE BOILER'S WALL.
DECOPPERING PHASE :-
ANODIC OXIDATION REACTIONS : COPPER DISSOLUTION
Cu + 2NH3 -------- > Cu(NH3)+
2 + e-
Cu(NH3)2 + 2NH3 ----------- > Cu(NH3)4++ + e-
Cu + Cu(NH3)4++ ------------- > 2Cu(NH3)2
+
CATHODIC REDUCTION REACTION
BrO3- + 3H2O + 6 e- --------- > Br- +6OH-
S2O8-2 + 2e- ----------- > 2SO4--
O2 + 2H2O + 4e- -------------> 4OH-
FINALLY THE FORMATION OF COPPER AMMONIA COMPLEX
OCCURS
Cu(NH3)4 (OH)2 OR Cu(NH3)4 . SO4
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CH2 - COOH CH2 - COO
OH OH
C + Fe --------------- > C COO ------------ > Fe
COOH COO
CH2 COOH CH2 - COO
TEMPORARY PASSIVATION
THE PROBABLE FUNCTION OF NITRITES CAN COM OUT
THROUGH THE SUBSEQUENT REACTIONS :
NaNO2 + H2O = NaOH + HNO3
2HNO3 = N2O3 + H2O
3HNO2 = HNO3 + 2NO + H2O
3Fe + 8HNO3 = 3Fe(NO3)2 + 2NO + 4H2O
2Fe + 2NO = 2FeO +N2
FeO + H2O = Fe(OH)2
AND Fe(OH)2 WOULD CHANGE INTO MAGNETITE
3Fe(OH)2 = Fe3O4 + H2 + 2H2O
MAGNETITE FORMED DURING THE TEMPORARY ASSIVATION
IS POWDERED AND NOT YET COMPACT.
ONLY OPERATION PASSIVATION WILL SHAPE A COMPACT
AND HOMOGENEOUS LAYER OF MAGNETITE.
CONTROLLED FINAL PASSIVATION
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THE AIM OF CONTROLLED PASIVATION IS THE FORMATION
OF A THIN COMPACT LONG LASTING LAYER OF PROTECTIVE
MAGNETITE. THIS FILM IS OBTAINED BY ACTING ON
SUBSEQUENT.
PARAMETERS : TEMPERATURE , HYDROTIVE , PH , REDUCING
ENVIRONMENT AND HYDROGEN DEVELOPMENT.
MAGNETITE IS OBTAINED FROM DIRECT REACTION OF
METAL WITH WATER :-
3Fe +4H2O --------- > Fe3O4 + 4H2
BY CONTROLING THE REACTION KINETICS , THAT'S TO SAY
BY PROMOTING DIRECT GROWTH UPON THE METAL AND BY
AVOIDING MAGNETITE REMOVAL.
A FEW MICRON MAGNETITE LAYER IS OBTAINED AND IT
PROVES TO BE AS PASSIVATING AND PROTECTIVE AS TO
REDUCE , IN OPERATIONAL PHASE , THE OXIDES FILM
GROWTH. TEMPERATURE: RANGE 200 300 OC
IF T < 200 OC METAL WATER REACTION IS VERY SLOW , FOR
T > 300 OC THE REACTION CANNOT BE CONTROLLED
BECAUSE OF HIGH RATE. MOREOVER THERMAL
DECOMPOSITION OF HYDRAZINE ( N2H4 ) DOESN'T SUPPLY
HYDROGEN ( H2 ) THAT IS IMPORTANT FOR THE CONTACT OF
REACTION(( 3Fe + 4H2O --------- > Fe3O4 + H2 ) )
HYDRAZINE (N2H4) : PREVENTS THE OXIDATION OF
PREVIOUSLY PASSIVATED SURFACES ACCORDING TO THE
REACTION :-
N2H4+ O2 ------------- > N2 + H2O
PH :- OPERATING PH MUST BE ALKALINE IN ORDER TO MAKE
IRON CORROSION IS DEAERATED WATER LOWEST.
THE REACTION (( 3Fe + 4H2O --------- > Fe3O4 + H2 ) ) ACCORDING
TO SCHIKORR IS THE RESULT OF THE SUBSEQUENT
REACTION :-
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3Fe + 6H2O = 3Fe(OH)2 + 3H2
3Fe(OH)2 = Fe3O4 +2H2O+H2
MOREOVER HYDRAZINE COMPLETES THE FORMATION OF
MAGNETITE BY CHANGING THE POSSIBLE RUST INTO Fe3O4
ACCORDING TO THE FOLLOWING REACTION :-
6Fe2O3 + N2H4 --------- > 4Fe3O4 + 2H2O + N2
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