Chemical Cleaning Process

8/4/2019 Chemical Cleaning Process

1/7

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


2/7

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 ).


3/7

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


4/7

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


5/7

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


6/7

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 :-


7/7

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

Chemical Cleaning Process

Documents

Transcript of Chemical Cleaning Process