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Download ASSDA Technical FAQ 5: Galling and its Control

WHAT IS GALLING?
Austenitic stainless steels are widely used for corrosion resistant bolting. One of the major problems in use is that diassembly is difficult because nuts and bolts seize. This phenomenon is known as galling and it is most prevalent with intermittently operated, slowly sliding surfaces. It is caused by cold welding of the high points of clean, oxide free metal left when the oxide film is dislodged by surfaces rubbing against each other. In high temperature service, there may also be a contribution from oxidation forming abrasive oxides and also reducing clearances. Diffusion bonding of the metal is also possible during long-term exposure but is not as likely to cause problems at reasonable temperatures.

Titanium and aluminium also suffer from galling. Specialist literature suggests that anodising or lubrication are both effective remedies. Anodising of stainless steel, however, is not regarded as a remedy since the chromium oxide film is orders of magnitude thinner than the oxide film on titanium or aluminium.

HOW TO CONTROL GALLING
The measures listed are roughly in order of importance and are explained in the next section.

  • Hardness: ensure a hardness difference of at least 50 Brinell between the but and bolt.
  • Design and quality control: confirm that the mechanical fit is correct, the surfaces are smooth but not mirror like, and that they are clean.
  • Reduce friction: select an appropriate lubricant and/or substrate material. Lubrication is required even if the joint is intended to be permanent.
  • Control: use a torque wrench and the correct force for the material class and surface conditions.

THE DETAILS

Surface Hardness
Surfaces treated so that they are harder, for example, hard chrome plated, nitrided, carburised or cold worked surfaces are usually less susceptible to galling.

However, the corrosion resistance of nitrided or carburised surfaces is less than standard pickled and passivated stainless steel. Hard chromium electroplated coatings (3μm to 0.5mm) are not recommended for high temperature or high stress service as they may crack and spall.

A shot peened (and hence cold worked) surface would not normally be suitable. It will be rougher and thus be more susceptible to galling because of the roughness despite the increased hardness induced by the compressive stress from the peening. Cold rolled surfaces will be harder and usually more galling resistant than machined surfaces.

There is a rule of thumb that a hardness difference of 50 Brinell is required for effective prevention of galling and this is illustrated in the experimental data shown below.

Design and quality control

Surface finish
Highly polished surfaces (Ra<0.25μm) or very rough surfaces (Ra<1.5μm) tend to gall more. Cold rolled surfaces are better as they tend to be smoother than machined surfaces. Machined threads should be carefully deburred before assembly. Inspection with a low power lens (x5 or x10) is a useful control methods or, as a practical test, if a cloth snags when wiped over the thread surface, then the bolt is likely to gall when assembled. Significant galling problems may also arise with electropolished surfaces which can be very smooth, i.e. Ra<0.25μm. However, electropolishing smooths rough edges and for this reason is an advantage in reducing galling tendency.

Goodness of fit
Parts should be dimensionally tight enough to prevent vibration and wear with consequent roughness which might facilitate galling. However, sufficient clearance is required to avoid fouling during assembly.

Quality control
Good housekeeping is needed to exclude dirt or abrasive materials from between mating surfaces. It is also vital that the thread profile match, i.e. diameter, clearance and thread form.

Reduce friction

Lubrication
There are a wide range of lubricants containing nickel, molybdenum, copper, silver or graphite, or combinations of these. They can be messy. Those primarily containing nickel are more suitable for high temperatures. Two suppliers of nickel and graphite containing anti-seize compounds are Devcon and Loctite. Their websites offer selection tables depending on service requirements. Unasco also offers both a nickel and a silver impregnated Teflon® tape. Molybdenum disulphide may also be suitable.

The graph below shows that using a lubricant makes a substantial difference to the torque value for the bolt condition.

Be aware of possible crevice corrosion between the grease and the steel if the grease dries out and cracks or, if the grease does not properly wet the surface so that a cavity forms allowing water to accumulate. If graphite is used in large quantities then galvanic corrosion of the stainless steel is possible. Nickel plating has been used as a protection against such corrosion. Silver plating has also been used as a lubricant to prevent galling of nickel-copper (Monel) alloys in oxygen service.

Material selection
If the system is not lubricated or has insufficient lubrication, proper grade selection is vital to limit galling. There can be an improvement obtained by using two different stainless steels across the interface. This is largely related to the magnitude of the hardness difference between the two steels and some experimental information is shown below. There also grades which are self-lubricating such as the high nickel grade Waukesha 88 which contains tin and bismuth or Waukesha 54C which contains tin and silver. The sulphide particles in free machining 303 also appears to provide lubrication. The corrosion resistance of these grades should be carfeully considered. In addition, some lower nickel, 200 series grades with nitrogen, manganese and silicon additions, such as ARMCO's Nitronic® 60 or Carpenter Technology's Gall-Tough® and Gall-Tough Plus®, strongly resist galling.

It should be noted that while using 304 bolts with 316 nuts is often quoted as a cure for galling, the hardness difference is not necessarily sufficient to prevent galling even with a cold rolled, harder 304 thread used against an annealed, softer 316 nut. There are reports that A2-80 and A4-80 bolts are more resistant to galling than 70 class bolts, but there are other reports (www.cartech.com "Galling and stainless steels") that cold work increases risk.

If an austenitic bolt such as 304 or 316 is used, then galling could be minimised by using a hard nut of duplex stainless steel or martensitic 431 stainless steel. Alternatively, a soft (also less corrosion resistant) but of aluminium bronze could be used, provided that the environment was not so corrosive that it caused rapid wastage of the aluminium bronze nut.

 

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The magnetic properties of materials are affected by their composition, metallic structure, processing methods and physical condition. Ferromagnetic materials are strongly attracted to a permanent magnet and may also be magnetised to act as a permanent magnet.

Permeability is the property used to measure how well a material concentrates the magnetic field. It gives an indication of the strength of the attraction to a magnet.

It is more usual to refer to the relative permeability. This is measured relative to the value for air or vacuum taken as 1.

Cold work

Wrought, austenitic stainless steels, such as 304 and 316, are generally regarded as non-magnetic in the annealed condition, ie they are not attracted significantly by a magnet. However, if they are cold worked they will be attracted to a permanent magnet. The change occurs because the cold work deformation induces a transformation of the microstructure from austenite to martensite. The effect is less marked in alloys with high concentrations of austenite stabilisers such as nickel, nitrogen and carbon. Once the martensite is formed, it may also become magnetised sufficiently to pick-up light objects such as paper clips.

Magnetic attraction effects are most often noticed in heavily cold worked fabrications such as wire or the dished end of a pressure vessel. It is possible to remove the magnetic effects by solution annealing and water quenching but this will also reduce the tensile properties and may give rise to distortion.

Magnetic effects in annealed stainless steels

In contrast to the austenitic alloys, ferritic stainless steels such as 409 or 3Cr12/5Cr12 and martensitic stainless steels such as 420, are strongly attracted to a magnet even in the annealed state. The duplex and super-duplex stainless steels will also be strongly attracted because they contain about 50% ferrite in their microstructure.

The electrical demagnetising treatments applied when it is necessary to avoid permanent magnetic fields are fairly effective with magnetically soft materials such as ferrite. However, the strain induced martensite in an austenitic stainless steel and the normal martensitic structure of (say) a 420, are reasonably magnetically hard and, once they are magnetised, it is difficult to electrically remove the permanent magnetic effect.

Heat treatment, welding and magnetic attraction

Poor heat treatment or high heat input welding of normal or high carbon austenitic stainless steels will cause sensitisation, ie formation of chromium carbides. The formation of carbides not only reduces the corrosion resistance of the stainless steel but also tends to form martensite around the carbide. This martensite is magnetic and the more severe the sensitisation, the stronger are the magnetic properties. This effect should not be confused with the intentional formation of a few percent of magnetic ferrite in notionally austenitic welds. This low ferrite concentration is required to give hot strength, ie to stop hot cracking during welding. The magnetic effects of weld ferrite are usually insignificant because welds are only a small part of a structure.

Castings

Castings have subtly different compositions than the "equivalent" wrought alloys. The austenitic alloys typically have a few percent ferrite and are weakly attracted to a magnet, ie they are ferromagnetic. Ferritic, martensitic and duplex casting alloys have similar magnetic properties to their wrought counterparts.

Effect of cold work on austenitic stainless steels

The table below shows the relative permeability of 304 and 316 at a low magnetic field strength and various cold reductions. Highly alloyed austenitics including the high nitrogen grades do not develop low carbon martensite on cold work and so their relative permeability typically remains below 1.02. The values may be compared with mild or carbon steel which has a ferritic structure and a relative permeability of at least 200. Transformer steel has a relative permeability of at least several thousand.

The effect of composition and degree of cold work (measured by the tensile strength) on the permeability, and hence the strength of any magnetic attraction, is plotted below right for a series of austenitic alloys. It shows that increasing the nickel content reduces the effect of cold work on magnetic properties. Measurements were made on hot rolled sheet 2.4 to 3.2mm thick which was cold rolled to specific strength.

FAQ3 Magnetic pic1.jpg

FAQ3 Magnetic graph.jpg

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Important Disclaimer
The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

Download Technical FAQ 1

Contact between dissimilar metals occurs frequently but is often not a problem. The aluminium head on a cast iron block, the solder on a copper pipe, galvanising on a steel purlin and the steel fastener in an aluminium sheet are common examples.

What causes galvanic corrosion?

For galvanic or dissimilar or electrolytic corrosion to occur, three conditions must be met:

  • The metal join must be wet with a conductive liquid
  • There must be metal to metal contact
  • The metals must have sufficiently different potentials

Wetting the join

The conductive liquid (or electrolyte) could be rainwater or water absorbed into surface deposits if the relative humidity (RH) is high enough or even simple condensation. If the deposits are sea salt, then they will start to dissolve if the RH exceeds 34% because of the magnesium chloride. The greater the conductivity the more severe the galvanic effects. Salt or industrial pollution significantly increases the conductivity of water so galvanic effects are normally more severe near the coast or in heavy industrial areas. Low conductivity, pure rainwater will only cause slight galvanic effects. One complication is that during evaporation, water films become more conductive so initially benign water may cause quite active galvanic effects as the liquid in the crevice under a bolt or clamp becomes more concentrated. Water may be excluded by design or the use of adhesive sealants or by painting the noble metal for 30 to 50mm beyond the join to prevent charged atom (ion) transport in any thin water film. Painting the active metal (carbon steel or aluminium or zinc) can cause deep holes at coating defects.

Metal to metal contact

Galvanic corrosion can only occur if the dissimilar metals are in electrical contact. The contact may be direct or by an external pipe or wire or bolt. If the dissimilar metals are insulated from each other by suitable plastic strips, washers or sleeves then galvanic corrosion cannot occur. Paint is not a reliable electrical insulator especially under bolt heads or nuts or washers or near edges of sheets of metal. The paint is usually damaged on installation or by subsequent movement. Note that the chromium oxide film layer on the stainless steel is very thin and not an electrical insulator. Therefore the chromium oxide film will not prevent galvanic corrosion.

Potential differences

All metals dissolve to some extent when they are wetted with a conductive liquid. The degree of dissolution is greatest with active or sacrificial metals such as magnesium and zinc and they have the most negative potential. In contrast, noble or passive metals such as gold or graphite are relatively inert and have a more positive potential. Stainless steel is in the middle although it is more noble than carbon steel. The potential can be measured with a reference electrode and is used to construct a galvanic series as shown on page 2 (ASTM Standard G82).

When two metals are connected and in contact with a conducting liquid, the more active metal will corrode and protect the noble metal. Zinc is more negative than steel and so the zinc coating of galvanised steel will corrode to protect the steel at scratches or cut edges. The stainless steels, including 304 and 316, are more positive than zinc and steel, so when stainless steel is in contact with galvanised steel and is wet, the zinc will corrode first, followed by the steel, while the stainless steel will be protected by this galvanic activity and will not corrode. The rate of galvanic attack is governed by the size of the potential difference.

The graph shows that stainless steels have two ranges of potential. The usual, passive behaviour is shown by the light hatching. However, if the passive film breaks down, the stainless steel corrodes and its potential is in the dark bar range.

As a rule of thumb, if the potential difference is less than 0.1 volt, then it is unlikely that galvanic corrosion will be significant.

If all three conditions are met then galvanic corrosion is probable and the rate of corrosion will be influenced by the relative area and the current density delivered by the noble metal.

Relative wetted surface area

If a noble metal like stainless steel has a large surface area in contact with the electrolyte while the sacrificial metal (such as aluminium) has a very small surface area in contact with the electrolyte, then the stainless steel will generate a large corrosion current which will be concentrated on a small area of sacrificial metal. The aluminium will corrode quickly, and so aluminium fasteners in stainless steel are not acceptable. However, a stainless screw in aluminium is frequently used although corrosion of the aluminium immediately around the stainless is quite possible. This is because the ratio of wetted noble fastener in an active metal might change from a 1:50 ratio to 1:1 during drying after a rainstorm. If contaminants are significant this means that avoiding dissimilar metal pairs may be a preferred option to prevent galvanic attack.

Galvanised fasteners in stainless steel will also lose zinc more rapidly than stand alone exposures. An added disadvantage is that the corrosion product will turn from white to orange when the corrosion reaches the zinc-iron alloy near the bottom of the galvanised layer. After that, corrosion of the carbon steel fastener commences - again at a faster rate than stand alone exposures.

As a rule of thumb, if the wetted area of the corroding metal is 10 times the wetted area of the noble metal, then galvanic effects are not serious although the larger the ratio the less the effect.

Available current density

Stainless steel has an effective passive film so the available corrosion current able to be carried by charged atoms (ions) is quite low. If the behaviour of a copper/steel and a stainless steel/steel couple is compared, the copper/steel coupling is a more significant galvanic problem despite the similar potential separation of 0.35 volts.

Examples of acceptable galvanic pairs include:

  • The copper alloy potential is more active than the stainless steel and it provides cathodic protection current to limit pitting of the stainless steel shaft or crevice attack at the bearing sleeve. The depth of loss of the copper alloy is low because it has a very large area compared to the exposed shaft.
  • Galvanised steel pipe hangers are used to hang stainless steel piping externally around chemical plants. The surface area ratio is bad with large area of stainless steel to small area of active zinc/steel but the rainwater is usually of quite low conductivity and 20 year service life is normal.
  • In the water industries, galling between stainless steel threads and nuts has been avoided by using aluminium bronze nuts on stainless steel studs or bolts. Although aluminium bronze is more active than stainless steel, the conductivity of the water, and hence the corrosion rate, is generally quite low. The nuts will require replacement but only at times of major overhaul.
  • The potential difference between passive 304 and passive 316 is small so galvanic corrosion of the 304 is not expected, even with large area ratios.

Unacceptable material pairs include: a rubber seal with a carbon black loading so high (for UV resistance) that it is conductive and causes galvanic attack of a stainless screw or pin. Gaskets incorporating graphite have caused similar problems for stainless steel flanges and must not be used for seawater regardless of the stainless steel alloy. Uninsulated stainless steel fixings are not permitted for Colorbond® wall or roof sheeting as the galvanic current from the corroding Zincalume® blisters the paint.

Galvanic Series

 

 Important Disclaimer

The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

Download Technical FAQ 1

FAQ2 Cleaning pic.jpg

Download Technical FAQ 2

Quick and Easy Tips for Keeping That Shine

Retaining a sparkling finish on stainless steel surfaces is just a matter of a few simple steps. And you don't need expensive products or special equipment - ordinary household cleaners are usually all that's required.

You just need to bear in mind a few easy DOs and DON'Ts...

It'll come out in the wash

Stainless steel looks best if it's cleaned regularly with plenty of water. Drying afterwards makes sure streaky marks aren't left behind.

Remember that simply wiping with a damp cloth is not as effective as it can smear dirt without removing it.

Routine cleaning prevents any stubborn stains building up.

So what will you need?

You don't need any fancy equipment. For day to day cleaning, plenty of water, some mild detergent and a cloth or soft brush will do the job. You can use a 1% ammonia solution but don't use bleach ? it's just too easy to make the solution too strong and too hard to rinse it properly afterwards.

After washing, rinse in clean water and wipe the surface dry with a soft absorbent cloth. On brushed stainless steel, follow the direction of the polish for best results.

An excellent cloth to use is 3M's Scotch-Brite high performance cleaning cloth.

Watch out for scratches!

The important thing to remember is that stainless steel can be scratched by careless handling or aggressive scrubbing. Just like you would take care of a polished timber finish, avoid dragging rough items across the surface and be aware that grit trapped under other objects can be the culprit.

Avoid bad chemistry

Stainless steel may discolour if left in contact with salts or acids for extended periods. Also avoid leaving carbon steel items in contact with stainless steel, particularly if wet. But if you observe ordinary hygiene measures, like timely cleaning-up in food preparation areas, you won't have any problems.

How to handle the tough customers

Sometimes you need a tougher approach. Here's how to get rid of the most common offenders:

Fingerprints, oil & grease marks

If a mild detergent or dishwashing detergent doesn't shift unsightly fingermarks, get rid of them with a bit of glass cleaner on a soft cloth. You can also use a small amount of alcohol, methylated spirits, acetone or mineral turpentine. Then rinse with clean water and dry.

You can give longer protection to high traffic areas by lightly rubbing with olive oil or baby oil followed by a polish and shine using a soft cloth.

Tea & coffee stains

Discolouration from tea and coffee stains can be removed by soaking in a solution of boiling water and baking powder. Remember to rinse well and wipe dry.

Sticky labels

Remove sticky labels as soon as possible. Gentle heat from a hair dryer or a glue gun generally softens the glue for easy removal, or you can warm stainless steel pots and pans in the oven before peeling off the labels. Eucalyptus oil based cleaners (or eucalyptus oil on its own) often work well to remove adhesives.

Ensure you don't leave any glue on the surface ? it could trap dirt or break down and cause staining.

Rust marks

Apply cream cleanser with a soft damp cloth and rub gently.

If the mark still won't shift, it might be necessary to use a proprietary stainless steel cleaner. These are usually based on dangerous chemicals (such as phosphoric, oxalic or sulphamic acids) and must be handled with care according to the manufacturer's directions.

After cleaning it is important to neutralise the acid with a 1% ammonia or baking powder solution, rinse with clean water and wipe dry. If the rust has worn away the surface, don't despair! Bad rusting can be repaired with professional polishing but you will need to get expert advice.

Paint

Apply paint stripper, taking care to follow the safety instructions. You may need to use a nylon brush or scouring pad, but avoid metal scrapers at all costs - they will damage the surface.

Hard water scale

Heavy limescale from hard water can be loosened by soaking in a hot water and 25% vinegar solution. Rinse well with a solution of baking powder or 1% ammonia and then with clean water. Always wipe dry.

Cement and mortar

Cement and mortar splashes should be washed off before they set. Mild acids such as vinegar may be needed but not those using chloride rich chemicals. Never use brick cleaning liquids which contain hydrochloric acid. Be very careful that loosened particles don't scratch the steel surface.

Don't go against the grain

Always rub stainless steel in the same direction as the grain. Rubbing against the grain will spoil the finish and stainless will lose its shine. Worse, rubbing against the grain can damage the surface by creating microscopic crevices where dirt can collect. This can lead to corrosion spots.

Fortunately, it's usually easy to tell which is the right direction. You need to watch out for items like round handrails, which are often polished around their circumference when they're manufactured, rather than up and down the length of the tube.

If you have to scrub a stain to remove it, make sure you use a clean nylon scourer or a cloth with chalk-based cream cleaner. But test an inconspicuous area first as you could end up with a bright polished spot which doesn't match the rest of the surface.

NEVER EVER use steel wool (wire wool) to clean stainless steel.
It is usually made of carbon steel and any fragments left behind will rust onto the stainless steel surface. Using any kind of scourer which has previously been used on ordinary (carbon) steel is also a no-no for the same reason.

Stainless steel wool scouring pads are available for heavy duty work, like removing burnt food from stainless steel saucepans. These will scratch the stainless steel surface, but won't leave fragments to go rusty.

Download Technical FAQ 2

Important Disclaimer
The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

FAQ 1: Galvanic/Dissimilar Metal Corrosion

What is it and how to avoid it

Contact between dissimilar metals occurs frequently but is often not a problem. The aluminium head on a cast iron block, the solder on a copper pipe, galvanising on a steel purlin and the steel fastener in an aluminium sheet are common examples.

Continue reading here or download PDF copy.


FAQ 2: Cleaning Your Indoor Stainless Steel

Quick and easy tips for keeping that shine

Retaining a sparkling finish on stainless steel surfaces is just a matter of a few simple steps. And you don't need expensive products or special equipment - ordinary household cleaners are usually all that's required. You just need to bear in mind a few easy DOs and DON'Ts...

Continue reading here or download PDF copy.


FAQ 3: Magnetic Effects of Stainless Steels

The magnetic properties of materials are affected by their composition, metallic structure, processing methods and physical condition. Ferromagnetic materials are strongly attached to a permanent magnet and may also be magnetised to act as a permanent magnet.

Continue reading here or download PDF copy.


FAQ 4: Testing for Grade Confirmation

Raw material price fluctuations and increasing demand for stainless steel have driven demand for lower cost alloys as alternative to the traditional "300" series steels. This has been met through a range of existing and new, innovative steels with difference properties, performance and availability. But as with the traditional stainless steels, you can't tell what they are by looking at them. This article describes a range of test methods available for grade confirmation.

Continue reading here or download PDF copy.


FAQ 5: Galling and its Control

Austenitic stainless steels are widely used for corrosion resistant bolting. One of the major problems in use is that disassembly is difficult because nuts and bolt seize. This phenomenon is known as galling and it is the most prevalent with intermittently operated, slowly sliding surfaces. It is caused by cold welding of the high points of clean, oxide free metal left when the oxide film is dislodged by surfaces rubbing against each other.

Continue reading here or download PDF copy.


FAQ 6: Preventing Coastal Corrosion (Tea Staining)

When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments. But, like all materials, stainless steel may become discoloured over time, impairing the overall look. This brown discolouration - tea staining - has been identified in coastal applications in Australia and overseas.

Continue reading here or download PDF copy.


FAQ 7: Guidelines for Use of Stainless Steel Underground

Stainless steel can provide excellent service underground. It is stronger that polymers and copper, and its resistance to chlorides and acidic soils is significantly better than carbon or galvanised steels.

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FAQ 8: General Corrosion Resistance

The normal state for stainless

Stainless steels resist corrosion because they have self-repairing "passive" oxide film on the surface. As long as there is sufficient oxygen to maintain this film and provided that the level of corrosives is below the steel's capacity of the particular material to repair itself, no corrosion occurs. If there is too high a level of (say) chlorides, pitting occurs. As an example, 316 works well in tap water (<250ppm) all over Australia, but will rapidly corrode in seawater because seawater has very high chloride levels (20,000ppm).

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FAQ 9: Welding Dissimilar Metals

Welding the common austenitic stainless steels such as 304 and 316 to each other or themselves is routine and the easiest of fusion welding. Nevertheless, there are many situations where it is necessary to weld stainless steel to carbon steel. Two common examples are balustrade posts connecting supports to stainless steel vessels. There are differences in physical properties such as thermal conductivity and expansionl, magnetic properties, metallurgical structure and corrosion resistance, which all require attention. This article outlines the necessary procedures for satisfactory welding, including reference to standards, and explains necessary precautions.

Continue reading here or download PDF copy.


 

 

 

 

 

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