Corrosion

TA Banner CorrosionSynopsis

During heat treatment surface oxide layers, usually called heat tints, are formed on precipitation-hardening nickel-based alloys like UNS N07718. These oxide layers are composed of elements that have been selectively oxidised from the base metal, principally nickel, chromium and iron.

The same phenomenon has been intensively studied on austenitic stainless steels. It is well known that the region beneath the oxide layer is depleted in one or more of the elements that are involved in the scale formation.

Consequently, reduced corrosion resistance can be expected. 

Causes, consequences and prevention of corrosion during welding of nickel based alloys 

We have all seen the effect of welds carried out without adequate gas coverage

– the hot metal simply oxidises in contact with air. It comes as a surprise to most that oxygen contents as low as 50 ppm (0.005%) may
   produce discolouration or heat tint in stainless steels. Reduction in corrosion resistance can be significant 1 - 5.

Whilst it is not too difficult to protect the outside surface of a weld by using an inert gas as coverage, preventing oxidation and loss of corrosion resistance on the inside is often overlooked.

The technique of inside protection is known as ‘weld purging’ and uses inert gas to flush out air and thus oxygen before and during welding. 

Fig 1.
The result of unprotected underbead. Some operators still tolerate this and absorb the cost of removing it after welding.

Fig. 2
Even relatively low oxygen content in the purge gas can lead to ‘heat tint’.
This example is the result of using an oxygen content as low as 50 ppm

Fig. 3
To ensure no heat tint occurs the oxygen content needs to be reduced to 20 ppm (0.002).

The Mechanism of Corrosion

Stainless steels owe their resistance to corrosion to the formation of a very thin (10-5 mm), transparent surface layer of chromum oxide. This provides a passive film that acts as a barrier to penetration by an invasive environment. When heated to a high temperature in the presence of oxygen this film increases in thickness until it becomes visible – the colour becomes darker with increasing film thickness.
At a critical film thickness the film becomes unstable and begins to break down. The fractured zones created offer sites for localised corrosion.

Four principle mechanisms are involved;

  • Pitting corrosion.
  • Stress corrosion cracking.
  • Crevice corrosion.
  • Microbiologically Induced Corrosion (MIC). 

Pitting corrosion

Pitting corrosion is a form of localised corrosion. It produces attacks in the form of spots or pits. This type of corrosion attacks most often take place at points where the passive layer might be weakened and it occurs in stainless steels where oxidation has reduced the passivity. Once the attack has started, the material can be completely penetrated within a short time.

Stress Corrosion Cracking

Stress corrosion cracking is characterised by cracks propagating either transgranularly (through) or intergranularly (along) grain boundaries. It results from the combined action of tensile stresses in the material and the presence of a corrosive medium. It can be induced in some stainless steels by adverse heat treatments such as those occurring in weld heat affected zones.

Crevice Corrosion

Localised corrosion of a metal surface that is attributable to close proximity of another metal such as a weld. It is a locally accelerated type of corrosion and is one of the major corrosion hazards in stainless steels.

Microbiologically induced corrosion

Corrosion promoted or caused by micro-organisms, typically in industries related to food, beverage and chemical processing. It is usually referred to by the acronym ’MIC’ and is common in welded sections.

To avoid these forms of corrosion it is essential that heat tints are properly removed before the stainless steel equipment or piping is exposed to aggressive or aqueous environments.
The alternative to removal, often costly or difficult, is to prevent heat tinting during the welding process by using an inert environment to protect the surface.

Removal of Heat Tint

Light discolouration can be removed by bright annealing or acid pickling but heavier deposits may require machining such as grinding and polishing.
Removal clearly requires access to the area in question, not only for treatment but also for debris removal.
Even when access is available none of these treatments is easy and most can be very expensive.

Weld Purging

The alternative to potentially time-consuming, difficult and expensive cleaning of heat tint is to avoid it during the welding process. This can be undertaken by protecting the joint from oxidation by using an inert gas such as argon. The upper side of the joint is protected by the inert gas used in the torch. The underside, known as the underbead, needs separate treatment. This technique is referred to as weld purging. (Fig. 4).

Seals are inserted on either side of the weld root and inert gas is admitted to displace air in the space between them. A wide range of options for sealing are available but the only totally reliable and sufficiently versatile systems are those based on inflatable seals.

QuickPurge APSQ3 DIA 01 with Legend

Fig. 4 The inert gas weld purging concept.

Equipment has been developed over the past decade to make purging much easier. Currently available systems are robust and suitable for multi-use applications. They can be supplied to cover the size range between 25 and 2,235 mm diameter.

The most effective devices are those based on connected inflatable dams (Fig. 5). These can be programmed to control gas flow and pressure during inflation and purging and once placed in position require little more input from an operator. The dams are fabricated using advanced engineering polymers and are thus suitable for use where elimination of contamination is essential.
Purge gas oxygen content can be controlled by using an oygen monitor. These instruments not only measure oxygen levels, but can be programmed to stop the welding process if the level rises above that set by the procedure. Recording and analysing software provides information for quality control purposes.

APSQ Schematic

Fig. 5 Advanced integrated weld purge system base on inflatable dams.

Considerable design effort has been applied by the designers and manufacturers of these solutions over the past decade or so and currently available systems6 address the problems of controlled inert gas pressure and flow, the need for easy and rapid deployment and removal to limit overall welding time, thermal resistance and leak-tight access for oxygen monitoring equipment. They also provide a large pipe contact area and therefore excellent and reliable sealing.

Coupled with these advantages comes flexibility to allow access and removal through pipe bends, abrasion resistance and the use of materials that meet food, semiconductor and nuclear compliance standards.

Clearly a knowledge of the oxygen level in the purge gas is essential, recognising that a level as low as 20ppm may be necessary. This can be accommodated with a weld purge oxygen monitor and advanced versions of these have been developed for the welding industry (Fig 6).

Metal water connectors for the AUDI W12 engine

Fig. 6
Integrated weld purge monitor designed for site use. These instruments can combine monitoring
down to 10 ppm with software for data recording, analysis and quality control.

References

  1. Microbiologically influenced corrosion of stainless steel - 2nd symposium on orbital welding in high purity industries, La Baule, France
  2. Effects of purge gas purity and Chelant passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing. Pharmaceutical Engineering. Vol 17 Nos 1 & 2 1997
  3. Considerations for Orbital Welding of Corrosion Resistant Materials to the ASME Bioprocessing Equipment Standard. Stainless Steel America conference 2008
  4. Heat Tint Poses Corrosion Hazard in Stainless Steel. Welding Journal December 2014
  5. ASM International. Corrosion in Weldments. 2006
  6. www.huntingdonfusion.com



Biographymichael fletcher
Dr. Fletcher is a qualified metallurgist with extensive experience in welding and non-destructive testing.
He works as an independent consultant providing support to a wide range of manufacturing industry on a global basis.

 

 

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