Friday, September 14, 2007

Stainless steel and the need for cleaning

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 Sheffield Corrosion

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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