Pickling and passivation: final surface treatment for alloyed stainless steels

18.8.2020

Alloyed stainless steels are increasingly used where stainless properties are suitable or required without the use of a mechanical anticorrosive barrier.

Naturally, the question of corrosion resistance of such steels keeps coming up. Corrosion often occurs in welded areas that were exposed to intensive heat, but also in seemingly unaffected places where carbon material pieces land or are rubbed, surfaces with inclusions, and places where heat, chemical or mechanical action alters the chemical composition or the structure of the surface.

What is the cause?
Mostly a disrupted or poor quality passive layer. Corrosion resistance of alloyed stainless steels is determined by a microscopically thin layer of oxides of alloying elements – passive layer. This is mostly disrupted by:

heat in a welded area and an affected surrounding area, or different heat action
rubbing action, material depositing, or contaminant material inclusion
chemical action

If the passive layer is disrupted in any way, a new, quality passive layer needs to be created. A prerequisite for the creation of a perfect and effective passive layer is a metallically clean surface, free from scales and colouring caused by welding or heat treatment, inclusions, abrasive wear and deposits of carbon or other contaminant material, mechanical impurities including paint marking or the original disrupted passive layer.

How to ensure a metallically clean surface?
In most cases, the most suitable and effective method is the chemical cleaning called pickling.

Machining usually doesn’t result in a perfectly clean surface. Although grinding removes part of the scales, colouring and carbon material, the remaining contaminant material is spread all over surface and, if combined with elevated humidity, corrosion may occur. Grinding with a blunt tool, coarse grain and an excessive depth of cut can create spots with high stress prone to pitting corrosion in an environment containing a high level chlorides. Blasting leaves particles of the abrasive material lodged in the surface with impurities underneath and a disrupted passive layer, which we wanted to remove. We not only don’t attain a clean surface, but blasting also makes the surface area far larger, thus increasing the risk of corrosion.

Pickling, by contrast, thanks to its vast variability, makes it possible to attain a perfect metallically clean surface on almost any product or device, at a reasonable cost. Another advantage of blasting is the unification of the surface appearance after a correctly performed pickling.

How is a passive layer created?
A passive layer is created on a metallically clean pickled surface in two ways:

The reaction between a metallically clean surface and an air oxygen creates a passive layer within several days (so-called self-passivation). This method is sufficient for normal use of alloyed stainless steels.
Using a passivating agent will result in the immediate creation of a passive layer, whose thickness is a multiple of that of a self-passivation layer. It is applied to alloyed stainless steels to be used in the power engineering, chemical industry etc. When using a passivation solution, the procedure is identical to spray pickling, or bath pickling (see below).

What is the correct pickling procedure?
Pickling – irrespective of the pickling agent application method used – must be performed as follows:

Surface degreasing and removing mechanical impurities.
Application of a pickling agent.
Action of the pickling agent (the length of action depends largely on the type of alloyed stainless steel, a degree of surface contamination, and the pickling agent used).
Rinsing with pressure water of at least 12 MPa (the pressure is vital for a perfect rinse to remove the pickling agent and impurities from places that are difficult to access. When pickling small and simple parts, mechanical cleaning with a rag or a brush and water can be used instead of pressure water.

What does the surface look like after pickling?
After pickling, the surface of alloyed stainless steel must be metallically clean without any traces of impurities of any kind (rust, carbon material residues, inclusions, colouring and scales after welding and heat action, mechanical dirt). Pickling will result in a matt and unified surface.

How to use pickling and passivating agents?
ANTOX is one of the best-known and best-selling pickling brands, for its high quality, effectiveness and high surface area coverage. FK system Brno, s.r.o. is an exclusive representative of the brand.

There are three basic methods of applying pickling agents:

Bath pickling
Paste pickling
Spray pickling

All of these three methods are equivalent in terms of quality, provided they are performed correctly; bath pickling, however, is usually notably cheaper and allows for the best surface unification.

Bath pickling is done by dipping parts in a pickling bath. It is used for pickling components of different shapes and sizes; limitations include the tank size and handling equipment capacity, and sometimes the shape of a component. Bath pickling is done using ANTOX 80 E.
Paste pickling is done by applying pickling paste with a brush. It is suitable for pickling welds and small parts. ANTOX 71 E and ANTOX 71 E EXTRA pastes are used for this method; ANTOX 3d is used for pickling polished components.
Spray pickling is done by spraying a part with pickling gel, using either a special application device or a simple bottle with a pump (see figure 2). This method is used for vast areas. A coloured indicator can be added to a pickling agent in order to make it easier to distinguish the sprayed area from the unsprayed area. ANTOX 73 E, ANTOX 73 E PLUS, and ANTOX 73 E EXTRA gels are intended for spray pickling.

Inner surfaces of pipes can also be pickled by circulating a pickling solution including degreasing and pickling to the oxygen degree of purity and pharmaceutical degree of purity.

Pickling agents are graded based on their chemical composition, ranging from the least concentrated (E) for pickling low-alloy chromium steels, to medium concentrated (E PLUS) for chromium-nickel austenitic steels, to high-concentration agents (E EXTRA) for pickling high-alloy steels.

Health and safety at work and the environment protection while pickling
Pickling agents contain nitric acid (HNO3) and hydrofluoric acid (HF). That is why they are classified as caustic substances and poisons. Staff must be instructed in HS rules and regulation. Special protective aids need to be used. A face shield, rubber overalls and gloves are sufficient for paste pickling. When spray or bath pickling is to be done, workers need to use a face mask with a filter protecting against acidic vapours and a coverall rubber suit.

Pickling and passivation waste water is acidic and contains dissolved metals form the base material. That is why it must be collected and disposed of in an environmentally-friendly way.

Can every company perform pickling and passivation on its own?
The final result of pickling depends on the choice of the right technological procedure, staff experience, quality equipment, the choice of the right pickling agent and other factors. At the same time, environmental protection regulations and a number of other regulations must be adhered to. That is why only companies that have their own approved pickling workplace may do spray pickling and passivation of larger components or bath pickling. We recommend that companies without their own pickling workplace only do paste or gel pickling of minor parts.

Our firm performs spray pickling of major parts and devices and bath pickling. Bath pickling is done in Brno, spray pickling of large components can be carried out at the client’s place or at an erection place (see fig. 3). In addition, we provide a comprehensive range of technical and consulting services regarding pickling and passivation. We perform work including collection and disposal of liquid waste. We have all the certificates and permits required to do pickling at the client’s place or an erection place.

Final word
To sum up, correctly performed pickling and passivation combined with the right type of alloyed stainless steel are a guarantee of corrosion resistance and a unified, metallically clean appearance of products.

Ing. Petr Kalný, FK system - povrchové úpravy, s.r.o.