1.1 INTRODUCTION
A stainless steel surface should appear clean, smooth
and faultless. This is obvious when the steel is used
for such purposes as façades or in applications with
stringent hygienic requirements, but a fine surface
finish is also crucial to corrosion resistance.
Stainless steel is protected from corrosion by a thin,
impervious, invisible surface layer – the passive layer
– that consists mainly of chromium oxide. The oxygen
content of the atmosphere or aerated aqueous
solutions is normally sufficient to create and maintain
this passive layer. Unfortunately, surface defects and
imperfections introduced during manufacturing
operations may drastically disturb this ”self-healing”
process and reduce resistance to several types of local
corrosion. This means that a final cleaning process
will often be required to restore an acceptable surface
quality with regard to hygiene and corrosion.
The extent of and methods for post-manufacture
treatment will be determined by the corrosivity of the
environment, the corrosion resistance of the steel
grade, hygienic requirements (e.g. in the
pharmaceutical and food industries) or by purely
aesthetic considerations. Consideration must also be
paid to local environmental requirements. Both
chemical and mechanical cleaning methods are
available. Good design, planning and methods of
manufacture can reduce the need for finishing work
and thus reduce costs. The influence of defects, and
ultimately their removal, must be considered when
manufacturing to specifications that relate to certain
surface quality requirements. For further details and
explanations, the ”Avesta
Handbook” is recommended.
1.2 TYPICAL DEFECTS
1.2.1 Heat tint and oxide scale
High temperature oxidation – caused by processes
such as heat treatment or welding – produces an
oxide layer with inferior protective properties,
compared with those of the original passive layer.
A corresponding chromium depletion in the metal
immediately below the oxide also occurs. The
chromium-depleted zone under normal welding heat
tint is very thin and can normally be removed
together with the tint. It is, however, necessary to
remove this layer in order to completely restore
corrosion resistance.
1.2.2 Weld defects
Incomplete penetration, undercut, pores, slag
inclusions, weld spatter and arc strikes are typical
examples of weld defects.
These defects have negative effects on mechanical
properties, resistance to local corrosion and make it
difficult to maintain a clean surface. The defects must
therefore be removed, normally by grinding, although
sometimes repair welding is also necessary.
1.2.3 Iron contamination
Iron particles can originate from machining, cold
forming and cutting tools, blasting grits/sand or
grinding discs contaminated with lower alloyed
material, transport or handling in mixed manufacture,
or simply from iron-containing dust. These particles
corrode in humid air and damage the passive layer.
Larger particles may also cause crevices. Reduced
corrosion resistance will result in both cases. This type
of corrosion produces unsightly discoloration and
may also contaminate media used in the equipment
in question. Iron contamination can be detected using
the ferroxyl test; see chapter 5.
1.2.4 Rough surface
Uneven weld beads and grinding or blasting too
heavily will result in rough surfaces. A rough surface
collects deposits more easily, thereby increasing the
risk of both corrosion and product contamination.
Heavy grinding also introduces high tensile stresses,
which increase the risk of stress corrosion cracking
and pitting corrosion. There is a maximum allowed
surface roughness (Ra-value) for many applications,
and manufacturing methods that result in rough
surfaces should generally be avoided.
1.2.5 Organic contamination
Organic contaminants in the form of grease, oil, paint,
footprints, glue residues and dirt can cause crevice
corrosion in aggressive environments, render surface
pickling activities ineffective, and pollute products
handled in the equipment. Organic contaminants
should be removed using a suitable pre-cleaning/
degreasing agent (chlorine-free). In simple cases, a
high-pressure water jet can be used.
Different chemical and mechanical methods, and
sometimes a combination of both, can be used to
remove the defects mentioned. Generally, cleaning
based on chemical methods can be expected to
produce superior results since most effective
mechanical methods tend to produce a rougher
surface whilst chemical cleaning methods reduce the
risk of surface contamination. Local regulations in
respect of environmental and industrial safety as well
as waste disposal problems may, however, limit their
application.
2.1 MECHANICAL METHODS
2.1.1 Grinding
Grinding is normally the only method that can be
used to remove defects and deep scratches. A
grinding disc is usually adequate for treating defects
of this type. The grinding methods used should never
be rougher than necessary, and a flapper wheel is
often sufficient for removing weld tint or surface
contamination.
The following points must always be considered:
• Use the correct grinding tools – self-sharpening,
iron-free discs should always be used for stainless
steel – and never use discs that have previously
been used for grinding low alloy steels.
• Avoid producing a surface that is too rough. Rough
grinding with a 40-60 grit disc should always be
followed by fine grinding using, for example, a
higher grip mop or belt to obtain a surface finish
corresponding to grit 180 or better. If surface
requirements are very exacting, polishing may be
necessary.
• Do not overheat the surface. Apply less pressure
when grinding in order to avoid creating further
heat tint.
• Always check that the entire defect has been removed.
2.1.2 Blasting
Sand and grit blasting (peening) can be used to
remove high temperature oxide as well as iron
contamination. However, care must be taken to
ensure that the sand (preferably of olivine type) or
grit is perfectly clean. The blasting material must
therefore not have been previously used for carbon
steel; not should the sand or grit be too old, since it
becomes increasingly polluted, even if it has only
been used for blasting contaminated stainless steel
surfaces. The surface roughness is the limiting factor
for these methods. Using low pressure and a small
angle of approach, a satisfactory result can be
achieved for most applications. For the removal of
heat tint, shot peening using smooth glass beads
produces a good surface finish and introduces
compressive stresses which improve stress corrosion
cracking resistance and resistance to fatigue.
2.1.3 Brushing
For the removal of heat tint, brushing using stainless
steel or nylon brushes usually provides a satisfactory
result. These methods do not cause any serious
roughening of the surface, but do not guarantee
complete removal of the chromium-depleted zone.
As regards the other mechanical methods, the risk of
contamination is high, and it is therefore important
that clean tools that have not been used for
processing carbon steels are used.
2.1.4 Summary
A final mechanical cleaning stage following a typical
manufacturing programme could be as follows:
• Removal of welding defects by grinding;
• Removal of material affected by high temperatures
and, if possible, removal of iron impurities. The
surface must not become unacceptably rough;
• Removal of organic contaminants (see section 1.2.5);
• A final acid treatment – passivation/
decontamination – is strongly recommended. A
thorough rinsing with fresh water, preferably using
a high-pressure water jet must follow the acid
treatment. In exceptional cases, however, rinsing by
high-pressure water jet only may suffice as the final
treatment.
2.2 CHEMICAL METHODS
Chemical methods can remove high temperature
oxide and iron contamination without damaging the
surface finish. Electropolishing may improve the
surface finish. Since they remove the surface layer by
controlled corrosion, chemicals will also selectively
remove the least corrosion-resistant areas such as the
chromium-depleted zones.
After the removal of organic contaminants (section
1.2.5), the following procedures are commonly used.
2.2.1 Electropolishing
Electropolishing normally produces a surface that
guarantees optimal corrosion resistance. The material
gains a fine lustre, and, above all, an even microprofile
that meets extremely stringent hygienic
requirements.
2.2.2 Pickling
Pickling is the most common chemical procedure
used to remove oxides and iron contamination.
Thorough rinsing with clean tap water must follow
pickling. The water quality requirements, including
acceptable chloride content, increase with the surface
requirements. Pickling normally involves using an
acid mixture containing 8-20 vol% nitric acid (HNO3)
and 0.5-5 vol% hydrofluoric acid (HF). Chloridecontaining
agents such as hydrochloric acid (HCl)
should be avoided, since there is an obvious risk of
pitting corrosion.
The effectiveness of pickling depends on the following
factors:
• The surface. This must be free of organic
contamination;
• The temperature. The effectiveness of the acids
increases strongly with temperature. This means,
for example, that the pickling rate can be increased
considerably by increasing the temperature. There
are, however, upper temperature limits that must
also be considered. See below.
• The composition and concentration of the acid
mixture.
• The steel grade. Highly alloyed grades need a more
aggressive acid mixture and/or higher temperature
in order to avoid an excessively long pickling time.
See table 1.
• The thickness and type of the oxide layer. This
depends largely on the welding procedure used.
Welding using an effective shielding gas will
produce a minimum of weld oxides. Such a gas
should be as free of oxygen as possible. For further
information, see the AvestaPolarit Welding
Handbook on welding stainless steel. Mechanical
pre-treatment to break or remove the oxide might be
advisable, particularly when pickling highly alloyed
steel grades.
• The surface finish. A rough hot rolled surface may
be harder to pickle than a smooth cold rolled one.
A number of different pickling methods can be used:
• Pickling in a bath is a convenient method if suitable
equipment is available. The composition of the acid
mixture and the bath temperature (20-65ºC) are
chosen with regard to the stainless steel grade and
the type of heat oxide. Overpickling, resulting in a
rough surface, may result when pickling the lowest
alloyed stainless grades at excessive temperatures.
The effectiveness of pickling is influenced not only
by the acid concentration and the temperature, but
also by the free metal content (mainly iron) in the
bath. An increased iron content requires a higher
bath temperature. A rough guideline is that the
free iron (Fe) content measured in g/l should not
exceed the bath temperature (ºC). When metal
contents in the bath reach excessive levels
(40-50 g/l), the bath solution can be partially or
totally emptied out and fresh acid added.
• Pickling with pickling paste. Pickling paste
for stainless steels consists of an acid mixture
(normally HF/HNO3) with added binding agents.
It is suitable for pickling limited areas, e.g.
weld-affected zones. It is normally applied using an
acid-resistant brush. The paste is not effective at
low temperatures (5-10ºC). The risk of overpickling
at high temperatures is less than when using bath
pickling. A greater risk is that of the paste drying
out due to evaporation, resulting in reduced
pickling effect and rinsing difficulties. Objects
should therefore not be pickled at temperatures
higher than 40ºC or in direct sunlight. Rinsing with
water should be carried out before the paste dries.
Even if neutralisation of the pickling paste is
carried out on the metal surface for environmental
and practical reasons, a thorough rinsing with
water is vital.
• Pickling with pickling solution. Pickling solution
(or pickling gel in spray form) normally consists of
a mixture of nitric acid and hydrofluoric acids
(phosphoric acid can be used to obtain mild
pickling properties), with binding agents and
surface-active agents to obtain good thixotropy and
the right viscosity. It is suitable for pickling large
surfaces, e.g. when the removal of iron
contamination is also desired.
2.2.3 Summary
A final pickling/cleaning operation following a typical
manufacturing programme could be:
• Grinding for removal of defects caused by welding.
It is important that slag is removed after welding.
• Removal of organic contamination (section 1.2.5).
• Pickling using a bath, paste or solution, possibly in
combination with a careful mechanical treatment to
break oxides.
• A thorough rinsing with water, preferably using a
high-pressure water jet.
2.2.4 Passivation and decontamination
This procedure is carried out in a manner similar to
pickling, but in this case the active agent is nitric acid
only, 18-30 weight % at around 20ºC. The acid is
applied by immersion or spraying. This treatment
strengthens the passive layer. The treatment is more
important after mechanical cleaning and operations
involving a risk of iron contamination, since the acid
also removes iron impurities from the surface.
Consequently, the method could also be referred to
as decontamination. As after every acid treatment,
rinsing with water is vital.
2.3 CHOICE OF METHOD
The choice of method and the extent of final cleaning
required will depend on the need for corrosion
resistance, hygienic considerations (pharmaceuticals,
food) or whether visual appearance is the sole
criterion. The routine removal of welding defects,
welding oxides, organic substances and iron
contaminants is normally a basic requirement and
usually allows a comparatively free choice of final
treatment. Provided that the surface roughness so
permits, both mechanical and chemical methods can
be used. However, if an entirely mechanical cleaning
method is considered, the manufacturing stage has to
be very well planned in order to avoid iron
contamination, since decontamination, probably with
nitric acid, will otherwise be necessary (section 2.2.4).
When requirements as to surface finish and
corrosion resistance are exacting, the choice of
method is more critical. A treatment sequence based
on pickling (section 2.2.3) will in such cases provide
the best chances of a superior result.
3 Chemical methods in practice
3.1 GENERAL REQUIREMENTS
The practical use of cleaning chemicals is demanding,
and certain working procedures need to be followed.
The choice of chemical cleaning process mainly
depends on the type of contaminants and heat oxides
to be removed, the degree of cleanness required and
the cost. This chapter gives guidelines for the
application of suitable chemical cleaning procedures.
In order to avoid health hazards or environmental
problems, pickling should be carried out in a special
pickling area indoors. In this context, the following
recommendations should be met:
• Handling instructions, essential product
information, such as product labels, and safety data
sheets for the various products must be available.
Local and national regulations should also be
available. See also section 6.1.
• Responsible staff should be familiar with the
health hazards associated with the products and
how these should be handled.
• Personal safety equipment, including suitable
protective clothing and facemask should be used.
See also section 6.2.
• When pickling indoors, the workplace should be
separated from other workshop operations in order
to avoid contamination and health hazards and to
ensure a controlled temperature.
• The area should be ventilated and provided with
fume extraction apparatus.
• Walls, floors, roofs, vessels, etc. that are subject to
splashing should be protected by acid-resistant
material.
• A washing facility should be available, preferably
including a high-pressure water jet.
• A first aid spray must be available. See also section
6.1.
• A facility for the collection and neutralisation of
rinsing water should be available. See also section
4.1.
• If the rinsing water is recycled, care must be taken
to ensure that the final rinse is performed using
de-ionised water. This is particularly important in
the case of sensitive surfaces and applications.
• A storage facility should be available. See also
section 6.3.
3.2 PRE-CLEANING/DEGREASING
Organic contaminants such as grease, oil and paint,
and also soil, grit, etc., have to be removed. This can be
done using the product Avesta Cleaner (contains
phosphoric acid), which is sprayed onto the surface to
be pickled using an acid-resistant pump. The Avesta
Spray-Pickle Pump is recommended for this purpose.
Avesta Cleaner provides a mild degreasing effect and
is sufficiently effective in most cases. However,
heavily contaminated surfaces may require a stronger
(non-chlorinated) solvent. Avesta Cleaner also
removes surface rust and brightens the surface. After
the use, the surface must be rinsed with clean water.
The use of a high-pressure water jet supports rinsing
and in some cases can also be an alternative to
chemical products for removing lightly attached
grease, oils and chemical deposits.
The water-break test is a simple way of assessing
the effectiveness of degreasing. See also section 5.1.
3.3 PICKLING
Pickling products can be applied in three different
ways:
• Brushing, using a pickling paste/gel
• Spraying, using a pickling solution
• Immersion in a pickling bath
The different methods are presented below.
Source: Avesta Polarit
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