Chemical Treatment Enhanc...

Chemical Treatment Enhances Stainless Steel Fabrication Quality
Pickling, grinding, or polishing during fabrication and maintenance can maximize corrosion resistance
By John W. Hill

Stainless steel has found widespread industrial applications in areas such as processing plants and equipment, building facades, railings, storage tanks, and piping in food and pharmaceutical factories. Chemical treatment of this metal is important to maximize 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. Stainless steel is often perceived as being resistant to corrosion, relatively inert, and requiring minimal treatment in fabrication and little maintenance.


Fig. 1 - The weld scale and heat tint associated with GMAW of stainless steel and the visual effect of pickled welds at the top of the tank.
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 nanometer-thick film of chromium 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 after its removal over a period of at least 824 h. This means stainless steels are self passivating under favorable conditions. Disruption of the passive film by chemicals, mechanical action, embedded iron particles, or oxygen starvation can easily 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, 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 decomposition of organic compounds, such as PVC and marking materials.

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
  • treatment of acidic waste for trade waste compliance
  • cleaning of finished product
  • deoxidizing of surface corrosion for maintenance.

Chemical Treatment Agents for the Stainless Steel Fabricator
Agents for chemical treatment of stainless steel include

  • solvent degreaser/cleaner
  • antispatter spray
  • pickling solution or paste
  • passivating solution or paste
  • neutralizing 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 were routinely used for surface cleaning and degreasing. In many countries today, however, occupational health, safety, and environmental regulations have focused attention on safer nontoxic and biodegradable alternatives to these flammable and/or toxic chemicals.


Fig. 2 - Exterior cleaning process to remove surface contamination and maximize corrosion resistance and appearance.
Solvents are generally preferred over water-based detergents to minimize the presence of water that may aid corrosion, especially when held in crevices and corners. Typical applications can include removal of adhesive protective laminates, fingerprints, permanent marker, pen, crayon marks, and machine oil.

Antispatter Chemicals
Welding often results in molten spatter falling on the surface of the steel being welded. This spatter sometimes solidifies, leaving hard, unsightly deposits that can be removed only by abrasion or polishing.

Mechanical removal of spatter deposits is time consuming and costly. Such rework can be avoided by using special antispatter sprays. Antispatter spray leaves a protective film on the steel's surface to prevent spatter from sticking once solidified. Solvent-based products are preferred for stainless steel, while water-based products are now emerging as low-cost products for the treatment of mild steel.

Pickling Solutions and Gels
Pickling is the removal of oxide film from the surface of the metal by chemical means. Exposure to high temperatures (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. ASTM 380-94a, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems (Ref. 1), recommends pickling using formulations of hydrofluoric (HF) and nitric (HNO3) acids to remove scale and help restore corrosion resistance through autopassivation, provided ambient conditions are appropriate. For applications in aggressive environments, it is advisable to develop full corrosion resistance by a passivation treatment subsequent to the pickling operation using a nitric acid-based gel or solution.

Pickling agents can be sprayed over an entire surface, used in dip applications, or applied as a gel to the weld and heat-affected zone. Pickling of stainless steel welds helps remove both oxide scale and free iron contamination. Once the pickling agent 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. Newer generation gels are proving to be far more effective and easier to use than older paste-based products.

Where the aesthetic appearance of the stainless steel is important, the pickling process will remove unsightly discoloration, leaving a clean, professional finish. Several papers have highlighted the importance of pickling as a superior postweld surface treatment method as compared to mechanical treatment methods such as grinding and sandblasting for improved corrosion resistance (Refs. 25) - Fig. 1.

Why Passivate?
Proper passivation with a nitric acid-based agent will dissolve surface contamination and assist in the optimal restoration of the chrome-oxide passive layer. 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. Failure of stainless components due to corrosion can reach catastrophic proportions. The cost in time, materials, rectification, and lost production can be tremendous.


Fig. 3 - Stainless steel wine tank highlighting the quality surface finish achieved through chemical cleaning.
It should be understood this passive surface condition is not a static situation. The chrome-oxide layer is constantly affected by the environment and will eventually be lost, but, at the same time, it reacts with oxygen to re-form. The process is in dynamic equilibrium. Only when the balance is brought toward loss of the passive film does corrosion occur (Ref. 3).

Passivating Solutions and Gels
Pickling removes not only the scale around the heat-affected area but also the chrome-oxide and surface iron and metal contamination, leaving the steel's surface available to react with atmospheric oxygen to restore the passive film (i.e., autopassivation).

Passive film restoration is dependent on the availability of oxygen for its formation and is 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.

When chromium content is in excess of 12%, the formation of a chromium-oxide passive film on the surface is possible either through autopassivation 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 prevent future corrosion after 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., the 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 deoxidization. In either case, the surface of the steel will be active and require a passivation treatment.

Neutralizing Agents for Trade Waste Compliance
Environmental and occupational health and safety issues are strongly impacting industrial firms that discharge liquid waste, especially pickling and passivating chemical waste. Many companies now face the implementation of ISO 14000 Environmental Management Systems and ISO 18000 Occupational Health and Safety Systems.

Low-cost alkaline solutions or pastes can be used to remove chemical waste from the surface of the steel or added to holding tanks containing the acidic waste prior to discharge into the sewer or water system. Because pickling chemicals contain hazardous hydrofluoric and nitric acid, removal of the pickling agent using a neutralizing solution also helps aid worker safety.

Postfabrication or Postinstallation Cleaning
Following the fabrication or installation of stainless steel, the plant and equipment require cleaning to remove contaminants that can cause corrosion of welds, unsightly surface corrosion, and other blemishes and soils such as machine oil, grease, permanent marker pen, crayon, etc.

Proven cleaning agents for the removal of organic contaminants such as grease, crayon, and marker pen include new generation, nontoxic solvents (aliphatic hydrocarbons and d-Limonene) and less worker-safe products such as white spirits and thinners.

Phosphoric acid-based surfactants are ideal for removing light soils, contaminants such as iron oxides, and iron chlorides that may lead to corrosion. These acid-based cleaners also have a mild passivating effect and brighten the surface to improve appearance.

Citric acid-based surfactants are also available for light surface cleaning. The chelating effect of citric acid removes surface-free iron, thus assisting the natural passivation process. Citric acid is not a passivating agent itself. These cleaners may be used where environmental and occupational health and safety requirements limit the use of acid-based chemistry - Fig. 2.

Summary

  • Quality finish during fabrication of stainless steel can be enhanced by chemical treatment prior to and after welding.
  • In particular, corrosion resistance can be maximized through the passivation of stainless steel following pickling, grinding, or polishing during fabrication or maintenance.
  • New chemical treatment technologies have proven more cost effective than traditional mechanical and labor-intensive methods for stainless steel and aluminium pre- and postweld treatment - Fig. 3.

    References
    1. 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. 2. Tuthill, A. H., and Avery, R. E. 1993. Corrosion behavior of stainless steel and high-alloy weldments in aggressive oxidizing environments. Welding Journal 72(2): 41-s to 49-s. 3. Guide to Stainless Steel Passivation, Halide Technology Pty. Ltd. 4. Proprietary test data. Sydney, Australia; Callington Haven Pty. Ltd. 5. Gumpel, Vollmer, et. al. 1995. Stainless Steel Europe.

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