CHAPTER
I
INTRODUCTION
1.1
Definition of corrosion
'Corrosion'
alloys
is
defined
as the destruction
of
metals
as a consequence of chemical. or electrochemical
or
reaction
with the environment.
In
nature
compounds,
metals
i.e.,
are
minerals.
present in
the
form
of
Some amount of energy is
chemical
needed
extract metals from their minerals and the same amount of
is
evolved
involving
during
the
corrosion.
chemical
or
electrochemical
to
energy
reaction
Thus, in corrosion phenomenon, the
metals
revert to their natural stäte (1).
1.2
1.2.1
Costs of corrosion
Economic losses
The
economic losses, due to corrosion, are divided into
direct losses and
2) indirect losses.
1)
Direct losses include the
costs of replacing the corroded structures and machinery or their
components.
Indirect losses include shut down, loss of products,
loss of efficiency, contamination of product and the costs due to
over design (2).
2
Were it not so insidious and ubiquitous, corrosion could
classified
as
humankind.
the
As
greatest
it is,
econoinic
calamity
corrosion is the
known
root
cause
to
be
the
of
many
Corrosion takes an even more significant toll on human
life
industrial econonüc calamities (3).
1.2.2
Human life'and safety
and safety.
fatal
Corrosion is scarcely reported as the cause of
accidents, but a few examples can be cited
spectacular
away
readily.
The
airline accident in which part of the fuselage
tore
during flight in Hawaii was in all probability due
combined
many
effects
of cyclic mechanical
stress
and
to
the
atmospheric
corrosion in a semitropical environment (4).
The
well-known
Virginia,
bridge
collapse
at
Pt.
Pleasant,
killed 46 people in 1967 and this has been
West
attributed
to stress corrosion cracking (5).
Selective corrosion of a welded liquid gasoline pipeline
Minnesota
in
1986 (6) resulted in massive fire
damage
to
in
the
entire town and also in the loss of two lives.
Again
in 1986,
a steam pipe
suffering from
the
combined
effects of corrosion and erosion, burst at a nuclear power
in Virginia, severely burning eight nearby workers, four of
died later.
plant
whom
3
These
are only a few examples of many such fatal
acciclents
caused by corrosion.
1.3
Methods of corrosion prevention and control
The
various
methods
employed for
corrosion
control
are
summarised in the form of a chart shown in Fig. 1.1.
1.4
1.4.1
Corrosion control by inhibitors
Corrosion inhibitor - definition
Corrosion inhibitor is defined as a chemical substance which
when
added
to
concentration,
the
corrosive
environment
at
an
optimum
decreases the corrosion rate of metals or
alloys
significantly.
1.4.2
Classification of corrosion inhibitors
et ai.,
Putilova
(7)
classified corrosion
inhibitors
into
three types :
Type 1 : Those which form a protective film on the metal surface.
Type 2 ': Those
which reduce the aggressiveness of the
corrosive
media.
Type 3 : Those which function by forming n protective film and at
the
same
corrosive
time
agent.
reducing
the
aggressiveness
of
the
5
Another
classification
is
based
on
the
environment.
Inhibitors have also been classified into three types, viz.,
inhibitors,
neutral
and alkaline inhibitors, and
vapour
acid
phase
inhibitors.
(8)
Recent classif ication of inhibitors is due to
Deans et cil . ,
who
based
mechanism
formers
have
of
2)
classified
them into four
inhibitive action.
neutralising.
types
They are :
inhibitors
1)
on
barrier
the
layer
3)
scavengers,
and
4)
is as anodic
inhibitors,
cathodic
miscellaneous.
Another
classification
inhibitors and mixed inhibitors. This classification requires the
knnwlodijo of pnl nr i u<\\' i nn ddtvi,
Figure 1.2 i 1 HiHtratos the i",choniatic ,u:tivation polariuation
curves
showing
interface
the pertinent processes at
the
under impressed electromotive force
metal/solution
(EMF)
conditions
(9).
Figure
I.3a illustrates the significant terms for a
corroding
metal.
line
represents
ECD
intersection
the
open
indicates
currunt).
of
The line EaD represents the anodic
the
cathodic
reactions..
anodic and cathodic reactions
The
*D'
circuit corrosion potential (E c o r r ) of the
the
magnitude
of
corrosion
at
icorr
freely
reactions;
point
of
establishes
metal
and
(corrosion
Figure I.3b is a schematic diagram showing the relation
of
metallic corrosion (D) , protection (F and G) and inhibition
(P) .
Generally, the immersed metal may be corroding by reaction
under
anodic control (EaF) by the use of anodic type inhibitors;
under
cathodic control (ECG) by the use of cathodic type inhibitors; or
by
mixed control (Ea-EcP) where the inhibitor controls both
anodic
and cathodic reactions.
It is roadily apparent
the
that
an
inhibitor whieh can control both tho maeti ons ii:; more ofluotive.
It is found that both anodic and cathodic type inhibitors
reduce
tho. corrosion curront (anodic : A i|? cathodic : ^ h ) / <*ncJ
thi.it
the
moro
"mixed"
inhibitor
rnducer.
tho
corrosion
curront
effectively ( A i 3 ) •
Anodic
inhibition
rutuU.ti; tn
(jrcntor
ijhiltu
in
anodic
Tafel slope and cathodic inhibition results in greater shifts
cathodic
anodic
Tafel
and
slope.
In the case of
mixed
cathodic slopes are shifted to an
inhibition
equal
extent
in
both
or
there is not much change in the Tafel slopes.
The anodic inhibitor shifts the corrosion potential
anodic
tiidu
potential
and
towards
thu cathodic inhibitor
cathodic side,
ühiltii
thu
whereas both shifts
rualiuod in thu c.uiu oi <i niixud J nhi bi tui:.
towards
uorrot.1 lon
will
be
1.5
Theories of inhibition of corrosion
v
Corrosion
Inhibitors' has been the central theme
of
many
reviews (7,10,11) and symposia (12,13).
The action of inhibitor in-acidic as well as in alkaline and
neutral media has been mainly explained by adsorption and/or film
formation. In the case of acid inhibitors, they can also act,
addition,
by
affecting
the
kinetics
of
hydrogen
in
evolution
(hydrogen overpotential theory).
1.5.1
Adsorption-theory and effect of molecular structure
Machu
predicts
substance
(14,15)
the
that
the
blocking
formation
with
characteristic
put
high
the
of a porous
electrical
case
chemical
of
layer
inhibitive
resistivity,
an
essential
(16)
considered
get adsorbed at the
inhibitors,
which
the
Uhlig
metal
active sites and affecting the
organic
theory
of
metal by virtue of their net charge.
the
adsorption.
of corrosion inhibition.
inhibitors
the
forward
surface,
potential
thus
of
Riggs (9) proposed that
adsorption ' depends
the
in
on
the
nature of the inhibitor molecule, the environment,
the
hature of the metal surface and the electrochemical potential
the metal/solution interface.
Adsorption was further
classified
as a) pi-bond orbital adsorption b) electrostatic adsorption
c) chemisorption.
of
and
10
Relationship
between the characteristics of the
electronic
interaction at the motal/solution intorfnno arul thn structuro
of
the inhibitor molecules has been discussed in detail (17 - 19).
The
of
study of structure - corrosion inhibition
relationship
orqanic coitipounds has received much attention frora
scientists.
Hackerman
inhibition
and
Hurd (20)
plotted
corrosion
the
for ring-substituted N-methylamines ' vs
degree
the
of
Hammett
substituent
constant. In 1965, Donahue and Nobe (21) proposed
correlation
between
inhibition by organic comppunds
Chemical structure and substituent constants.
investigations
substituents
amines,
were
carried
efficiencies
and electron
the
effects
pyridines,
benzoic acids, aliphatic sulphides and
inhibitive
their
Subsequently, many
out to correlate
in related molecules such as
and
a
of
aliphatic
thiophenes, with
densities
at
functional
groups as estimated from Hammett constants (17,22-27) .
1.5.2
Hydrogen overpotential theory
The
action
of acid inhibitors was
overpotential theory.
hydrogen
amines
are
the
and
thus,
increased
hydrogen
increased
the
cathodic
Hackerman and Sudbery (29) pointed
adsorbed on the anodic and cathodic regions
metal surface.
in
by
It was believed that inhibitors
overpotential,
polarisation (28) .
explained
out
on
that
the
Hoar and Holliday (30) pointed out that the shift
initial potential of the metal in
presence
of
organic
11
inhibitors
the
in the positive direction indicates that they act
anodic
sites. This theory does not. explain
the
on
inhibitive
action of all types of systems.
1.5.3
Film formation theory
In order to explain the action of inhibitors in neutral
alkaline
media, Evans (31) proposed the film
formation
Evans attributed the inhibition to the formation of an
film.
According
inhibition
is
to
Putilova et ai.,
(7),
in
acid
due to the formation of a layer pf
and
theory.
insoluble
solution,
insoluble
or
slightly soluble corrosion product on the metal surface.
1.5.4
Electrochemical polarisation theory
Explaining the action of passivating inhibitors, Stern
suggested
that
inhibitors
like chromate
get
reduced
at
(32)
the
cathodic sites and consequently raise the electrode potential
to
more
on
noble
value
bringing
about
passivation.
Studies
passivating inhibitors (7) reveal that only a small amount of the
total
current
inhibitors.
could be associated with
Thus, Stern's
theory
behaviour of all types of systems.
cathodic
also
cannot
reduction
explain
of
the
1.6
Synergistic effect and corrosion inhibition
Mixture of inhibitors frequently provides better
inhibition
than either of the individual components; i . e . , the mixtures
synergistic.
thirties,
This
was recognised by Speller (33) in
the
who reported finding "compound filnis", such as
are
mid-
formed
by phosphate - chromate mixtures, to be roore effective than those
of either alone.
The
synergistic effect of halides and organic compounds
inhibitors
37) .
has been frequently reported in the 'literature
as
(34-
It is suggested that the synergism could be interpreted
as
a co-adsorption of both halide anion and organic cation
directly
on
the
two
to
the
the
species
metal
on
surface (38).
At high
concentrations,
the iron surface transforra from
adsorption
formation of a complex film (37-39).
Synergistic
as
effect has been shown by various mixtures
such
K 2 Cr0 4 and urotropine, NaV03 and potassium antimonyl tartrate
(40) polyphosphate and orthophosphate (41) , disodium sebacate and
benzotriazole (42), potassium iodide' and dicyclohexylamine
(43),
halides and benzyl amine (44).
Synergistic effects of mixtures of corrosion inhibitors have
been
reviewed and discussed at length (45, 46).
effect
The
inhibiting
of different organic phosphorus-containing compounds
the synergistic effect of various additives have been studied
and
by
13
a new mathematical model (47).
Synergism was established in> the inhibition of corrosion
steel
in
aqueous chloride media by a combination
oxidiser and monoethanolamine (48).
effect
of
Molybdate shows
inorganic
synergistic
with various species such as nitrite (49, 50),
(49, 51, 52), Zn 2 + (51-54), phosphonic acid
(55),
of
phosphate
tolyltriazole
(50), silicate (56), polymeric dispersants (55),.monoethanolamine
(57), and
hydroxy carboxylates like
sodium
salicylate,
sodium
gluconate and sodium malate (58).
Sodium dodecyl succinate and sodium nitrite' show synergistic
effect in controlling the corrosion of steel in simulated cooling
water
circulates
benzotriazole
(59). . A
show
mixture
synergistic
effect
of
in
ethylxanthate
and
controlling
the
corrosion of copper in 0.IM NaCl at pH 7-11 (60) .
A
synergistic
inhibitory
benzylthiocyanate (CgH5CH2SCN)
effect
of
BiCl 3
and
on corrosion of iron in IN
H2S04
was
investigated using polarisation and impedance
measurements,
and
it was shown that a composite protective film was formed
on
the iron surface by a combination of both the conipounds (61).
Synergistic
Cu 2
+
, Ni 2
+
hydroquinone
on
effects of morganic cations like
2 +,
As 3 , + Sn
, H g 2 + and organic cations of phenol, formaldehyde and
on the inhibitive performance of propargyl
steel dissolution in boiling HC1 solution has been
alcohol
reported.
14
Propargyl alcohol controlled the corrosion rate by the
of a surface layer at the interface.
another
species
formation
Its effect was enhanced
was present in the solution that
promoted
if
the
film formation (62).
A mixture of borogluconate and corrin-type inhibitors showed
synergistic effect in preventing the corrosion of iron in neutral
media (63) .
The
combination consisting of 25 ppm
sodium
borogluconate
and
25 ppm nitrite ion showed synergistic effect in
the
corrosion of mild steel in aqueous solution.
controlling
containing
100
ppm Cl" (64).
It has been reported that the inhibitive power of gluconates
and
gluconic
acids increases with the addition of
because of their combined synergistic
A
borate
mixture
borate
effect (65).
consisting of sodium salt of
ATMP,
phosphates,
and silicate was found to be effective in inhibiting
corrosion
of steel,
Cr-Ni steel,
ions
east iron,
AlMg2
alloy
the
and
Al (66).
Phthalic
anhydride, ATMP and zinc ion as a three
component
inhibitor has good adsorption and corrosion inhibition properties
(67).
16
synergism
in
the
inhibition
process;
Health
and
safety
requirements control the selection and usc oi inhibitors.
1.7 oi
chromates
Chroraate
Chroraate
of
is
a
very
effective
passivating
inhibitor.
passivation usually involves formation and
maintenance
a protective ferric oxide film (76).
referred
has
to as anodic inhibitors.
also
Chromates
are
However, cathodic
been reported (77,78). Passivation
with
inhibition
chroraate
sensitive to various coramon anions such as chloride and
(79,
80).
declined
The
in
use
recent
of chromates
years
at
because
high
of
usually
sulphate
concentration
health
is
has
and
safety
recirculating
cooling
considerations.
1.7.2
Molybdates
Molybdate
systems
have
alternative
based
based treatments for open
become
popular
the
early
1980s
to the toxic and ecologically unacceptable
inhibitors.
Molybdate
prevent
the
aqueous
solutions (82).
inhibitors.
since
has been
used
corrosion of iron (81) and mild
as
steel
as
an
chroraate
inhibitor
in
Molybdates when used alone, are
to
neutral
anodic
Molybdate is found to be an efficient inhibitor
in
aerated distilled water but inefficient in deaerated water (83).
1.7.3
Pertechnetate
The
protection
reported.
A
surface.
of carbon steel in 13 ppm . KTcO^
protective
Technetium
tetravalent
is
oxide
film is
forraed
recluced from the
on
has
the
heptavalent
been
metal
to
state. Structures in'water can be protected
the
against
corrosion and fouling by coating the metal with " Te (84).
1.7.4
Nitrite
Nitrite ion is a well known oxidising type of cooling
water
inhibitor because of its unique roie in forming and repairing the
oxide
film
faetor
(85, 86).
which
discourages
recirculating
impaired
by
Bacterial decomposition of nitrite
systems
(87).
the
use
of
nitrites
The effectiveness
sulphate and nitrate;
in
of
the presence of aggressive ions such
is
open
nitrite
as
when these are present above the
a
is
chloride,
critical
concentration, localised attack will be developed (88) .
1.7.5
Phosphates
Phosphates
are frequently used in the
polyphosphate
form.
They control the corrosion of ferrous metals.
Polyphosphates aet
as
effectiveness
cathodic
polyphosphates
inhibitors
as
(89-91) .
The
inhibitors is inereased by
solution, but decreased by rise in temperature.
the
movement
of
of
1.7.6
Silicates
Silicates have the advantages of low eost and' non-toxicity.
Sodium
silicate has been used as a corrosion control
agent
for
open-circulating cooling water system (92).
1.7.7
Cations
Cations
in
inhibitors.
constituents
cathodes.
dissolved
adequate
The
of
cations
the
These
concentrations
form
solution
insoluble
in
insoluble
alkaline
deposits sereen
aet
with
medium
at
loeal
cathode
from
process,
and
the
Aluminium cations, for
are effective inhibitors for mild steel in water.
inhibition
oxide.
is
due
cathodic
salts
oxygen, thereby polarising the cathodic
thus, inhibiting the corrosion.
as
to the precipitation
of
the
example,
The
corrosion
hydrous
aluminium
The inhibition efficieney may be itnproved by the presence
of thiourea (93) .
1.7.8
Organic inhibitors
Several
corrosion
organic compounds have been used as
of
mild
steel
in
aqueous
inhibitors
neutral
of
environment.
Gluconates and gluconic acids are known to be effective non-toxic
inhibitors
for
iron and mild steel in cooling water
The organic surfactant, l-decylimidazole, inhibits the
(94,
95).
corrosion
19
of
iron in neutral solution of 3 percent sodium
The
chloride
inhibition of <*- and Y- pyrophthalones on the
iron
and mi Id steel in near neutral. aqueous
(96} .
corrosion
sulphate
has
been studied.
the
organic molecules on the metal surface as well as
of
solutions
The inhibition is explained by adsorption
by
of
their
incorporation into the oxide film (97).
1.7.9
Carboxylates
Several carboxylates such as sodium salicylate (98) ,
cinnamate
(99), anthranilate (100), and adipate (101) have
used as inhibitors.
and
iron
sodium
Inhibitive anions like benzoate,
phthalates
other caboxylates (102-104) stabilise the oxide film on
surface.
been
Presumably, their inhibitive action results
the
from
the bonding of the anion to the metal surface by excess electrons
on
are
the 0~.
Carboxylates are anodic inhibitors.
sensitive
to
the
presence
of
higher
Therefore
concentrations
they
of
aggressive iona.
1.7.10
Tannins
Tannins
inhibitors
environment.
are
of
vegetable origin and
(105-107).
They
are
are
good
inoffensive
Sodium tannate has been used in the
corrosion of mild steel in neutral solutions (108) .
corrosion
towards
the
inhibition
of
20
1.8
Review of literature on phosphonates as inhibitors
The
need
to
develop
chromates,
nitrites
and
stinmlated
research
work
inhibitor
inorganic
formulations
phosphorous
on . "
free
from
compounds
Phosphonates
as
has
corrosion
inhibitors '*.
Phosphonates
phosphonic
are
acids.
sodium, potassium or
Phosphonates
provide
ammonium
the
salts
following
of
key
aggregation of properties :
a)
They
form complexes with metal ions
at
stoichiometric
concentrations.
b)
Corrosion
inhibition
Chemicals
:
In
(synergists)
the
they
presence
of
provide
other
corrosion
inhibition.
c)
Seale inhibition : They inhibit precipitation of typical
scalants at substoichiometric inhibitor concentrations.
d)
Hydrolytic
aqueous
stability
solutions
: They
even at
resist
elevated
decomposition
temperatures
in
and
extremes in pH.
e)
Chlorine
stability
: They resist
degradation
by
ppm
levels of available chlorine.
Because
inhibitors
importance.
of
in
these properties, studies
cooling water systems
have
on
phosphonates
gained
as
considerable
21
Phosphonates
inhibitors.
by
themselves
effective
corrosion
However, several Chemicals synergise the
corrosion
inhibition by phosphonates.
with
are
not
Phosphonates show synergistic effect
2-aminothi.ophenol (109), zinc
(1:10-114),
dichromate
(110,115-117) and carboxylic acids (111).
Among
(ATMP)
the phosphonates, amino(trimethylene phosphonic acid)
and
l-hydroxy-ethane-1,l-diphosphonic
acid
(HEDP)
are
widely studied.
Although the structure of ATMP is quite different, it shares
many of the properties of the polyphosphates.
1..
It
forms complexes with calcium and
öther
polyvalent
metal cations.
2.
It is a good peptizing agent and dispersant.
3.
It stabiilses iron and manganese-bearing water (118).
4.
It
stabilises supersaturation with respect to
calcium
carbonate and calcium -sulphate (119,120).
5.
It
forms
protective
films
on
metal
surfaces
(110,115,121) .
Kubicki
property
aqueous
ai., have investigated
(111)
the
inhibitive
of ATMP in preventing the corrosion oi. iron in
solution;
carboxylic
layer
et
acids
capacitance
The influence of ATMP, in
neutral
combination
and zinc salts upon the change of
the
with
double
on iron and the inhibitive properties of
the
22
mixture were examined.
The greatest decrease of capacity and the
greatest protective effectiveness were obtained for tri-component
compositions
containing ATMP, phthalic acid or oxalic
acid, and
Zn 2 + ions.
The
tests
to
results
of the potentiodynamic
cathodic
polarisation
of Carter and Denohue (122) show that increase in pH
8.0)
and calcium (as CaC03) increase
corrosion
(6.5
inhibition.
They also observed that with ATMP, the metal surface did not have
any
visible film.
colours,
Film
indicating
formed
under
With chromate based treatments,
with
adherent protective
ATMP alone is thin and
films,
interference
are
rapidly
breaks
severe conditions such as high flow rate, low pH
transfer.
present.
down
or
heat
To overcome these inherent disadvantages, phosphonates
were formulated along with zinc or organic inhibitors.
The
influence of the inhibitor system consisting
of
ATMP,
oxalic acid or phthalic anhydride, and zinc sulphate on the
of
corrosion of steel in simulated industrial water of
Chemical
composition
and in water being
studied by gravimetric method.
a
perspective
inhibitor
for
chlorinated
rate
variable
has
been
This formulation is considered as
the
corrosion
protection
of
industrial cooling and heating water installation (123).
It has been observed that the addition of sodium sulphite to
Zn2+-ATMP formulation increases the degree of protection of
iron
23
in a certain range of inhibitor concentration.
by
This is
attained
the effective slowing down of the reduction of oxygen on
the
steel (124) .
The
influence
of
divalent
ions
and
ATMP
on
corrosion
inhibition of low carbon steel in neutral water (NaCl 30 + N a 2 S 0 4
70
mg/1)
has
weight-loss
been evaluated
method.
by
polarisation
resistance
It has been reported that
the
and
protective
properties depend on the solubility products of hydroxides of the
metal-complexing agents and on the stability of the coniplexes
of
the metals with the ligand. An equation was obtained relating the
minimal
protective
indicated
the
parameters.
protective
that
concentration
based
on
properties has
of
the
inhibitor
A comparative analysis was also
properties of complexonates based on
HEDP.
with
The source of the
the
made
of
ATMP
with
in
their
differences
been indicated (125).
HEDP has been used as corrosion and seale inhibitor for mild
steel
(126).
Addition
of
triazole
to
' phosphonate-zinc
combination is shown to improve the corrosion protection of
steel.
improved
The
corrosion
inhibition
efficieney
of
HEDP
by combination with dichromate (115-117) or zinc
mild
can
be
salts
(71,127) or C a 2 + (127).
The corrosion of steel and iron has been studied in
neutral
solutions containing NaCl, Na 2 S0 4 and HEDP and its complexes with
transition
metals.
potential
were
The
measured
corrosion
as
a
composition and temperature.
to
hydroxide
corrosion
have
of
the
elcctrode
the
electrolyte
The possibility of passivation
have
also
been
of
OH~
discussed
aqueous solution containing HC0 3 ~, C a 2 + , Cl"
investigated
combinations
was
(129) .
The
influence
of HEDP and molybdate species on the
studied in both buffered and
the
(128).
sa.lt
HEDP
and
corrosion
unbuffered
in
S042-
and
of
due
in
inhibiting properties of HEDP and its sodium
been
iron
function
and
and the participation
protection
Corrosion
neutral
formation
rate
of
electrolytes
(130) .
The
mixture. consisting
gluconate
salt
of
HEDP,
and zinc sulphate effectively inhibited the
of steel in water (131).
of
of sodium
phosphonate
sodium
corrosion
It has been observed that decomposition
industrial
cooling
waters is decreased or prevented by treatment with water
soluble
sulfamic
acid
inhibitors
at
by chlorine
0.25 ppm/ppm
in
chlorine.
These
phosphonates
include HEDP and tris(aminomethäne phosphonic acid) (132).
The corrosion of iron in 0.1M Na 2 S0 4 exposed to air at pH
was
inhibited by 1,6-hexane diphosphonic acid (133).
organic
products
are
containers
after
inhibitors
optiohally
laurylhydroxy
stored
addition
of
serving
diphosphonic
in
non-aqueous
100 to
as a
1500
colour
phase
ppm
of
Corrosive
in
steel
corrosion
stabiliser.
acid reduced the corrosion
6
Thus,
rate
of
25
steel immersed in fatty acids (134) .
Phosphonate-terminated carboxylate-based telomers
have
been used as scale/corrosion inhibitors and/or fouling agents
in
aqueous
by
systems (135). Corrosion inhibition of carbon
steel
2-phosphonobutane-l,2,4-tricarboxylic acid is based on
of an inhibiting film (136).
of
carbon
steel
phosphono
tricarboxylic
manganese
acid)
ion
It has been reported that corrosion
can be inhibited by
carboxylic
acid
(e.g.,
and/or
(137).
formation
addition
an
organic
2-phosphonobutane-l,2,4-
phosphino
Corrosion
of
carboxylic
inhibiting
acid
and
compositions
containing carboxylated phosphor.ic acid (hydroxy phosphono acetic
acid
or
seguestrant
acid,
2-phosphonobutane-l, 2,4-tricarboxylic
acid)
selected from tartaric acid, citric
acid,
and
a
gluconic
and sulphur-containing lignosulphonates have been
studied
(138).
The
composition
containirig
the steel corrosion
inhibitor
hydroxyphosphono acetic acid and the' copper corrosion
inhibitor,
tolyltriazole,
(139).
corrosion
(tap
has
been
used
as
seale
inhibitor
inhibitor for iron-based alloys in an
aqueous
water) comprises hydroxyphosphono acetic acid or its
soluble
ammonium
or alkali metal salt and a
co-polymer
A
system
water
formed
from 2-acrylamido-2-methylpropane sulphonic acid' and acrylic acid
or methacrylic acid (140).
is
a
Carboxyhydroxymethyl phosphonic acid
suitable ferrous alloy corrosion
inhibitor,
for
cooling
26
towers.
This
decomposes
to
substituted
inhibitor
is
nontoxic . to
aquatic
life
simple compounds (141). Hydroxy- and/or
phosphonic
acid and diphosphonic
acids
and
carboxy-
usable
as
corrosion and seale inhibitors have been reported (142).
Aminophosphonic
acid/phosphate
mixtures
for
controlling
corrosion of steel and inhibiting calcium phosphate precipitation
in
cooling water has been reported (143). Water in open
system
is
phosphonic
treated
as
diethylenetriamine-penta(methylene)
with
ethanolamino
esters to prevent corrosion and sealing.
Optionally,
the effect is inereased by adding anionic polymeric
dispersants,
phosphate
acid
with
cooling
its sodium salt,
along
azole compounds, zinc salts, chromates, phosphates, ete.
Seale and corrosion of metal surfaces in contact with
(144).
industrial
process waters is inhibited by treated waters with an amido alkyl
phosphonate
containing
inhibiting
polyamides
water
properties
in
acrylic
of
acid
phosphonic
polymer
acid
the inhibition of corrosion of
has been studied (146) .
(145).
The
derivatives
of
carbon
steel
The formulation consisting
of
in
a
vater soluble organic phosphonate and an N-substituted acrylamide
tpolymer' has
been
used for inhibiting
seale
formation
and/or
cäispersing ions in reverse osmosis systems (147).
Phosphonoalkyl
benzene
has- been
Corrosion and seale on metals (148) .
used
as
inhibitor
of
Bis(2-chloroethyl) phosphonate
vinyl
and
bis ( 2-chloroethyl)
phosphonate have been used in corrosion inhibiting
compounds
for
phosphonomethylated
steel
surface
derivative
(149).
A
rubber
phosphono-N-
of caproamide has been
used
as
inhibitor for metal corrosion (150) . Phosphonated hydrolysed poly
(maleic anhydride) is an excellent anti-corrosion agent and seale
inhibitor
Cl-
in a cooling water system containing < 40,000 ppm
(151).
The
of
effect of zinc phosphonates on the cathodic
steel
in
sea water has
been
diseussed
protection
(152) .
Corrosion
protection of aluminium alloys in aqueous solutions of
chloride,
by zinc phosphonates has been reported (153) .
The
corrosion behaviour of L-62 brass was studied in
water
containing 230 ppm Cl - and 620 ppm S 0 4 2 ~ , and in the presence of
phosphonic
acids (154). The effects of corrosion
prevention
of
benzotriazole phosphonic acid derivatives on elution of copper in
acidic or sodium chloride solution have been diseussed (155).
In recent years various phosphonates and polymeric compounds
tlave
been
this
regard
(156)
has
elaimed to show corrosion-inhibiting
properties.
phosphonic acid like hydroxyphosphono
been used.
The use of polymers
acetic
include
acid
polyacrylic
*cid, polymethacrylic acid (157), sulphonate containing
'l58, 159), the most important ones being based on
In
polymers
2-acrylamido-
?. H
2-methylpropanesulphonic
acid
(160).
N-Phosphonomethylated
derivatives of polyamides have been used as inhibitors for
corrosion
programme
(162).
(161).
with
The
An
alkaline
cooling
water
ATMP and sodium polyacrylate has
perf
ormances
of
HEDP
and
metal
treatment
been
acrylic
reported
acid
based
terpolymer have been studied (163).
The
calcium
co-polymer,
is
phosphonate
stabilising
capacity
of
acrylic acid-hydroxyl ethylacrylate sulphate
a useful property in all organic cooling
water
the
ester,
fonnulations
(164).
A
brief
inhibition
review
on the mechanistic
aspects
is also worth mcntioning here.
of
The
corrosion
influence
of
salts of various phosphonic acids on the chemical passivation
of
pure iron and mild steel in neutral aqueous solution by dissolved
oxygen
has
passivation
been investigated.
It is shown
that
is not attainable in the presence
of
the
chemical
phosphonates.
In fact, the passive state of iron or mild steel surface achieved
in
air-saturated
sodium benzoate solution is destroyed
phosphonates (165) .
The
corrosion
the
roie
of
the
•
the
ligand nature
in
inhibition
by phosphonates has been reviewed.
corrosion
by
inhibition
is
due to
the
^omplexes with different solubilities (166).
of
It is
shown
formation
of
metal
that
metal
29
The use of phosphonic acids (e.g., Cublen K60, Skiantan W Kf
Trisphon)
cooling
as
corrosion inhibitors for carbon steel pipes
water
system
has
been
examined
and
the
of
a
inhibition
mochan i sm i n nn snd on tho formnt: i on • of oxidr-froo cnlcj.um
phosphonate protective layer (167).
Corrosion
by
disodium
inhibition of carbon steel (in 3% NaCl
monoalkyl
formation
of
monoalkyl
phosphonates
phosphonates
has
been
a relatively compact, adherent
solution)
reported.
film
of
The
disodium
is responsible for the decrease
in
the
corrosion rate (168).
Increase
in
corrosion
rate
with
increasing
inhibitor
(phosphonate) concentration beyond the optimum value is explained
by the formation
Aluminium
have
of corrosion product-inhibitor complexes (169) .
corrosion inhibitors based on
gem-diphosphonates
been prepared and examined in water/glycol
solution.
The
inhibitor protected aluminium by forming a waxy, hydrophobic film
(170).
It
has been reported that some phosphonic acids
inhibition
effect in HC1 solution.
may
exert
These effects depend on
štructure of phosphonic acids, i.e., the numbers and position
nitrogen atoms and phosphono groups (171).
the
of
29
The use of phosphonic acids (e.gr., Cublen K60, Skiantan W K,
Trisphon)
cooling
as
corrosion inhibitors for carbon steel pipes
water
mnchnn.inm
system
i r.
bnr.fd
has
on
been
tho
examined
formation
and
oi"
the
of
a
inhibition
oxido-freo
cnlcium
phosphonate protective layer (167).
Corrosion
by
disodium
inhibition of carbon steel (in 3% NaCl
monoalkyl
formation
of
monoalkyl
phosphonates
phosphonates
has
been
a relatively compact, adherent
solution)
reported.
film
of
The
disodium
is responsible for the decrease
in
the
corrosion rate (168).
Increase
in
corrosion
rate
with
increasing
inhibitor
(phosphonate) concentration beyond the optimum value is explained
by the formation
Aluminium
have
of corrosion product-inhibitor complexes (169).
corrosion inhibitors based on
gem-diphosphonates
been prepared and examined in water/glycol
solution.
The
inhibitor protected aluminium by forming a waxy, hydrophobic film
(170).
It
has been reported that some phosphonic acids
inhibition
effect in HC1 solution.
may
exert
These effects depend on
štructure of phosphonic acids, i.e., the numbers and position
nitrogen atoms and phosphono groups (171).
the
of
30
The
tjueous
gised
corrosion
niedia
of
iron and low
in the presence of
on them has been studied.
j.nc
carbon
steel
complexons
and
On the basis of
in
neutral
compositions
studies
using
phosphonate inhibitors of corrosion, it was found that
jitremely
effective
protection of steel over a wide
pH
an
range,
jow rate and temperature is possible owing to the formation of a
elatively thin but insoluble films on the metal surface (172).
The
2S04
nd
corrosion rate of steel has been investigated
as a function of pH.
in
The influence of adding 0.1
0.1M solution of Fe 2+ , Co 2+ , Mg 2 f , Z n 2 + , A l 3 + and
xamined.
As
a result of the formation
of
Cr3+
insoluble
ompounds in slightly acidic or near neutral sulphate
M
0. 5M
HEDP
was
surface
solutions,
he rate of corrosion significantly decreased (173).
The
Ca
lon
concentrations have
orrosion
inhibition
of
HEDP.
orrosion
inhibition
is
different
oncentrations.
n
little-
However,
frora
the
high
concentrations
^ 2 + deposited
on
the
mechanism
of
to
low
Ca 2+
When Ca 2 + concentration is low, HEDP is adsorbed
the surface of steel and performs as a mixed
igher
effect
on
inhibitor.
of Ca 2+ , HEDP forms a complex
the steel surface, and acts
rihibitor to prevent corrosion (174) .
as
film
a
At
with
cathodic
31
Increased
cathodic polarisation is shown as
the
principal
inhibitive function of the ATMP - zinc mixture (110) . It has been
shown
by infrared and X-ray photoelectron
spectroscopies,
that
ATMP reacts with aluminium surface to form a coating which is
an
effective inhibitor (175).
Our
review
of literature indicates
corrosion
inhibition are
that
largely
the
mechanistic
aspects
of
based
results
of the weight-loss studies and electrochemical
upon
the
methods.
Surface exanunation techniques like X-ray diffraction, uv-visible
reflectance
FTIR
spectroscopy,
studies
and
spectroscopy have rarely been used so far in examining
the
nature of the film.
studies
have
monophosphonates
so
uv-visible luminescence
The review of literature also shows that
far
like
been
carried
ethyl
phosphonate and phenyl phosphonate.
out
phosphonate,
on
the
no
simple
2-chloroethyl