KD Industribejdsning

Facts about:Pickling of stainless and acid proof steel

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Preface ..................................................................................................................................................... 2

Stainless and acid proof steel and corrosion types .................................................................................. 4

Mechanical surface treatment .................................................................................................................. 5

Chemical surface treatment................................................................................................................... 6-7

Pickling is a special task, production ........................................................................................................ 8

The pickling process, about KD Industribejdsning.................................................................................... 9

Internal working environment and external environment ........................................................................ 10

Analysis report ........................................................................................................................................ 11

KD Industribejdsning.

Table of content

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This brochure is meant as a help/guidance to all that work with stainless steel, either as advisor, constructor, smith on the floor or with some other need for a little insight into stainless steel.

We have chosen to call the booklet: Facts about pickling of rust and acid proof steel.

The use of stainless steel is increasing rapidly in Denmark as well as globally.

This is of course because of the advantages there

are when using stainless steel, and not least the design trend. Today stainless steel is used as skin cladding for a wide range of products and buil-dings.

In the booklet we mainly look at chemical surface treatment (pickling) of stainless steel, why and how this is done!

Wishing you happy reading.KD Industribejdsning ApS

Preface

Brochures KD Industribejdsning:

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For a large part of the equipment produced it is suffi-cient to use the traditional authentic standard alloys (Aisi 304, 316) or variations of such, which, despite the limited durability, are sufficient in the areas of foodstuff, biotechnology, medicine and other indu-stries.

From time to time you run into corrosion damage on stainless installations which occur shortly after the installation has been put into service. Often, it is damages that are based on the way the material has been affected during storage, handling or du-ring manufacture of the installation which has led to a reduction of the steel’s natural corrosion durability.

The main alloy element in stainless steel is chro-mium which the steel per definition has at least 12% of, and the steel’s often good corrosion durability is linked with the chromium’s ability to passivate.

Passivation is when the steel makes a protective

layer of insoluble oxides and this layer which is only a few nanometers thick is so durable that it can often protect the steel from corrosion in corrosive medias.

Corrosion typesWhich corrosion types the steel is attacked by de-pends a lot on the medium’s acidity (pH).Under very acidic conditions with pH under 2-2.5 chromium passivates poorly, and therefore it is especially general corrosion which is a risk, but also stress and intergranular corrosion are risks.In the large area with pH between 2.5 and 14 it is especially local corrosion, pitting corrosion and co-vering/crevice corrosion which can happen, but also stress corrosion is a risk here. In practice the main part of the industrially used aqueous solutions are in this large pH area.For the very alkaline area with pH over 14 it is pri-marily stress corrosion (caustic embrittlement) that must be considered.

Stainless steel is only rust-proof on the surface!

Rust and acid proof steel

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Of mechanical methods for surface treatments can be mentioned: sandblasting, glass blasting, wire brushing, grinding and polishing.Mechanical methods have the purpose of changing the surface’s mechanical-topographic abilities for aesthetic or hygienic reasons. The influence on the steel’s corrosion abilities can be both positive and negative. Some of the small positive features are that glass blasting introduces compressive stress in the surface of the steel which can give a reduction of the risk of stress corrosion.Common for these methods of surface treating stainless steel is that it is often a time-consuming and thus expensive process while all methods re-duce the corrosion durability of the steel.

Generally it is not advised to use mechanical sur-face treatment for stainless steel, and if it is a de-mand for the look, to use either glass blasting or grinding/polishing with a subsequent passivation to quickly regenerate the rust-preventing oxide layer. 1. Pickled.

2. Ground and pickled.3. Glass blasted and pickled.

Mechanical surface treatment

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Item before pickling. Item after pickling

Chemical surface treatment

Electro polished supporting beam (APV).

Chemical surface treatment methods for stainless steel primarily aim to make metallically clean and passive surfaces.So they have a significant effect in getting the op-timum corrosion durability from a given stainless steel type.Of chemical surface treatment we can mention: decontamination, passivation, pickling, electro po-lishing and chemical polishing.

Decontamination is used to clean the stainless steel’s surface of impurities, including smudging with black steel, and is often done with 5-10% nitric acid.

Passivation is done with at least 20% nitric acid and is used to quickly establish a stable passive

layer (Can be used after glass blasting, grinding and polishing).

Pickling the purpose of pickling is to remove wea-thering from the surface, weathering can come from welding, grinding, cutting and mechanical and other mechanical processing processes. The pickling should remove both the oxides and the de-chro-mated layer directly below the oxides.

The pickling acid normally consists of a 20% nitric acid, 5% hydrofluoric acid, demineralised water and surfactants, and most often takes places at tempe-ratures between 20 and 60 degrees.

Item after pickling.

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Pickling is a special taskThere are quite a lot of environmental problems connected with pickling of stainless steel. Problems in the working environment consist of the fact that the pickling acids are corrosive and therefore re-quire protection. Furthermore during the pickling process toxic nitric oxides are produced which the employees also have to be protected against. As the used pickling acid usually contains significant amounts of chrome and nickel removal or proces-sing is an environmental problem that has to be handled. Therefore pickling must be described as a task for a specialist company and skilled people who know how to handle both the personal protec-tion and removal of used acid.

ProductionIn order to ensure the optimum corrosion durability in stainless steel it is important to ensure the opti-mum treatment of the steel already in connection with storage, handling as well as the subsequent processing.Storage should be done so the steel does not come into contact with black steel, preferably on wooden or plastic covered shelves.Transport and handling should also be done so the steel is not scratched more than absolutely neces-

sary, it is important to make sure that the supplier does not simply throw the steel of the car!When processing, both laser cutting, cutting and folding it must be ensured that the material does not get scratched.When tack-welding it is very important to reduce the heat source as much as possible and to protect against spatter from the welding process.

ALL SUBSEQUENT GRINDING MUST BE AVOIDED!

If it is necessary to grind it must be done in as small an area as possible and with minimum grain 180.Grinding and scratch marks cannot be removed with pickling, on the contrary it is often more visible after the pickling and therefore gives a negative ge-neral impression of the item.In some cases it is possible to reduce this with glass blasting before pickling.Correct handling and preliminary work is crucial for the finished item’s corrosion durability and life.

DO IT RIGHT THE FIRST TIME, IT IS ALWAYS CHEAPER!

KD Industribejdsning production.

Advantages of using KD Industribejdsning• Environmentally approved pickling centres • Experienced and skilled employees

• Competitive prices

• High quality (own quality control)

• Meets all agreed delivery times

• Flexible

KD Maskinfabrik production hall.

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The pickling processKD Industribejdsning’s pickling centres are laid out in consultation with The Danish Working Env-ironment Service, BST, the local authorities and consulting engineers in regards to ventilation and wastewater.

KD Industribejdsning has a sourer with a dimension of 2,5x3x10 meters which holds more than 30,000 litres pickling acid. The items ar lo-wered into it. It can be as individual items or more items in specially designed steeping bowls, racks and the like.

The different customer items are marked while the material quality is noted, the higher the alloy the longer time the pickling process takes.The temperature in the sourer is regulated so the most optimum acid temperature is reached at about 25 degrees, at higher temperatures the risk of nitrous gases is increased.

After the item has been in the sourer for the cor-rect amount of time (depending on alloy) the item is taken out and left to drip before it is lowered into a rinsing bowl to wash of the last remains of acid. After this process the item is lifted to the washing area where the item is cleaned with high-pressure water with demineralised water.

Lifting Capacity: KD Bejds Syd: 10 t.

After pickling the employee checks the item to ensure that the quality is as promised and quality documentation is made on request.The centre manager makes a measurement report which is the basis for invoices, as well as neces-sary delivery notes, transport papers etc.The pickling acid is checked regularly for the con-tents of metallic salts and acid composition.

About KD IndustribejdsningKD Industribejdsning was established as a part of KD Maskinfabrik in 1992. The cause was that KD Maskinfabrik who primarily produces machine equipment to both council and industrial water purifying plants was met with increasingly high demands for handling and treatment of stainless/black steel, increasing demands for formiergas covering etc.

It was in the beginning of the 90’s focus was really put on stainless steel and especially the problems concerning surface treatment, therefore KD Ma-skinfabrik chose to establish its own pickling centre which was to take care of both the pickling of own materials and customer jobs.

It was quickly necessary to hire people that only dealt with pickling jobs, and at the same time the pickling activities were put into an independent company: KD Industribejdsning.

KD Industribejdsning was quickly known as a seri-ous pickling centre that was in control.

KD Industribejdsning can offer the following surface treatment:

- Glass blasting.- Decontamination.- Passivation.- Dip pickling in sour 2,5x3x10 meters- Spray pickling.- Mobile pickling (done at the customer’s premises).

Dip pickling

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Internal working environment and external environmentAt arbejde i et bejdsecenter, betyder omgang med To work in a pickling centre means to be around strong and unhealthy acids, therefore it is crucial that all our employees have all had thorough trai-ning in working with these and in what precautions that must be taken in connection with pickling.

A pickling centre is therefore equipped with very powerful ventilation which ensures that the air is circulated approximately every 3 minutes. Further-more respiratory protective equipment is used, as well as complete protective gear, to ensure as safe a working environment as possible. For big spray tasks there are always 2 men present to ensure air supply etc.

The handling of heavy items is done with forklift trucks or cranes to prevent heavy lifting.After pickling the item is lowered into a rinsing bowl to wash of the worst of the acid before the final cleaning takes place with high-pressure cleaners.All equipment is checked regularly by responsible authorities, just as it is continuously ensured that all equipment is safe and in usable condition.

External environmentKD Industribejdsning puts great emphasis on the external environment and the environmental strain from a pickling centre is, besides the use of forklift trucks and vans/lorries and heating, primarily air, waste water and heavy metal-containing sludge.Both the floor in the pickling building, reception pool, sourer and the water purifying plant are equipped with leak detectors so a contamination from e.g. a leak is discovered immediately.Here we will only mention air, wastewater and sludge.

AirIn connection with the powerful exhaust from the pickling building mist eliminators have been mounted on the ventilation pipes which separate hydrous particles in the air and lead them to the internal purification plant. The replacement air is filtered and heated at low surrounding temperatu-res, all ventilation is secured with a flow watch. Air for the respiratory protective equipment is filtered through carbon filters, heated and pressurised, according to the rules from The Danish Working Environment Service.

WastewaterAll waste water is led to a reception pool in the pickling building, from here it is pumped to a che-mical purifying plant where the waste water’s pH is controlled in order to ensure settling, primarily from chrome and nickel, the heavy metals settle with the help of a polymer additive and is drained afterwards.After the chemical purifying plant the water is pumped into a buffer tank where all the waste water is densified by using vacuum and heating, the permeate is drained off, the purified demine-ralised water is then pumped into the heating ring in the sourer (only during winter months) and then to a consumption tank for reuse for high-pressure cleaning. Thus considerable amounts of water are saved.

SludgeAll the sludge from the purification process is drained off and dried and then it is put into storage barrels. The sludge is stored in a locked container and removed to Kommunekemi in Nyborg.The sludge is danger class X3.

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Internal working environment and external environment

Sources: DTI Surface treatment R3, Force by/ E. Rislund Corrosion resistant Stainless steel.

Water purifying plant.

KD Industribejdsning ApS · Hesselly 12 · DK-6000 KoldingTlf: +45 75 82 67 99 · lp@kd-bejds.dk· www.kd-bejds.dk · Part of KD Group

Correct pickling:

”Not just for your sake, - butalso for Mother Earth”

We are always at your disposal when

a specific task needs to be discussed,

often many unfortunate joints and

other things can be avoided by di-

scussing them with our centre mana-

gers that have many years experience

in the handling and pickling of many

different tasks.

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