Stainless Steel - Design Considerations

Topics Covered

Background

Grade Selection

Design to Avoid Corrosion

Specific Design Points – To Retain Corrosion Resistance

Invert Structural Members

Ensure Tanks and Pipes Drain Fully When Idle

Raise Tanks Off the Floor

Smooth, Rounded Corners Inside Tanks

Insulation or Lagging of Pipelines and Vessels

Incomplete Filling Problems

Inlet Location

Reduce Splashing Within Tanks

Heater Location

Avoid Settling in Pipes and Vessels

Pipe Welds

Structural Attachments

Welding Mild Steel to Stainless Steel

Weld Repair to Avoid Crevices

Preparation for Welding

De-scaling of Welds and Surface Cleaning

Avoid Sharp Machined Corners

Avoid Sleeving of Shafts

Background

Whatever the intended function of a component it is essential that the design also consider the way in which it will be constructed. Some of these considerations such as allowing sufficient access for welders and for the tightening of bolts, are common to other metals, but in other respects the unique properties of stainless steels need to be considered.

Because of stainless steels' high expansion rate and low thermal conductivity it is important to not unduly restrain components during welding, and to not constrain carbon steel and stainless steel which behave differently when heated.

The machinability of most stainless steels is somewhat lower than that of many other metals, so there is extra incentive to reduce the amount of machining required. One way in which this can be achieved is to use bright (ie cold finished) bar of a size such that no machining is required over the largest component size; this same result may be achieved by using bright finished hexagonal or other shaped bar. Use of hollow bar can also substantially reduce the amount of machining required to produce certain components, particularly flanges, bushes etc.

If machining is to be carried out it is important that sufficient clean-up be allowed so that a reasonable cut is achieved; very light cuts can result in the tool skidding across a very heavily cold worked surface; good machining practice for stainless steels, and the austenitic (300 series) grades in particular, is to use heavier feeds and lower speeds than for carbon steels.

Grade Selection

If extensive machining is to be carried out the use of free machining grades 303 or 416 should be considered, but consideration must also be given to the relatively low corrosion resistance, weldability and formability of these grades.

Improved Machinability grades such as the "Ugima" range (eg Ugima 304 and Ugima 316) are now available - these offer better machinability than standard stainless steels but still retain the excellent corrosion resistance, weldability and formability of their standard grade equivalents.

Stainless steel grades can also be selected for ease of cold forming; Grade 302HQ (UNS S30430) is a low work hardening rate grade available in wire form specifically for the cold forming of fasteners such as bolts and screws. By contrast Grade 301 and 304 have a very high work hardening rate and can be supplied in a heavily cold worked condition suitable for the manufacture of springs; these require no hardening treatment after forming.

Components to be welded must be fabricated from a grade selected on that basis; to avoid problems associated with "sensitisation" - caused by holding in the temperature range of about 450 to 850°C - it may be necessary to use a low carbon "L" grade or a stabilised grade such as Grade 321. In the case of all welding it is essential that welding consumables are selected to match the grade being welded.

Design to Avoid Corrosion

When designing for stainless steel fabrication it is necessary to be aware of the factors which can cause premature corrosion failures. The principal problems are:

         General corrosion - a widespread wall thinning caused typically by exposure to strong reducing acids particularly at high concentration or temperature

         Pitting corrosion - related to chlorides, even in low concentrations and particularly at slightly elevated temperatures

         Crevice corrosion - also related to chlorides but made worse by small crevices in which liquid is trapped

         Intergranular corrosion due to prolonged heating in either welding or in application in conjunction with incorrect grade selection

         Stress corrosion cracking due to applied tensile stress, again in conjunction with chlorides and raised temperature

         Galvanic corrosion due to proximity of metals widely spaced in the electrochemical series

         Contact corrosion due to contamination of the stainless steel by a material such as mild steel particles.

Often measures to prevent several of these problems are similar. Design of stainless steel components must be made to prevent build-up of stagnant water, to encourage circulation of liquids, to discourage evaporation-concentration and to keep stresses and temperatures as low as possible.

Good design alone is not sufficient to prevent problems; fabricators must also be aware of these problems and may need to modify their practices accordingly

Specific Design Points – To Retain Corrosion Resistance

Invert Structural Members

Avoid entrapment of moisture within members and within attachments. Stagnant liquid remnants are likely to concentrate and to lead to pitting corrosion.

Ensure Tanks and Pipes Drain Fully When Idle

Tanks and pipelines left with small residual fluid quantities also encourage pitting corrosion. The problem is made worse if the fluid is spread to a thin film.

Raise Tanks Off the Floor

Tank bottoms placed directly on concrete floors will create crevices; ideal sites for corrosion in the event of liquid spillage. Sealing the gap improves the position, but is subject to misapplication and deterioration. A drip skirt prevents liquid collecting beneath the tank, while raising the tank on legs removes the crevice entirely.

Smooth, Rounded Corners Inside Tanks

Efficient maintenance cleaning of tanks is often important to remove built-up debris or stains; this reduces the likelihood of crevice corrosion under sediments and also may be important to retain hygienic conditions or prevent product contamination. All internal tank corners should if possible be well rounded and smooth. Welds should be in tank sides, not at corners. Welds should also be ground smooth (much easier if the weld is in the side, not corner) and full penetration or from both sides. All of these measures improve the fatigue resistance of the structure, as well as removing crevices.

Insulation or Lagging of Pipelines and Vessels

Thermal insulation of tanks and pipes should be free of chlorides. The insulation should be clad to totally prevent entry of water, as pitting corrosion or stress corrosion cracking can occur in the warm, moist environment. The outside of a hot tank or pipe can be a highly corrosive environment because of evaporation of liquid resulting in very high localised chloride contents. The outside may in fact be a more corrosive environment than the inside!

Incomplete Filling Problems

Vapour phases given off by some fluids can be quite corrosive; in these cases if the tank cannot be filled completely the vapour space should be well ventilated to remove the vapour.

Inlet Location

When dosing or making up a tank a highly corrosive chemical may be added. In these instances it is important that the inlet be located away from side walls and in a moving liquid stream, so that the addition is quickly diluted.

Reduce Splashing Within Tanks

A further problem may be caused by splashing during filling or mixing - splash drops on the inside tank walls will undergo evaporation and hence concentration of the corrosive species. Splashing should therefore be avoided, perhaps by ensuring that the inlet pipe terminates beneath the liquid level or by running mixing propellers slowly and fully submerged.

Heater Location

Corrosion of all types proceeds more rapidly at higher temperatures. It is therefore important that immersion heaters in vessels are placed so as not to locally heat any section of the vessel wall, and processes should be run at the lowest constant temperature possible.

Avoid Settling in Pipes and Vessels

Crevice corrosion can occur beneath debris which settles out of stagnant or slow moving liquid, and in some environments low liquid velocity also permits marine organisms to grow on the steel, with similar increase in crevice corrosion initiation. Designs should both maintain a reasonable flow rate (about 1 m/sec has been shown to substantially decrease the pitting rate in sea water) and result in total draining when the operation ceases. Dosing the fluid with a biocide may be a solution to the fouling problem, but chlorinated biocides such as hypochlorites can themselves be highly corrosive to stainless steels. Over-dosing must be avoided.

Pipe Welds

When joining pipe runs or using butt welding or socket weld fittings it is highly desirable to achieve full penetration welds. Incomplete weld penetration will result in a joint which appears good on the outside but has severe crevices at the root of the weld. Full penetration can be facilitated by the use of consumable inserts or by GTAW with hand fed filler metal.

Structural Attachments

Crevices are readily created when supporting rings, attachment pads etc are welded to stainless steel vessels. Intermittent weld runs may give adequate mechanical strength but only fully sealed welds give freedom from crevices.

Welding Mild Steel to Stainless Steel

Mixed-metal welding can be satisfactory, generally using an over-alloyed welding consumable such as Grade 309L, but caution must be exercised to prevent carbon migration into the part of the stainless steel exposed to the corrosive environment. Differential expansion rates can also lead to excessive stresses, particularly at tack welds. Stainless steel attachment pads between a stainless steel vessel and its mild steel support can assist in reducing these problems.

Weld Repair to Avoid Crevices

If a leaking tank is patched this should be done in a manner which avoids creating further crevices, on either inside or outside. For example a butt welded patch should be used and the weld dressed rather than using a lap welded patch.

Preparation for Welding

Minimum amounts of energy should always be put into stainless steel welds. The material should be carefully prepared and shaped before welding commences. The welder should avoid chill casting the first metal laid down and shrinkage cracks in the final weld pools.

De-scaling of Welds and Surface Cleaning

To achieve maximum corrosion resistance all weld scale should be removed and the surface ground, polished and possibly buffed to the specified finish. Best resistance to corrosion is achieved when the steel surface is mirror smooth and totally free of scale or other contaminants. Pickling or passivating will assist in this process, and "pickling paste" is available to easily carry it out.

Avoid Sharp Machined Corners

Machining should avoid sharp internal corners at sites such as changes of shaft diameter and the corners of keyways, as these may act as crevices both by their own geometry and by the tendency during service for material to build up in these sites (and perhaps be difficult to clean out). Radiusing of corners will also improve the fatigue resistance of any component subject to fluctuating stresses, as it does also for other materials.

Avoid Sleeving of Shafts

Fitting of sleeves to shafts creates possible crevice sites between the inner and outer components. A better solution from this point of view is to machine from a solid bar.

 

Source: Atlas Steels Australia

 

For more information on this source please visit Atlas Steels Australia

 

Date Added: Jan 2, 2002 | Updated: Jun 11, 2013
Comments
  1. pip beveridge pip beveridge Japan says:

    If a fuel tank (Low Sulphur FO) is constructed of mild steel, with a 316L small hatch and combing, would you expect corrosion at the interface (309L welding consumable), or anywhere else in the tank over the course of 25 years?  Is it safe to use Stainless for the manhole/coaming or should Mild Steel be used?

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