Stainless Steel Pickling

Definition

As per ASTM A380; Pickling is a chemical treatment utilising strong, corrosive acids that remove metallic contamination, welding and heat-treatment scales. 

Fabrication processes such as cutting, forming, bending and grinding can introduce metallic contamination, and embed it into the stainless steel surface. If this foreign material is carbon steel, then corrosion at a later date is all but inevitable. Embedded iron dissolves readily in the pickling chemicals (much more easily than the stainless alloy itself), so pickling is the ideal method in which to eliminate this problem before the component goes into service. 

Welding and heat-treatment processes produce a scale on the surface. The scale is rich in chromium, which has been drawn from the underlying alloy. This leaves a chromium depleted zone directly beneath which is vulnerable to attack, particularly in the aggressive environments found in the oil and gas, petrochemical, food manufacturing, waste treatment and nuclear industries. After dissolving the scale, the pickling process then removes the chromium depleted zone, and exposes the base material, creating a good surface for subsequent re-passivation. 

I should add at this point, that the oxidised layer and chromium depleted zone is very, very thin! It is typically somewhere between just a few nanometers to a few micrometers thick, so pickling will not affect your tolerances.

Specifications

With regards to specifications, the industry go-to is ASTM A380 ‘Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment and Systems’. Virtually all industry-specific specifications are either based upon it or at the very least, refer to it. As such, it is the one that INOX Passivation’s pickling processes conform with.

The inspection section of this document, however, is principally focussed on general cleanliness and passivity, and shares little opinion on what is ‘acceptable’ with regards to pickling – other than “over-pickling should be avoided” and that “surfaces should be thoroughly rinsed”.

In order to quantify an acceptance level, most industries define their minimum standard in terms of the colour of the weld-heat-tint after pickling. The argument being, if the weld-oxide has been cleaned to a certain extent, enough material has been removed and it will suffice for the application. 

Traditionally, the “AWS D18.2: Heat-tint Levels on the Inside of Welded 316L Austenitic Stainless Steel Tube” image has been used as the guide.

Depending on the environment that the stainless steel will be exposed to, fabricators will specify a minimum standard between number 1 (no discolouration) to number 3 (a pale ‘straw-yellow’ colour).

Effectively, this means that the pickling contractor will subject the fabrication to the pickling process for a time long enough to remove all traces of the discolouration to at least the standard specified by the client.

Pickling Colour Chart

Other colour reference charts are sometimes referred to. The oil and gas industry often use the ones depicted by Force Technology. These differ slightly, in that there are a number of charts – for different materials (not just 316L tube). 

It should be noted that like the AWS chart, they were created to determine whether or not the welding process has been performed satisfactorily. The various colours represent the effectiveness of the gas purging and are not strictly representative of something that has been partially or fully pickled.

Despite this, it’s as good a way as any to at least draw a line in the sand and it provides a clear, visual indication of pass / fail for the pickling process for both parties.

Process Options

Successful pickling is a function of acid strength, acid temperature and contact time. All three variables can be manipulated (within reason) in order to suit the grade of stainless steel, the component being processed and of course, the client’s schedule. 

INOX Passivation specialise in the circulation and spray application of pickling products

Circulation

In the case of circulation, a temporary loop is created using the client’s system and our own hoses, fittings and pumps through which we transfer our various chemical products. Usually, the system is degreased, pickled and then chemically passivated before being thoroughly flushed with purified water. In the case of vessels, this method can be employed using spray-balls and is much more effective (both cost-wise and quality) than completely filling them with the chemical.

Spray Pickling

Alternatively, we can spray on a gel-version of the pickling liquid. This product clings to the fabrication for a sufficient period of time to remove the contaminants. It is then rinsed off using high-pressure water, with the spent chemicals and rinse-water being collected for subsequent disposal. 

Spray-pickling is a hazardous process. The chemical is applied via a spray-pump and a fine chemical mist is produced. The process is also more susceptible to creating fumes than other methods. Suitable precautions should be taken at all times.

Contact Time

The grade of stainless steel is a critical consideration when determining the contact time and acid strengths. For example, 304 grade pickles very easily when compared to super-duplex.

Depending upon the thickness of the material and severity of the weld-heat-tint, standard pickling solutions could process 304 in an hour or so. Super-Duplex could take in excess of 36 hours to fully remove the discolouration in the same solution. 

One way to speed the process up is to strengthen the pickling chemical. Most pickling companies will use a standard blend of hydrofluoric and nitric acids – at a relatively weak concentration.

This ensures that the process is easily managed, is repeatable and reduces any risk of over-pickling. If the client’s schedule dictates a faster turnaround than usual is required, the chemical concentration can be raised to increase its potency. 

Using a stronger blend when processing the more resistant alloys, can help to speed up it up. Using it on 304 could lead to over-pickling if due care is not taken.

There is a limit to how much extra hydrofluoric and nitric acid you can add. At a certain point, it stops making any difference to the contact time and instead becomes a likely source of damage – not to mention an increased health and safety hazard.

Alternatively (or simultaneously), the chemical temperature can be raised. As a (very) rough rule of thumb, the contact time can be halved for every additional 10 ℃ of heat applied. 

i.e., what takes 4 hours at 20 , will take 2 hours at 30 and 1 hour at 40 .

The significant downside to this approach – other than the heating bill – is the increased level of hazardous fuming, particularly nitrous oxides (NOx), that increase as the chemical gets hotter.  

Health & Safety

Hydrofluoric (HF) and nitric acids are strong corrosive chemicals. They are potentially very damaging to the non-compatible materials (carbon steel, for example, is very vulnerable to HF/nitric pickling solution) and extremely hazardous to personnel. 

Hydrofluoric is toxic and all operatives who are expected to handle or come into contact with it should be well trained in the associated risk mitigation techniques, correct PPE requirements/ usage, and all spill and first-aid responses.

HF is particularly unpleasant in that you may not even notice at first that you’ve been exposed to it. In the meantime, the fluoride ions work their way through your skin and start attacking the calcium in your bones and blood. By the time you feel a burning sensation, it may be too late to wash it off. If you have been exposed to enough of it, you will be at risk of going into toxic shock and cardiac arrest, amongst other life-threatening effects.

Thus, there are various hydrofluoric acid ‘antidote’ products on the market. Most commonly used is calcium gluconate gel. This works by offering an alternative source of calcium for the HF to attack (other than your bodily fluids). It should be rubbed onto the affected area generously – and repeatedly. 

Other popular products have hexafluorine as their key ingredient. These are supposedly faster acting than calcium gluconate and, as it comes as a liquid, easier to apply to sensitive areas (it’s easier to spray hexafluorine into your eyes than to rub a gel into them for example!).

Nitric acid produces NOx fumes. The quantity of which are directly affected by the dissolved iron concentration within the pickling solution and as mentioned previously, the acid temperature. These fumes are very noxious and hazardous to health. They should be controlled where possible by employing suitable LEV equipment and a scrubber system to neutralise them. Operatives at risk of exposure to these fumes should wear appropriate RPE.

In short, pickling products are pretty nasty stuff and should only be handled by trained personnel.

Nitric-free Solution

Like all other industries, the metal finishing one has been subject to significant increases in demand relating to quality and speed of service. 

As our clients improve their own processes and work towards optimising effectiveness and longevity of their equipment, we have seen a rise in the use of high-alloy grades of stainless steel and a reduced compromise in the colour acceptance chart.

It is not unusual for the project specification to state that all grades of stainless steel shall be pickled in accordance with number 1 on the AWS chart. (no discolouration at all). 

Whilst this is possible, it can be very time-consuming to achieve on the duplex grades, particularly on circulation projects where it may be impossible to rinse the heat-affected areas with high-pressure water. 

In some cases, this can have a massive implication on the project schedule. Systems fabricated from super duplex could take days of exposure to pickling liquid in order to achieve the correct finish. Increasing the acid concentration and temperature can speed this up, but it will bring about the significant increased health and safety risks described above.

INOX Passivation is able to offer a nitric acid-free pickling liquid. This means that we can heat the chemical up to 50 ℃ with absolutely no NOx fumes. Because of the high temperature (and increased effectivity), it also means we can keep the hydrofluoric concentration relatively low. There is no need to ‘spike’ it too dangerous levels. 

With this product, we are able to dramatically reduce the contact time. 

Using the rough formula described above, if it takes 36 hours to pickle super-duplex with conventional chemicals at 20 ℃, it would take just 4.5 hours to get the same finish at 50 ℃.

The benefits are obvious. Instead of having pickling liquid circulating around your plant for 2 days substantially increasing the labour cost element and usually creating an enormous “off-limits” area for the duration, the process could be wrapped up before dinner time.

Protect your investment

If your stainless steel components will be used in a hostile environment, it is essential that they are pickled to restore the alloy’s natural corrosion-resistant qualities. Many of the industries listed earlier use stainless steel in applications that make it all but impossible to carry out routine maintenance.

The warranty you give your client is presumably based partly on the corrosion resistance of whatever grade steel you used. By failing to repair the damage created during fabrication before it goes into service, you are potentially installing equipment that does not meet the criteria listed in your material certification.