Shreir's Corrosion VOL IV

870 Pages·1998·45.29 MB·English

by  Editor-in-Chief: Tony J.A. Richardson

4.01 Surface Pretreatment
G. L. Higgins
Chemetall Ltd, Aylesbury, UK
R. S. Hullcoop
Ray Hullcoop and Associates, High Wycombe, UK
S. Turgoose
CAPCIS-INTETECH, Manchester, UK
W. Bullough
Corus Research Centre, Rotherham, UK
This article is a revision of the Third Edition articles 11.1 and 11.2 by G. L. Higgins and R. S. Hullcoop & S. Turgoose and
W. Bullough, volume 2, pp 11:3–11:23, ß 2010 Elsevier B.V.
4.01.1 Introduction 2484
4.01.2 Chemical Cleaning 2484
4.01.2.1 Solvent Cleaning 2484
4.01.2.1.1 Vapor degreasing 2484
4.01.2.1.2 Emulsifiable cleaners (water rinsable cold solvent cleaning) 2485
4.01.2.1.3 Emulsion cleaners 2485
4.01.2.2 Neutral Cleaners 2485
4.01.2.3 Acidic Cleaners 2485
4.01.2.4 Alkaline Cleaners 2486
4.01.2.5 Acid Pickling of Ferrous Materials 2487
4.01.2.5.1 Formation of millscale 2487
4.01.2.5.2 Mechanism of scale removal 2488
4.01.2.5.3 Hydrogen embrittlement 2489
4.01.2.5.4 Pickling of alloy steels 2489
4.01.2.5.5 Pickling inhibitors 2489
4.01.2.6 Acid Pickling of Nonferrous Metals 2491
4.01.2.6.1 Electrochemical cleaning 2491
4.01.2.7 Ultrasonic Cleaning 2491
4.01.3 Mechanical Cleaning 2491
4.01.3.1 Flame Cleaning 2491
4.01.3.2 Manual Cleaning 2491
4.01.3.3 Abrasive Blast Cleaning 2492
4.01.3.3.1 Surface finish 2492
4.01.3.3.2 Surface profile 2492
4.01.3.4 Ultrahigh Pressure Water Jetting 2492
4.01.3.5 Health, Safety, and the Environment 2492
4.01.4 Standards 2493
References 2493
Workpiece The item, component, or
Glossary
material that is undergoing surface
Millscale An oxide that forms on steel during high
pretreatment.
temperature processing (e.g., hot rolling) and
must be effectively removed prior to the
application of any subsequent surface
protection.
Surfactant A substance that lowers the surface Abbreviations
tension of a liquid, allowing easier spreading BS British Standard
across the surface of a material. EDTA Ethylene diamine tetra-acetic acid
2483
2484 Surface Treatment and Modification
removed dissolve in the cleaning medium, for exam-
EN European Norm
ple mineral oil in chlorinated solvents. Detergency is
ISO International Standards Organisation
the ‘lifting’ action attributed to some alkalis and to
TLV Threshold limit values
special surface-active agents commonly referred to as
VOC Volatile organic compounds
surfactants. Chemical reaction is characterized by, for
example, the saponification of some oils in strong
alkali, or the reaction of rust with acid solutions.
4.01.1 Introduction The main types of cleaners used for the removal of
organic contaminants are solvent cleaners, neutral
The attainment of a clean surface prior to the applica- cleaners, acid cleaners, and alkali cleaners.
tion of any subsequent treatment or coating is essential,
whether this subsequent operation is electroplating,
4.01.2.1 Solvent Cleaning
anodizing, chemical treatment, or organic coating. The
standard of cleanlinesswhichmust be achieved has been The traditional use of flammable hydrocarbon
stated to be ‘‘that which will allow the subsequent solvents (e.g., white spirit or paraffin) used either
process to be carried out satisfactorily’’; however, this by immersion or by manual application is not
is an almost meaningless statement. Often it is industry to be recommended as an effective or particularly
experience that provides a guide to the standard of safe method of degreasing. When used by immersion,
surface preparation required. As an example, the degree the holding tank can become heavily contaminated
of cleanliness required to satisfactorily zinc plate from and the contaminants will remain on the work after
an acid solution is somewhat higher than that required the solvent has evaporated. The use of solvent-soaked
prior to zinc plate from a high-cyanide alkali zinc rags, although a time-honored procedure, is now
solution. However, this should never be taken as a being frowned upon on the grounds of operator
license to skimp on surface preparation and in another safety; aqueous based prewipes are available.
example the arguments over the degree of pretreatment
required for ‘surface-tolerant’ paint coatings abound 4.01.2.1.1 Vapor degreasing
and will probably continue. It should be remembered The use of hot/boiling solvents, with either immer-
that it is to a large extent true that problems of early sion of the articles to be cleaned in the bulk solvent
failure in metal finishing are traceable to incorrect or or in the overlying vapor using specially designed
insufficient surface preparation. installations, is an effective use of solvents for clean-
ing purposes. The solvent, which traditionally was of
a halogenated hydrocarbon type, is held in a sump at
4.01.2 Chemical Cleaning the base, which is heated by any suitable means and
under thermostatic control. Above this may be a wire
Chemical contamination consists of oils, greases, pre- mesh on which the workpieces are rested. The solvent
servatives, or old paint coatings whichmust be removed is condensed near the top of the chamber by cooling
prior to further finishing. Paint coatings can be removed coils; thus, between the mesh and the coils is created a
by chemical paint strippers (e.g., solvent or alkali region where the solvent is in vapor form. When cold
based); however, these methods will not be discussed items are introduced, the vapor condenses upon them
further here. Sources of most organic contamination and liquid solvent flows off, thereby removing contam-
are, for example, cutting and machining fluids, preser- ination. To a large extent, only clean solvent is vapor-
vatives, oils, and greases from, for example, rolling ized, thus ensuring that only fresh solvent is used to
operations, press lubricants, and mechanical or manual clean the workpiece until the sump becomes overcon-
handling operations. Various means of removal have taminated, when the solvent must be cleaned or
been proposed: mechanical action; solvency; deter- replaced. Care should be exercised with some metals,
gency; and chemical reaction. In all cleaning operations notably aluminum, that solvent with free chloride is not
one or more of these mechanisms will contribute more used, as this could lead to pitting of the metal surface.
or less to the overall cleaning procedure, dependent Currently, the use of chlorinated solvents is
upon the cleaning method and solution employed. increasingly deprecated in view of the extent of
With chemical cleaning, performance will be release of volatile organic compounds (VOC). There-
enhanced by the use of mechanical action, such as fore, the advice of the manufacturers of the installa-
brushing, air agitation, spraying, electrolysis or ultra- tion and the suppliers of the solvent should always be
sonic agitation. Solvency is where the materials to be heeded in the operation of these installations to ensure
Surface Pretreatment 2485
their trouble-free running. Effective fume control by spray, as either a preclean in a multistage pretreat-
must be available above the installation and the work ment line, or as the cleaner in an industrial washing
must be removed slowly enough to ensure that all the machine. Such washing machines are often used to
solvent has evaporated from the work before it leaves clean parts which are contaminated with cutting oils
the extracted area. Also, as per governing legislative and which require inspection before storage. Like the
rules, exposure limits for solvents are increasingly emulsifiable cleaners, the emulsion cleaners, after rins-
being reduced and current advice must be sought. ing, often leave a hydrophobic surface which is resis-
tant to short-term corrosion. Emulsion cleaners can be
4.01.2.1.2 Emulsifiable cleaners (water used hot or cold. Heat generally improves the cleaning
rinsable cold solvent cleaning) action but, in most cases, leads to an objectionable
Emulsifiable cleaners (sometimes incorrectly ref- increase in the smell associated with solvent products.
erred to as emulsion cleaners) are blends of organic
solvent (often hydrocarbons) with surface active 4.01.2.2 Neutral Cleaners
agents and dispersed in an aqueous medium. The
These are rapidly replacing emulsion and alkali pro-
work is immersed in the unheated solution for a
ducts for use in industrial washing machines and are
sufficient time for the cleaner to penetrate the surface
generally used at pH7.5–9, which is considerably lower
thoroughly. The articles are then removed and water
than that of the corresponding alkali products. Neutral
rinsed. Additives in the cleaner allow the solvent to
cleaners are on the basis of chemical soaps, with addi-
emulsify in the water thus removing the contamina-
tions of surfactant (to improve cleaning, wetting, pene-
tion. Spray rinsing or agitation in an immersion rinse
tration, and defoaming), corrosion inhibitors (which
will aid removal of the residues. This form of cleaning
may be nitrite or organic), and a bactericide.
creates disposal problems as the rinse water cannot be
Neutral cleaners provide the benefits of generally
merely disposed of into sewerage systems. Thus, the
lower operating temperatures, reduced odor, easier
effluent will require storage and the emulsion is bro-
effluent treatment, and improved health and safety
ken up before discharge of the water layer and
considerations over the alkali or emulsion products.
approved disposal of the organic material. As with all
Because of the inhibited nature of the surface pro-
solvent-based materials, the need to observe thresh-
duced, such products are used for inter stage cleaning
old limit values (TLV) for personal exposure and the
and prior to assembly. The surface is generally not
need for the work to be carried out only under effec-
suitable for immediate painting.
tive fume extraction must be taken into account when
considering this type of cleaning product.
4.01.2.3 Acidic Cleaners
The cleanliness of the surface produced by emul-
sifiable cleaners is not of the highest standard, and The vast majority of acid-based cleaning products are
additional cleaning may well be necessary before designed for the removal of scale, rust, and other oxide
further finishing operations. However, in the use of films. These products may also contain solvents and
this method prior to some chemical conversion coat- surfactants to degrease and derust simultaneously.
ing processes (e.g., immersion in phosphate baths), There are, however, certain acid-based materials which
the crystal growth can be quite refined because of the can primarily be construed as cleaners. One such type of
absence of the passivation effect often encountered material is used in the cleaning of aluminum cans prior
with some heavy-duty alkali cleaners. Needless to to treating and lacquering. Such cleaners are normally
say, manufacturers’ recommendations should be fol- on the basis of sulfuric or phosphoric acid, with, gener-
lowed at all times and the suitability of any particular ally, additions of hydrofluoric acid and surfactants.
cleaning/pretreatment combination should be con- These materials are sprayed on to preformed cans to
firmed. Another benefit gained from the use of emul- remove the lubricant used during the can-forming oper-
sifiable cleaners is that the surface produced is ation. The fluoride is present to enhance the removal of
usually hydrophobic and therefore, to an extent, fine metal swarf as well as to remove the oxide film.
resistant to tarnishing and corrosion in storage. Fluoride-free acid cleaners are used for general
pretreatment cleaning of aluminum as an alternative
4.01.2.1.3 Emulsion cleaners to strong alkali materials. Although more expensive in
These are materials, containing blends of organic terms of initial make-up and plant requirements the
solvents and surfactants, which are added to water rate of material loss through cleaning and etching is
to form an emulsion. Typical concentrations are in reduced. Furthermore, the need for an acid de-smut
the range 0.5–5%. Such emulsions are normally used (required after alkali cleaning) can be eliminated.
2486 Surface Treatment and Modification
As the surface smoothing and leveling effects are 2. The soaps produced by saponification may give
somewhat limited, the use of acid cleaners prior to excessive foam during spray cleaning or react with
anodizing or electropainting, where surface defects hard-water salts to form scum and scale.
can be enhanced, is not common. Care must be taken 3. Amphoteric metals, such as zinc and aluminum, can
here not to confuse acid cleaners with high-strength, be attacked (i.e., corroded) more than is desirable.
phosphoric acid-based chemical polishes and chemical 4. Powder products formulated with too much
brighteners, which are used to obtain specific types of hydroxide can be hygroscopic and thus tend to
attractive surface finish. Also in the category of acid form solid blocks in storage rather than remaining
cleaners could be considered the lightweight alkali– as free-flowing powders.
metal phosphating cleaner–coater solutions; a fuller 5. Spray cleaners based on hydroxide can pick up
discussion of such materials is best left to specialist carbon dioxide from the atmosphere and such
publications on metal pretreatment chemicals. carbonated solutions become less effective.
4.01.2.4 Alkaline Cleaners Carbonates and bicarbonates are used as lower
alkalinity adjuncts or substitutes for hydroxide and
Alkali cleaners can be used before almost every con-
are more resistant to carbonation during spraying
ceivable metal finishing operation at one stage or
than hydroxide-only solutions. Powder products
another; they can be used by spray, by immersion,
blended with light sodium carbonate are much less
by manual application or by all three, or maybe by
hygroscopic and the carbonate can be a useful ‘carrier’
two out of the three methods. They can come as
for liquid additives, such as surfactants and solvents.
powders or as made-up liquids. They may be single
Silicates can offer an almost complete cleaning
or multipack, to be used as supplied or at a range of
system on their own. Sodium silicate, the most com-
dilutions. They may require high temperatures or
monly used of these materials, has a high enough pH
work successfully at ambient temperature. They
value to cause saponification on its own and the anion
may be suitable for cleaning one metal only or have
formed gives a certain degree of detergency and
multimetal capability. The user thus has an immense
inhibition. Thus, silicates are often found in multi-
range from which to choose.
metal cleaners. Light metals, such as zinc and alumi-
Consideration will first be given to the inorganic
num will generally not be attacked if the silicate level
species used to produce the base material. The pH
is sufficiently high and the free hydroxide level suffi-
values of several commonly used materials are shown
ciently low. However, cleaners containing silicate can
in Table 1. Hydroxides are the simplest, strongest
cause problems. They should not be used prior to an
alkalis and most commonly used. A major effect of
alkaline process on aluminum, owing to the forma-
hydroxides in cleaning is saponification: the conver-
tion on the surface of alkali-insoluble aluminum sili-
sion of certain oils and greases to water-soluble soap-
cate. Cleaners containing silicates can also cause
type materials.
problems if used prior to some surface-sensitive
Beside the benefits of hydroxides must be placed
treatments, such as use of zinc phosphating solutions.
certain disadvantages:
Other common ingredients of alkaline cleaners
1. The possible passivation of iron and steel surfaces; include phosphates, which have both detergent and
this can be a problem prior to chemical conversion dispersive properties; also, the polyphosphates have
coatings. water softening capabilities. In addition, borates are
often the base for light-duty cleaners, because of their
Table 1 pH values of selected alkalis as 1 wt% solution inhibiting action and less alkaline pH. They can also be

at 50 C
used, to a certain extent, as a substitute for phos-
phates when a phosphate-free product is required.
Alkali pH
The organic acid salts, such as EDTA and heptonate,
NaOH 12.7
are included for water softening properties and to assist
Na2CO3 11.3
in the removal of solid particles. Gluconate and hepto-
Na2SiO3 12.2
Na3PO4 11.8 nate, in particular, are effective in the highly alkaline
Na2P2O7 10.6 solutions used for etching aluminum and prevent the
Na5P3O10 9.8 precipitation of aluminum hydroxide scale and sludge.
Na2B4O7 9.3
Surfactants are probably the materials which most
NaC7H13O8 7.8
affect the performance of alkali cleaners. Surfactants
Surface Pretreatment 2487
are chemicals that modify the solubility of various of the molecule (typical examples of each are given in
materials in, and their surface affinity for, oil and Figure 1):
water. The diverse composite which makes up the
1. cationic, where the residual charge is positive;
surface of a metal object must be fully wetted out if
2. anionic, where the residual charge is negative;
the cleaner is to perform properly. Surfactants lower
3. amphoteric, where there exists both positive and
the surface tension to allow wetting out to occur. Oils
negative charge centers;
and greases must either be dissolved off the surface or
4. nonionic, where there is no residual change.
lifted from it; surfactants assist in both areas.
There are four broad categories of surfactant on Care must be taken with some surfactant-containing
the basis of the charge associated with the active part cleaners not to exceed certain temperature and con-
centration limits. Thus, it is not always true that the
hotter and more concentrated the solution, the better
CH
3
the cleaning. Many commonly used surfactants have
+ −
C
16H33 N CH3 Cl limited solubility in alkali and become less soluble as
the alkalinity, ionic strength, and temperature rise.
CH
3 A point can, therefore, be reached when the surfac-
Cetyl trimethyl ammonium chloride (cationic) tants come out of solution and, in immersion clean-
ing especially, performance will suffer drastically.
Similarly, some spray cleaners are designed to work
above a certain minimum temperature and strength.
− + In this case, a surfactant is designed to come out of
C
12H25 SO3 Na
solution to act as a defoamer for the system. Exam-
ples of typical simple formulations for various types
Sodium dodecyl benzene sulphonate (anionic)
of alkali cleaners are given in Table 2.
4.01.2.5 Acid Pickling of Ferrous Materials
CH
3
The hot rolling of steel produces a surface layer of
+ −
C
14H29 N CH2COO complex oxides known as ‘millscale.’ It is unstable
and must be removed prior to painting if reliable
CH
3 performance is to be obtained. Removal of millscale
may be accomplished by a variety of methods but it is
Myristyl dimethyl betaine (amphoteric)
traditionally removed by pickling in strong acid con-
taining one or more corrosion inhibitors.
4.01.2.5.1 Formation of millscale
C
9H19 (OCH2CH2) nOH When mild steel is heated in air an oxide or scale will
form on the steel surface. This scale consists of three
well defined layers, whose thickness and composition
Polyethoxylated nonyl phenol (nonionic)
depend on the duration and temperature of heating.
Figure 1 Common surfactants of each main category. In general, the layers, from the steel base outwards,
Table 2 Typical formulations for alkali cleaners
Constituent Composition (%)
NaOH 20 60 20 0 0
Na2SiO3 50 20 40 60 0
Na2CO3/NaHCO3 20 10 20 10 20
Na3PO4 – – 12 20 20
EDTANa4 5 5 5 5 8
Na2B4O7 – – – – 50
Surfactant 5 5 3 5 2
Substrate Steel Steel Zinc Multimetal Aluminium
application immersion electrocleaning electrocleaning immersion spray
2488 Surface Treatment and Modification
comprise a relatively thick layer of wu¨stite, the com- exposed metal and the magnetite and hematite layers
position of which approximates to the formula FeO, a which have not been detached.
layer of magnetite (Fe3O4), and a thin layer of hema- The mechanisms of oxide dissolution and scale
tite (Fe2O3). removal have been widely studied and have been
2
When the steel is rapidly cooled, the thickness thoroughly reviewed by Frenier and Growcock,
3
and composition of these layers remain more or less who concluded, in agreement with others, that oxide
unchanged, but when it is slowly cooled through removal from the surface of steel occurs predomi-

575 C the scale becomes enriched in oxygen and the nantly by a process of reductive dissolution, rather
remaining wu¨stite layer breaks down to some extent than by chemical dissolution, which is slow in mineral
into an intimate mixture of finely divided iron and acids. In this process the reduction of the ferric com-
1
magnetite. Holding the temperature between 400 ponents of the scale is coupled to oxidation of the base

and 575 C causes the iron particles to agglomerate metal, both reactions yielding ferrous species readily
and the scale becomes further enriched in oxygen. As soluble in the acid. For magnetite the processes are:

wu¨stite is unstable below 575 C, scales produced at
þ 2þ
1. Fe3O4 þ 8H þ 2e ¼ 3Fe þ 4H2O
temperatures lower than this contain magnetite and
2þ
2. Fe ¼ Fe þ 2e
hematite only. In addition, the scales are often cracked
and porous. This is due to the difference in contraction Scale removal is also assisted by the dissolution of
between scale and metal on cooling and to the change the underlying metal by normal acid corrosion pro-
in volume when the metal is oxidized. cesses, which undermines the scale, and by the physical
effect of hydrogen gas evolved in this latter reaction.
4.01.2.5.2 Mechanism of scale removal
In general there does not appear to be any direct
When steel that has been slowly cooled through
correlation between the rate of the chemical dissolu-

575 C is immersed in mineral acid, the acid pene-
tion of oxides and the rate of scale removal, although
trates through the cracks and pores in the upper
most work on oxide dissolution has concentrated on
layers of scale and rapidly attacks the oxide layers
magnetite. For example, Gorichev and coworkers have
(Figure 2). The iron, being anodic, dissolves to form
studied the kinetics and mechanisms of dissolution
ferrous ions and the magnetite, being cathodic, is 4,5
of magnetite in acids and found that it is faster
reduced, forming more ferrous ions. As the three
in phosphoric acid than in hydrochloric, while scale
constituents of these cells are good electrical con-
removal is slower in the former. Also, ferrous ions accel-
ductors, their resistance is sufficiently small so that
erate the dissolution of magnetite in sulfuric, phospho-
the rate of dissolution is largely governed by the rate
ric and hydrochloric acids, whereas the scale removal
at which acid diffuses in through the cracks and the
rate is reduced by the addition of ferrous ions. These
rate at which spent acid diffuses out through them.
observations appear to emphasize the importance
A similar but slower action occurs between the
of reductive dissolution and undermining in scale
removal, as opposed to direct chemical dissolution.
3
Acid As further confirmation of this Rozenfeld has
reviewed Russian work on this subject and reports
Fe2O3
that in pickling with sulfuric acid the amount of acid
Fe3O4
used in scale dissolution is only about one-tenth that
Partially
decomposed consumed by the dissolution (corrosion) of the
wüstite
underlying metal. However, in hydrochloric acid the
Fe
direct scale dissolution occurs to a much greater
degree, and is responsible for about 40% of the acid
(a)
consumption.
Acid
A mechanism such as that given above provides
Fe2O3
explanations for the known effects of many process
Fe3O4
variables. The reductive dissolution and undermining
Fe processes require access of the acid to the metal
surface, hence the benefits obtained by the deliberate
(b) introduction of cracks in the oxide by cold-working
Figure 2 Mechanism of scale removal with acid: (a) high prior to pickling. Also the increase in pickling rate
temperature scale and (b) low-temperature scale. with agitation or strip velocity can be explained in
Surface Pretreatment 2489
terms of the avoidance of acid depletion at the oxide– as a scale conditioner on heavily scaled chromium
solution interface. steels and a nitric–hydrochloric acid mixture for
scale removal. A ferric sulfate–hydrofluoric acid mix-
4.01.2.5.3 Hydrogen embrittlement
ture has advantages over a nitric–hydrofluoric acid
Although the majority of the hydrogen produced on
mixture in that the loss of metal is reduced and the
the cathodic areas is evolved as gas and assists the
pickling time is shorter, but strict chemical control of
removal of scale, some of it diffuses into the steel in
the bath is necessary.
the atomic form and can render it brittle. Hardened
Electrolytic pickling of stainless steel in 5–10%
and higher carbon ferritic steels are particularly sus-

(w/v) sulfuric acid at 50 C can be used for removing
ceptible to this hydrogen embrittlement and cracks
the majority of the scale. The strip is first made anodic,
may appear during pickling. Austenitic steels, how-
when a little metal dissolves, and then cathodic, when
ever, are much more resistant to embrittlement
the evolved hydrogen removes the loosened scale.
because of the lower diffusivity of hydrogen in the
To complete the pickling, a nitric plus hydrofluoric
face-centered cubic lattice. Certain impurities (e.g.,
acid dip is given for austenitic steels and a nitric acid
arsenic, sulfur, antimony, etc.) promote the dissolution
dip for ferritic steels.
of hydrogen into the metal (or inhibit the hydrogen
The chromium-containing oxides on stainless steels
recombination reaction generating hydrogen gas,
are more resistant to reductive dissolution and harder
which amounts to the same thing). Consequently, this
to remove than oxides on mild steel. Typically mixed
increases the quantity of hydrogen that diffuses into the
acids and multistage treatments are used and many
steel and the consequent risk of embrittlement. Several 9
formulations have been reported. Scale conditioning
theories of the mechanism of embrittlement have
6,7 can be carried out in acids, in molten salts (e.g., sodium
been put forward and these are discussed in more
hydroxide plus sodium nitrate) or in alkaline solutions
detail elsewhere in this volume.
(e.g., alkaline permanganate). Scale removal can be
As well as causing brittleness, absorbed gas may
obtained with a variety of acids, the commonest being
combine to form molecular hydrogen on the surface 3
a nitric/hydrofluoric mixture. Rozenfeld also reports
of inclusions and voids within the steel. Thus a gas
effective pickling with ferric sulfate plus sulfuric acid
pressure is set up in the voids and this may be suffi-
mixtures and considers that the effect of the ferric
cient to cause blisters to appear either during pick-
ions is to speed up the dissolution of the underlying
ling or during subsequent processing such as hot-dip
metal. Organic acids, such as citric acid, also have a
galvanizing. The embrittlement effect can be largely 10
role in the cleaning of lightly corroded alloy steels.

removed by heat treatment of the steel at 150–200 C,
but even then the original ductility may not be
4.01.2.5.5 Pickling inhibitors
entirely restored. In estimating the degree of embrit-
tlement, the temperature and rate of testing have an
Organic inhibitors
important effect. Thus the embrittlement tends to
During the pickling of scaled steel, the thinner and
disappear at very low and very high temperatures
more soluble scale is removed before the thicker and
and is reduced at high strain rates.
less soluble scale. Consequently, some exposed base
4.01.2.5.4 Pickling of alloy steels metal is attacked before the pickling operation is
The furnace scales which form on alloy steels are complete. In order to reduce this acid attack to a
thin, adherent, complex in composition, and more minimum, corrosion inhibitors are used. Although
difficult to remove than from nonalloy steels. Several an effective inhibitor reduces acid attack, it does not
mixed acid pickles have been recommended for prevent the attack of oxidizing agents on the exposed
stainless steel, the type of pickle depending on the base metal. Thus, the ferric ions resulting from the
8
composition and thickness of the scale. For lightly- gradual dissolution of the detached magnetite and
scaled stainless steel, a nitric–hydrofluoric acid mix- hematite may, even in the presence of an inhibitor,
ture is suitable, the ratio of the acids being varied to attack the exposed steel causing pitting.
suit the type of scale. Strict chemical control of this Clearly, the selected inhibitor should not decom-
mixture is necessary, as it tends to pit the steel when pose during the life of the pickle nor decrease the rate
the acid is nearing exhaustion. For heavy scale, two of scale removal appreciably. Some highly efficient
separate pickles are often used. The first conditions inhibitors, however, do reduce pickling speed a little.
the scale and the second removes it. For example, a It would be expected that as the hydrogen evolution
sulfuric–hydrochloric acid mixture is recommended is reduced the amount of hydrogen absorption and
2490 Surface Treatment and Modification
embrittlement would also be reduced. This is not
100
a b
always the case; thiocyanate inhibitors, for example,
c
actually increase the absorption of hydrogen.
d
Many organic substances soluble in acid or colloid-
ally dispersible have been shown to have inhibiting 95
properties. The most effective types contain a non-
polar group such as a hydrocarbon chain and a polar
group such as an amine. They contain oxygen, nitro-
e
90
gen, sulfur, or other elements of the fifth and sixth
groups of the Periodic Table. They include alcohols,
aldehydes, ketones, amines, proteins, amino acids, het-
erocyclic nitrogen compounds, mercaptans, sulfoxides,
85
sulfides, substituted ureas, thioureas, and thioazoles. 0 0.01 0.02 0.03 0.04
The efficiency of an inhibitor (I ) under a given set % Inhibitor concentration in pickle
of conditions can be expressed by the ratio of the
Figure 3 Relationship between % inhibitor efficiency and
decreased corrosion rate as a function of the original
inhibitor concentration in 6wt% H2SO4: (a) di-o-tolyl
uninhibited corrosion rate (as a percentage): thiourea; (b) mono-o-tolyl thiourea; (c) commercial inhibitor
containing 20% di-o-tolyl thiourea; (d) commercial inhibitor
ðUnhibited corrosion rate containing 20% diphenyl thiourea; and (e) gelatin.
Inhibited corrosion rateÞ
I ¼ 100%
Inhibited corrosion rate
hydrogen evolution in scale removal, inhibitors
In general the efficiency increases with an increase in might be expected to affect the rate of this removal.
inhibitor concentration; typically an effective inhibi- Also, if the inhibitor adsorbs on the oxide surface
tor gives >95% inhibition at a concentration of then the rate of chemical dissolution of the oxide
0.01 wt% and 90% at 0.005wt%. Provided the may be affected. Experimental evidence suggests
inhibitor is stable, increase in temperature usually that these effects may occur, depending on the acid
13
increases the efficiency although the actual acid and the inhibitor. Cumper has shown that pyrrole
attack may be greater. A change in acid concentra- and indole can increase the rate of dissolution of
14
tion, or in type of steel, may also alter the efficiency. magnetite in hydrochloric acid. It has been reported
Thus, the conditions of a laboratory determination of that commercial amine-based inhibitors can either
efficiency should closely simulate the conditions increase or decrease the rate of scale removal in the
expected in commercial practice. same acid. Other reports suggest that the presence of
Figure 3 shows the relationship between effi- inhibitor has little effect on scale removal rate in
ciency and concentration of some thiourea deriva- hydrochloric acid but markedly decreases it in sulfu-
tives and gelatin in the pickling of cold-reduced and ric acid. One area that has not been widely studied is

annealed strip in 6wt% sulfuric acid at 85 C. The the effectiveness of inhibitors on scaled surfaces, but
thiourea derivatives, diluted with sodium chloride, there is experimental evidence that the presence of
gelatin and a wetting agent, are used commercially. magnetite scales can significantly affect the perfor-
Mono- and di-O-tolyl thioureas are stable in this mance of nitrogen-based inhibitors in alkaline solu-
pickle for at least 50 h, but diphenyl thiourea and tions used for chemical cleaning.
gelatin decompose after 4 or 5 h.
The principles behind the selection of effective Inorganic inhibitors
inhibitors for steel in the various acids have been Inorganic inhibitors are salts of metals having a high
11 12
reviewed by Schmitt and Gardner. The selection hydrogen overvoltage, for example, antimony and
of an inhibitor is dependent on both the metal and the arsenic salts. The inhibiting action is associated with
acid. For steel, in general, nitrogen-based inhibitors the formation of a coating of the metal, which, being
(e.g., amines and heterocylic compounds) are used cathodic to the steel and having a high hydrogen
in hydrochloric acid, whereas sulfur-containing ones overvoltage, prevents the discharge of hydrogen ions
(e.g., thiourea and its derivatives) find more favor in and so stops the dissolution of the steel. These inhi-
sulfuric acid. bitors are seldom used in commercial practice, but
Given the reductive dissolution process involved antimony chloride dissolved in concentrated hydro-
and the contributions from undermining and chloric acid is used in the laboratory for stripping
% Inhibition efficiency
Surface Pretreatment 2491
deposits of zinc, cadmium, tin, and chromium from workpiece. Care must also be taken during cathodic
steel, and with the addition of stannous chloride for cleaning of steel alloys as hydrogen embrittlement,
15
removing scale and rust. whichmust always be considered a risk during cathodic
cleaning, must be either avoided or allowed for.
4.01.2.6 Acid Pickling of Nonferrous Metals
4.01.2.7 Ultrasonic Cleaning
Although less common, acids are also used for the
removal of oxide and scale from nonferrous metals Another method for introducing mechanical action
and alloys. Table 3 summarizes a number of pickling into immersion cleaning is by the use of ultrasonic
solutions for selected materials. agitation. Here, a high frequency vibration is imp-
arted to the solution. At the surface to be cleaned,
minute bubbles are formed by local cavitation of the
4.01.2.6.1 Electrochemical cleaning
fluid. When they collapse, they impart a high degree
A significant increase in immersion cleaning perfor-
of local agitation, thereby efficiently removing any
mance can be achieved by local agitation caused by
surface contamination.
vigorous gas evolution on the surface of the work-
piece. Thus, the use of an applied cathodic voltage
evolves hydrogen, which provides a ‘scrubbing’
4.01.3 Mechanical Cleaning
action, greatly enhancing the cleaning process. How-
ever, during cathodic cleaning any dissolved metal 4.01.3.1 Flame Cleaning
ions in solution will tend to plate-out on the metal
Now little used as a preparatory method, flame
surface, so the work will normally be given a short
cleaning is a process whereby an intensely hot flame
anodic cycle at the end of the cleaning time to dis-
is played on the surface of the steel. In theory, differ-
solve this contamination. Also, periodic reverse
ential expansion causes millscale to detach. In prac-
cathodic–anodic cycling is another way of achieving
tice, there is evidence that the treatment may not
the same effect. Anodic cleaning can also be per-
remove thin, tightly adhering millscale. Also, steel
formed as a small fraction of the metal is dissolved,
less than 5mm thick can buckle. Finally, the process
thereby simultaneously removing any contamination;
can ‘burn in’ chemicals deposited on the surface,
however, this should be done with care, as otherwise
causing premature paint failure.
the workpiece may be destroyed.
For electrocleaning, care must be taken that a solu-
4.01.3.2 Manual Cleaning
tion of sufficiently high conductivity is used to prevent
solution voltage drops and ‘burning’ of workpieces The term encompasses all manual and mechanical
in high current density areas. Clearly, excessively methods of cleaning other than blast-cleaning, for
high anode current densities should be avoided as example abrasive discs, wire brushes, scrapers, vibra-
these will cause excess corrosion/dissolution of the tory needle guns and chipping hammers are all
Table 3 Pickling solutions for nonferrous metals
Metal Acid Temperature Time

Copper and brass (60–90% Cu, 7–25% (w/w) H2SO4 or 15–25% (w/w) HCl 15–60 C 1–10min

10–40% Zn) 15 C 1–3min

Aluminimum bronze (82–95% Cu, Scale conditioned with 10% (w/w) NaOH followed by 75 C 2–5min
5–10% Al, 0–5% Fe, 0–5% Ni) H2SO4 or HCl as above

Copper–silicon alloys (96–97% Cu, 7–25% (w/w) H2SO4þ1–3% (w/w) HF 15 C 1–5min
1–3% Si)

Nickel–copper alloys (55–90% Cu, 10% (w/w) HClþ1.5% (w/w) CuCl2 80 C 30min
10–30% Ni, 0–27% Zn)

Nickel–chromium alloys (35–80% Ni, Scale conditioned with 20% NaOHþ5% (w/w) 100 C 1–2 h

16–20% Cr, 0–45% Fe, 0–2% Si) KMnO4 followed by 20% (w/w) HNO3þ4% (w/w) HF 50 C 5–30min

Aluminium alloys (0–10% Cu, 0–30% 25% (w/w) H2SO4þ5% (w/w) CrO3 65 C 20min

Mg, 0–6% Zn, 0–12% Si) 40% (w/w) HNO3þ1–5% (w/w) HF 15 C 1–5min

Magnesium alloys (0–10% Al, 0–3% 10–20% (w/w) CrO3þ3% (w/w) H2SO4 100 C 1–30min

Zn. 0–0.2% Mn) 25 C 15 s