SS Weld rust stain removal

Here is an old file saved in my computer that answers your question on cleaning of stainless steel. Aluminum is also discussed and may be useful to other readers as well. I am just posting the whole article since I cannot provide a link. I hope it is not too long.

The Importance of Chemical Treatment Associated with the Welding of Stainless Steel and Aluminium

Presented to the Philippine Welding Society, 11th October 1997 by John W. Hill BA (Chem) General Manager Specialty Products, Callington Haven Pty Ltd, Sydney, Australia.

1. INTRODUCTION

Stainless steel has found widespread applications in industry in areas such as process plant and equipment, building facades, railing, storage tanks and piping in food and pharmaceutical factories, and many others.

Stainless steel is often perceived as being resistant to corrosion, relatively inert and requiring minimal treatment in fabrication, and little maintenance.

The term passivity refers to the natural corrosion resistant property of many metals and alloys including chromium, titanium and stainless steels.

Passivity is conferred on stainless steel by an invisible nanometre thick film of chrome oxide. The stability of this film depends largely on the corrosive environment in which the stainless steel is found.

In a clean environment the passive film forms spontaneously following its removal over a period of at least 8 to 24 hours. This means that stainless steels are self passivating "under favourable conditions".

Disruption of the passive film by chemicals, mechanical action, embedded iron particles, or oxygen starvation can readily occur in the workshop or during fabrication. Surface free iron particles, dust, grit and iron oxide contaminants arise from handling, fabrication/forming, welding, grinding, machining, paint and crayon marks, polishing, tumbling and workshop cross contamination.

These contaminants penetrate the passive film, absorb and generate chlorides, ferric chloride or produce inorganic chlorides by the decomposition of organic compounds such as PVC and marking materials.

Chemical treatment of stainless steel is important to maximise product quality in all stages of the fabrication life cycle, from initial cleaning, to spatter protection, pickling, passivation, waste treatment and final cleaning of the finished product.

Aluminium also requires specific chemical treatment prior to welding, following welding to remove weld scale and burn marks, spatter protection and final surface cleaning of the finished product if required.

2. STAINLESS STEEL CLEANING PROCESSES

All stages of the fabrication life cycle of stainless steel require specific chemical cleaning treatment. These include :

• surface preparation, cleaning and / or degreasing
• weld spatter protection
• descaling / pickling
• passivation
• reatment of acidic waste for trade waste compliance
• cleaning of finished product
• deoxidising of surface corrosion for maintenance

Chemical treatment agents for the stainless steel fabricator

• solvent degreaser / cleaner
• anti spatter spray
• pickling solution or paste
• passivating solution or paste
• neutralising solution for waste acid treatment

Solvent cleaners

Traditionally, readily available and low cost petroleum based products such as white spirits, kerosene, thinners or chlorinated solvents have been routinely used for surface cleaning and degreasing.

In many countries today, occupational health and safety and environmental regulations have focused attention on safer non toxic and biodegradable alternatives to these flammable and/or toxic chemicals.

Solvents are generally preferred over water based detergents in order to minimise the presence of water which may aid corrosion in some cases, especially when held in crevices and corners.

Typical applications can include removal of adhesive protective laminates, finger prints, permanent marker pen or crayon marks and machine oil.

Anti Spatter Chemicals

Welding of metals often results in molten spatter falling on the surface of the steel being welded. This spatter solidifies leaving hard lumps which can only be removed by abrasion or polishing.

Mechanical treatment of the spatter deposits is time consuming and can be avoided by the use of special anti spatter sprays.

Solvent based products are preferred for stainless steel, while water based products are now emerging for treatment of mild steel.

Pickling solutions and pastes

Pickling is the removal of the oxide film from the surface of the metal by chemical means. An exposure to high temperature (e.g. during welding or heat treatment) will scale the surface. In the case of stainless steel, such high temperature scale has inferior corrosion resistance and must be removed. Pickling using formulations of hydrofluoric (HF) and nitric (HNO3) acids, removes the scale and helps to restore corrosion resistance through auto passivation providing the ambient conditions are appropriate. For applications in aggressive environments, it is advisable to develop full corrosion resistance by a passivation treatment subsequent to pickling operation.

Pickling agents can be sprayed over an entire surface, used in dip applications or as a paste applied to the weld and heat affected zone.

Pickling of stainless steel welds helps to remove both the oxide scale and free iron contamination. Once the pickling paste is completely rinsed away, the treated area will be chemically clean. As such, the surface has an enhanced ability to react with atmospheric oxygen to spontaneously passivate, providing the area is free from dirt and outside sources of contamination

Where the aesthetic appearance of the stainless steel is important, the pickling process will remove unsightly discoloration, leaving a clean professional finish.

Why passivate?

The failure of stainless components due to corrosion can reach catastrophic proportions. The cost in time, materials, rectification and lost production can be tremendous.

If passivation is not carried out stainless steel can rust due to surface free iron, and since chlorides are also often absorbed from the atmosphere, some ferric chloride will be produced.

Prolonged contact with ferric chloride will eventually initiate pitting and crevice corrosion on the stainless steel surface.

Therefore, proper passivation with a nitric acid based agent will dissolve the surface contamination and assist in the optimal restoration of the chrome oxide passive layer.

It should be understood that this passive surface condition is not a static situation. The chrome oxide layer is constantly affected by the environment and is slowly lost, but at the same time it reacts with oxygen to reform. The process is in dynamic equilibrium. Only when the balance is brought towards loss of the passive film does corrosion occur.

Passivating Solutions and Pastes

Pickling not only removes the scale around the heat affected area, but it also removes chrome oxide and surface iron and metal contamination, and therefore leaves the surface of the steel readily available to react with atmospheric oxygen to restore the passive field (i.e. auto passivation).

The passive film restoration is dependent on the availability of oxygen for its formation, and also subject to inhibition by atmospheric pollution, airborne chlorides (present as sodium chloride - NaCl - especially near coastal areas) and the presence of mild steel in the same workshop if not strictly controlled.

Where the chromium content is in excess of 12.0%, the formation of a chromium oxide passive film on the surface is possible either through auto-passivation or enhanced passivation with nitric acid.

Enhanced passivation with a nitric acid based passivating agent is recommended following grinding, pickling or polishing of stainless steel where the surface has been made more active by these processes. The passivating agent enhances the passivation process in a more uniform manner to restore the chrome oxide film and help to prevent future corrosion following installation of the fabricated product.

When stainless steel is commissioned into high corrosion environments, such as at sea, near the sea, in polluted environments etc., then use of a passivating agent is highly recommended to ensure a resilient passive film is achieved.

If stainless steel is corroded in situ, it may be repaired by either polishing or chemical deoxidisation. In either case, the surface of the steel will be active and require a passivation treatment.

Neutralising solutions for trade waste compliance

Many companies now face the implementation of ISO 14000 Environmental Management Systems and ISO 18000 Occupational Health & Safety Systems.

Environmental and occupational health and safety issues are strongly impacting industrial firms which discharge liquid waste, especially pickling and passivating chemical waste.

Low cost alkaline solutions or pastes can be used to remove these chemicals after use from the surface of the steel, or added to holding tanks containing the acidic waste prior to discharge into the sewer or water system.

Pickling chemicals contain hydrofluoric acid and nitric acid and are hazardous. Removal of the pickling agent using a neutralising solution also helps to aid worker safely.

3. ALUMINIUM CLEANING PROCESSES

Most stages of the fabrication life cycle require some form of chemical treatment, including:

• pre weld cleaning
• weld spatter protection
• treatment of acidic waste for trade waste compliance
• cleaning of finished product and prior to painting
• deoxidising of surface corrosion prior to welding for maintenance

Chemical treatment agents for the aluminium fabricator

• pre weld cleaner
• post weld pickling agent
• surface brighteners
• solvent degreaser

Pre weld cleaner

Pre weld cleaning is very important for aluminium to ensure an oxide and water break free surface is obtained to maximise the quality of the weld.

This can be accomplished with an aluminium compatible solvent or non etching acidic detergent.

Acid cleaners are generally composed of either organic or inorganic acids such as phosphoric or gluconic, for example, along with water miscible solvents and organic wetting and emulsifying agents. Soil is removed by etching, wetting, emulsification or dissolution.

Best results for pre weld cleaning usually achieved by hand wiping.

Post weld pickling agents

Mild etching of aluminium after welding can be used to remove heavy oxides, corrosion products and heat discoloration. Hydrofluoric and sulphuric acid solutions or pastes can effectively treat the weld area and heat affected zone.

It is important that stainless steel pickling agents are not used on aluminium as they are far to strong and would result in significant and excessive etching.

Aluminium Brighteners

Aluminium pickling acids or an appropriate acidic detergent will remove oxides and corrosion products without etching the aluminium surface and leave a clean bright surface.

Application by immersion, brushing, spraying or hand wiping.

Removal of corrosion prior to maintenance

Aluminium pickling solutions or pastes can also be used prior to welding to remove resilient surface corrosion and oxides which would interfere with weld quality.

SUMMARY

Quality finish during fabrication of stainless steel and aluminium can be enhanced by chemical treatment prior to and after welding.

In particular, corrosion resistance can be maximised through the passivation of stainless steel following pickling, grinding or polishing during fabrication or maintenance.

New chemical treatment technologies has proven to be more cost effective than traditional mechanical and labour intensive methods for stainless steel and aluminium pre and post weld treatment.

REFERENCES

Full details of the pickling and passivation processes are available in ASTM A380-94a, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems

The Australian Stainless Steel Development Association - Australian Stainless 1996 Reference Manual Guide to Stainless Steel Passivation - Halide Technology Pty Ltd Metal Finishing - Guidebook and Directory Issue '93 Proprietary Test Data - Callington Haven Pty Ltd, Sydney

John Hill
Callington Haven, AUSTRALIA