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Pickling of Stainless Steels
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Pickling of Stainless Steelsposted on 18 July 2021 | posted in Stainless steel
CHEMICAL PICKLING OF STAINLESS STEELS
PICKLING IS ONLY CARRIED OUT AFTER WELDING
Not for the removal of Iron contamination or cleaning!
Stainless Steels have been around for over 100 years but when fabricating and welding it inevitably means the formation of oxides, discoloration and heat scale on the surfaces in the proximity of the welds. Removing this to restore the original lustre has always been difficult, time-consuming and expensive. However, there are a number of products available depending on the circumstances which may provide an answer.
Pickling can be carried out using pastes, tapes or various liquid formulations depending on the limitations such as access, level of heat tint and limiting damage which may occur to surrounding areas.
The alternative to pickling is the manual removal of weld scale by grinding or wire brushing. This damages the surface leaving visible scratch marks and can result in loss of the final corrosion resistant properties of the metal. Electro-chemical weld cleaning machines are also available which use more environmentally friendly chemicals.
The most widely used by fabricators due to the simple application and low cost is in the form of a paste .Antox 71E Plus is a thixotropic pickling paste, based on hydrofluoric and nitric acid and especially formulated to simplify the cleaning of welded areas. It has been produced as a paste so that the hazards of using this type of chemical are significantly reduced. Unlike liquids, a paste will not splash. In addition, when applied to the area to be cleaned, the paste will remain only on that area and will not have any tendency to run into adjoining areas.
If used in accordance with the instructions, it is effective, reliable and much safer than other techniques.
WHERE TO USE ANTOX 71E PLUS
Antox 71E Plus is used to clean the welds and surrounding surface areas on virtually all grades of stainless steel. These fabrications are used in many industries of which the following are typical:
Chemical and pharmaceutical processing plant Food, drink and brewery plant
Petroleum refineries and off-shore structures Aircraft and aerospace
Nuclear construction and heavy industry Storage tanks and walkways
Shipbuilding Building and construction
HOW TO USE ANTOX 71E PLUS
Antox 71E Plus is used direct from the container after a good stirring or mixing to ensure that the paste is homogeneous. An even coat of paste is applied to the weld area using either the brush supplied with the paste, or alternatively, by using one of the specially designed Antox long-lasting acid resistant applicator brushes.
The paste is left on the surface for the minimum time necessary for cleaning to take place and is then neutralised with Antox NP (Neutralising Paste). This stops the reaction of the pickling paste,minimises acid contamination elsewhere on the fabrication which may cause corrosion and allows the surface to be simply washed off using clean cold water. The water should have a neutral pH as there are no acidic residues. This technique avoids the risks associated with handling strongly acidic products and leaves the surface clean, shiny and safe to touch.
The surface may require a further clean to remove Iron particles, dirt and grease as well as a Passivation process to restore the Cr-Oxide layer which gives Stainless Steel its corrosion resistant properties. Stainless Steels do passivate naturally over time with a good clean oxygen supply but most procedures advise on chemical passivation to hasten the process.
OTHER PICKLING PRODUCTS
Antox 73E spray pickle - This has a reaction similar to Antox 71E but has been developed for spraying. It is mainly for use with large fabrications where it can be used to both pickle the welds and clean the steel surface in one application. It requires all plastic acid resistant spray units and colour indicator can be used to reduce the nitric fumes and assist in even application.
Antox 75E Surface cleaner - This is a surface cleaner for removal of Iron contamination, brightening metal surfaces and removing dirt and grease.
Antox 80E Bath pickle– For complicated shapes or small parts. Removes welding scale and heat discoloration simultaneously by dipping or immersion. Requires suitable chemical resistant tanks.
Antox 90E Passivating Solution – used to replace the passive protective Cr-Oxide layer.
Antox NP Neutralising Paste - Slightly alkaline but with no toxic or harmful ingredients. Used for neutralising the residues of the Antox 71E Plus pickling process before washing with water and discharging of the effluent.
Antox 2001T Pickle and Polish – Chemical paste with fine particles which polish the surface. Suitable for highly polished surfaces where the welds can be polished to the same bright finish.
Pick & Clean Pickling Tape – Pick is pickling paste in the form of a tape perforated every 200mm. The tape is placed over the weld, removed after the required time, then neutralised with the Clean which is also supplied with the kit. The Clean can be supplied as a liquid or as a wipe for easy disposal.
We are continually developing products to meet our customer's wide ranging applications and would be glad to discuss your requirements in more detail.
Call one of our technical staff today to see what we can do for you.