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Is there a need to Pickle and Passivate after bead blasting?

The answer to this question is that pickling in compliance with ASTM A380 is required after bead blasting to provide a passive surface.

Why pickle in compliance with ASTM A380?

Bead blasting with glass is generally used to match the appearance of different components such as castings or plate/sheet or pipe. The beads deform the surface but do not remove much, if any, material. The process is therefore ineffective in removing heat tint or any chromium depleted layer underneath. The “splashing” of the metal can trap fragments from the blast media and provide micro-crevices which are potential corrosion initiating sites. In addition, the fresh metal exposed by the blasting will uncover the sulphide inclusions scattered throughout the metal and these can also be initiation points. Passivation to ASTM A967 would remove the sulphide inclusions but not the metal splashes. It requires pickling to ASTM A380 to dissolve the metal splashes and both release any trapped blast media, dissolve any exposed sulphide inclusions, and provide a passive surface.

Surface roughness!

A surface roughness of 0.5µm Ra (or ~20µ”) will be achieved by a 320-grit abrasive finish which is specified by a 2K finish in EN 10088.2 as suitable for exterior, corrosion resistant exposures. Pickling a mechanically roughened surface will usually slightly reduce the Ra. However, the Ra of blasted surfaces depends on blast time, blast pressure and media profile, for example bead or scrap glass, so the Ra of a blasted surface is less predictable. Pickling will improve a slightly smoother finish.

ICS has access to a dedicated stainless steel technical team so if you have any technical questions regarding any aspect of stainless-steel, please do not hesitate to contact us on 08 9497 3500.

ICS is a Member of the Australian Stainless Steel Development Association (ASSDA) and this article has been peer-reviewed by their technical specialist.

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Stainless Fabrication: Common Traps to Avoid

Errors in stainless-steel fabrication can be expensive and difficult to resolve. So a ‘Get it right the first time’ approach to stainless fabrication is necessary to gain the best result. Check the ASSDA website regularly for a local Stainless Steel Specialist.

ASSDA Accredited Fabricators – Ensuring the Best Result

ASSDA Accredited Fabricators are companies and individuals that have a common understanding of successful technical practices for fabricating stainless-steel.

To ensure the highest standard in quality, Accredited Fabricators belong to the ASSDA Accreditation Scheme, an ASSDA initiative that is intended to achieve self-regulation of the industry, for the benefit of both industry members and end users.

The Accreditation Scheme criteria requires all fabricators to conform to stringent standards of competence, training and education, personal and professional conduct, adhering to a Code of Ethics and a Code of Practice, and committing themselves to continuing competency development.

The Scheme gives owners and specifiers of stainless-steel greater certainty that fabrications using stainless-steel will be performed by technically competent industry specialists.

Common Traps to Avoid

Surface damage, defects and contamination arising during fabrication are all potentially harmful to the oxide film that protects stainless-steel in service. Once damaged, corrosion may initiate. Common causes of surface damage and defects during fabrication include:

Scratches and Mechanical Damage

Scratches and gouges form crevices on the steels surface, allowing entrapment of process reactants or contaminants, providing ideal locations for corrosion. Scratches may also contain carbon steel or other contaminants embedded by the object that caused the scratch.

Scratches will also raise customer concerns in situations where appearance is important. Mechanical cleaning is the most effective way to remove them. Prevention would be better.

Surface Contaminants

Common contaminants likely to attack stainless-steel include carbon steel and common salt. Dust and grime arising during fabrication may contain these contaminants and should be prevented from settling on stainless-steels.

Oil, grease, fingerprints, crayon, paint and chalk marks may also contain products that can provide crevices for localised corrosion and also act as shields to chemical and electrochemical cleaning. They should be removed.

Residual adhesives from tape and protective plastic sometimes remain on surfaces when they are stripped. Organic solvents should remove soft adhesive particles. If left to harden, adhesives form sites for crevice corrosion and are difficult to remove.

The most frequently encountered fabrication problem is embedded iron and loose iron particles, which rapidly rust and initiate corrosion. Other common sources of contamination are abrasives previously used on carbon steel, carbon steel wire brushes, grinding dust and weld spatter from carbon steel operations, introducing iron filings by walking on stainless-steel and iron embedded or smeared on surfaces during layout and handling. All should be avoided.

Welding

The high temperature characteristics of welding can introduce surface and other defects which must be addressed.

Undercut, spatter, slag and stray arc strikes must be minimised as they are potential sites of crevice corrosion. General cleanliness and removal of potential carbon contaminants such as crayon marks, oil or grease is important in obtaining good weld quality. It is also important to remove any zinc that might be present.

Heat Tint and Scale

Heat tint and scale occur when stainless-steel surfaces are heated to moderately high temperatures in air (3500C+) during welding.

Deleterious oxides of chromium may develop on each side and on the under surface of welds and ground areas. These oxides lower the corrosion resistance of the steel and during their formation the stainless-steel is depleted of chromium. The oxidation and the portion of the underlying metal surface with reduced chromium should all be removed by mechanical, chemical or electrochemical means to achieve the best corrosion resistance.

Distortion

Stainless-steel has a relatively high coefficient of thermal expansion coupled with low thermal conductivity, at least compared with carbon steel. So, stainless-steel expands rapidly with the input of heat that occurs during welding and the heat remains close to the heating source. Distortion can result. Distortion can be minimised through using lowest amperage consistent with good weld quality, controlling interpass temperatures and using controlled tack welding, clamping jigs with copper or aluminium backing bars as heat sinks on the welds.1

Removal of Surface Contamination

There are three methods of repairing the surface of stainless-steel.

Mechanical Cleaning

Wire brushing should only be done with stainless-steel bristles that have not been used on any other surface but stainless-steel. Clean abrasive disks and clean flapper wheels are commonly used to remove heat tint and other minor surface imperfections. Also effective is blasting with stainless-steel shot, cut wire or new, iron-free sand (garnet is a common choice).

Chemical and Electrochemical Cleaning

Embedded iron, heat tint and some other contaminants can be removed by acid pickling, usually with a nitric-hydrofluoric acid mixture or by electropolishing. These processes remove, in a controlled manner, from the affected areas, the dark oxide film and a thin layer of metal under it, leaving a clean, defect-free surface. The protective film reforms after exposure to air.

Passivation

Passivation involves treating stainless-steel surfaces with, usually, dilute nitric acid solutions or pastes. This process removes contaminants and allows for a passive film to be formed on a fresh surface, following grinding, machining etc.

Care must be taken. Nitric acid treatments will remove free iron, but not iron oxide contaminants. Passivating, unlike pickling, will not cause a marked change in the appearance of the steel surface.

 Installation

Stainless-steel is best installed last to avoid damage during construction. Also, careful storage and handling including protective coating films are required prior to and during installation to minimise risk of damage to the stainless steel structure.

A primary goal of the stainless steel industry is to have finished products put into service in a ‘passive’ condition (free of corrosive reactions). Stainless steel is a robust and relatively forgiving material, but adherence to informed, good practice will ensure satisfaction for customers and suppliers alike.

Understanding stainless steel is important to its successful application. Ask your stainless steel representatives whether they have successfully completed ASSDA’s Stainless Steel Specialist Course. Their commitment to product knowledge will be your key to success.

References

  1. NI & Euro Inox (1994) Design Manual for Structural Stainless Steel NI Ref. No. 11 013

Resources

ASSDA acknowledges the assistance and contribution of Mr Peter Moore, Technical Services Manager of Atlas Steels in the production of this article.

The Australian Stainless Steel Development Association (ASSDA)

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Common misconceptions about stainless steel

Everyone knows that stainless steel resists corrosion, but beyond that, an amazing range of half-truths and exaggerations have evolved – often misleading and sometimes simply wrong. This article examines some of the more common myths, explains why they are wrong, and more to the point, provides correct information.

MISCONCEPTION: There are only two types of stainless steel, 304 and 316.

FACT: There are hundreds of stainless steels from high strength duplex 2205 supporting bridges, to furnace ducts of ferritic 3/5Cr12 and the high temperature 310, but the most common types are the austenitic 300 series.

Stainless steels were invented a little over 100 years ago. The corrosion resistance, ease of cleanability, and bright appearance of stainless steels meant its compound growth since 1950 has been about 5% year-on-year. Because of the ease of forming and welding, about 70% of stainless steel use has been within the austenitic family. Within 30 years the accepted chromium level for good corrosion resistance settled at about 18% and “304” was born. Then stainless moved to the seaside and corroded, which led to the development of “316” by adding molybdenum. This in turn created the popular myth of two readily weldable and formable stainless steels despite the hundreds of austenitic grades recognised in standards.

About 25% of global use is seen in (mainly) thin sheet ferritics for cladding. The remaining 5% sees strong duplex, extra strong martensitic blades and wear resistance, and the precipitating hardening grades where strength/hardness is the priority.

MISCONCEPTION: 316 stainless steel is a marine grade and is suitable for seawater immersion. 

FACT: Seawater has about 20 times the chloride level that 316 can withstand and it is worse if the surface is rough or has a crevice (such as a nut and bolt). Seawater suitable stainless steels are the super austenitic or super duplex grades.

316 is often referred to as the ‘marine grade’ but this simply means that, provided it has a good finish and is washed by rain or under a proper maintenance regime, it will remain bright and shiny. In seawater it will rust especially around hard fouling or crevices – think seashells or bolts – and even under deposits in the splash zone. Furthermore, in severe coastal applications where salty ocean spray is allowed to build up over time, 316 can visibly corrode.

MISCONCEPTION:  If it has rust stains, it is not stainless steel.

FACT: Carbon steel contamination or choosing the wrong grade of stainless steel are the usual reasons for rust on stainless steels.

If the rust occurs within a few days or weeks, it is almost certainly due to carbon steel contamination from fabrication or the local environment. Longer initiation periods arise from surfaces that are too rough, aggressive environments (think 304 posts on a wharf), lack of washing (drainpipes under eves) or bar product that has not been passivated.

MISCONCEPTION:  Stainless steel reinforcement will cause accelerated galvanic corrosion of carbon steel reinforcement.

FACT: In concrete, carbon and stainless steels have similar galvanic potentials.

Galvanic interactions occur between connected metals that have different potentials when immersed in a liquid that will cause one of them to corrode. Hence 304 bolts in a 316 panel immersed in tap water will not show galvanic effects despite the difference in potentials. In contrast, carbon steel will corrode more rapidly when coupled to copper or stainless steel in water. It is different in alkaline concrete as both stainless and carbon steel are at the same potential. It is common practice to use stainless steel reinforcement in tidal and splash zones or around penetrations and couple it to the rest of the carbon steel reinforcement.

MISCONCEPTION:  Only non-magnetic stainless steels have good corrosion resistance.

FACT: Magnetism is not related to corrosion resistance.

Probably because the lower chromium stainless steels are all magnetic, e.g., the 3/5Cr12 utility grades or the 410 or 420 or 440 martensitic, a myth perpetuated that magnetism and corrosion resistance were related. And then along came duplex grades with their resistance to seawater (and more aggressive environments) plus a strong ferromagnetic effect. The weak magnetic effect of heavily cold worked 304 versus the negligible magnetic effect of cold worked 316 may also have contributed to the myth.

MISCONCEPTION: Low nickel in stainless steels means it will corrode.

FACT: Nickel only affects the microstructure form, NOT corrosion resistance.

Nickel is a friendly metal and is the predominant influence in turning  ferritic stainless steel into austenitic or duplex grades depending on how much nickel is added. It has no effect on corrosion resistance to initiation of corrosion, which is how the integrity of stainless steel is judged.

MISCONCEPTION: Well-polished stainless steel does not require maintenance. 

FACT: Maintenance is important for the long-term effectiveness of any product. Stainless steel requires minimal maintenance but relies on preserving its passive film with oxygen and water.

Maintenance of stainless steel is required i.e., cleaning to remove adherent deposits left after rain washing. High polish will ease maintenance cleaning, but in the long term, general grime can accumulate just like the detritus on coatings or concrete.

MISCONCEPTION: Using a 316 nut on a 304 bolt stops galling of fasteners.

FACT: Austenitic stainless steels are widely used for corrosion resistant bolting, but galling control requires consideration beyond materials selection, including hardness, design and quality control, lubrication and friction.

Galling of fasteners is simply the cold welding of clean stainless steel surfaces under load. It is worse with fine threads, tight clearances, poor profiles, lack of lubrication, accumulated dirt and over-tightening. Because 304 cold works more than 316, the rule of thumb that a 50HB difference in hardness would prevent galling leads to the 304 cold rolled bolts and 316 machined nuts combination as a “solution”. It may work but the list of caveats above shows its limitations.

MISCONCEPTION:  Stainless steel is expensive.

FACT: The initial capital cost of stainless steel material may be a few percent more but, when considering life-cycle costing, stainless steel delivers long-term performance with minimum downtime and low costs associated with maintenance.

Using stainless steel does not require coatings, has reduced maintenance requirements compared to repainting or patch repairs of coated or galvanised steel, and can either be repurposed or recycled after its practical life. For example, replacing galvanised steel with 304/316 stainless steel in one particular wastewater treatment plant reduced downtime for refurbishment or replacement from 22% to a mere 2%.

There is a stark contrast between the maintenance of iconic structures built from different materials. The Eiffel Tower in Paris was constructed in iron and Sydney’s Harbour Bridge is the world’s largest (wrought) steel arch bridge, but both structures require regular repainting as part of essential maintenance. New York’s Chrysler Building is clad in stainless steel and has only required two washings in its 90-year history.

The Schaffhausen Bridge in Switzerland was built in 1995 with duplex reinforcement in the lower 7.6m of the pylons and 304 in the longitudinal reinforcement because of concerns about road de-icing salts. A life-cycle costing over 80 years showed that with stainless steel used for about 5% of the steel tonnage, stainless steel delivered 13% lower life-cycle costs over carbon steel.

Looking at sustainability, the Tokyo Water Authority reduced leakage in their potable water distribution system from 15.4% in 1980 to 3.6% in 2019 primarily by replacing mains to meter connections with corrugated stainless steel tube. To put this into context, since 1994 Tokyo has reduced annual water leakage by nearly 142 million cubic metres, with savings in excess of US$200 million per year.

This article is featured in Australian Stainless Magazine issue 73, 2021.

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ASSDA Accreditation

What does it mean to be ASSDA accredited?

Accredited Fabricators belong to the ASSDA Accreditation Scheme, an ASSDA initiative that is intended to achieve self regulation of the industry, for the benefit of both industry members and end users.

The Scheme gives owners and specifiers of stainless-steel greater certainty that applications using stainless-steel will be performed by technically competent industry specialists.

Because International Corrosion Services are ASSDA accredited we have access to the entire ASSDA team.  This allows us to offer the benefit of independent technical advice on stainless-steel and pass it on to our customers, even if our customers are not members.

The Australian Stainless Steel Development Association (ASSDA)

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Sustainability

Did you know that stainless-steel is 100% recyclable?

Not only is stainless-steel durable and low maintenance, it’s also a 100% recyclable commodity.  The majority of stainless-steel produced today has already repurposed. And although some stainless-steel will find its way to landfills or other disposal sites, there are no expected detrimental effects to soil or ground water.

Stainless-steel products are designed to have a long life; often spanning over several decades. The main alloying elements of stainless-steel, (chromium, nickel and molybdenum), are all highly valuable and can easily be recovered and separated from other materials.

So how does stainless-steel improve modern day life?

Stainless steel has many environmental and social benefits. Stainless steel products enable us to lead a healthier life and are cleaner for the environment as well:

  • Improvements in air quality
  • Reduction in the use of fossil fuels
  • Keeps water clean
  • Reduces waste
  • Saved costs for consumers and producers.
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