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Picking the Right Materials for Desalination

Desalination is an important emerging process to bring fresh water to more and more parts of the world. As new processes emerge, we get the opportunity to explore newer and less corrosion-prone materials, meaning that desalination units last longer and work better.

The earliest desalination involved multi-stage flash, in which water is evaporated and then condensed in a series of flash chambers. Historically, these desalination tanks were made with shell materials that included mild steel or mild steel with a linking of copper-nickel, clad steel or stainless steel. Newer construction often includes solid duplex stainless steel instead.

Because of the risk of scaling with multi-stage flash, newer distillation processes have been created. In the new process, low temperature multi-effect distillation (LT-MED), distillation occurs under vacuum using lower temperatures, typically between 60 and 70 degrees Celsius. The lower temperatures involved means that less energy is used; however, this is a process that takes more time. The large surface areas needed can also mean higher fabrication costs. Over time, these distillation units have gone from being made with mild steel to being made with stainless steel shells with either titanium or copper-nickel tubes.

In the third most common desalination process, reverse osmosis is used. This process was first investigated in lab settings in the 1950s and came into commercial use in the 1970s. By the beginning of the 21st century, over 15,000 reverse osmosis plants were in operation. This filtering process is based on the principle of osmotic pressure; however, instead of osmosis causing particles to become fully dispersed, high pressure causes the opposite to happen. Selective membranes allow some materials through while keeping others behind. Water is pushed through the membranes of a filter while salt is prevented from moving through with it. Because of the high pressure put on piping, stronger materials are called for. Duplex steel, superduplex or alloys like 904L are used. 

As more areas deal with pressures such as drought and growing populations, more desalination processes will be needed. By picking the materials that are ideal to fabricate desalination units that are able to stand up to the rigors of processes such as sea water reverse osmosis, you can create items that are resistant to pitting, corrosion and other kinds of wear. There are more materials than ever that can help you balance effectiveness and durability with economy. 

Great Plains Stainless Steel has a wide range of options available, including strong and corrosion-resistant duplex stainless steel. Our highly knowledgeable staff can work with you to find the options that fit your needs the best. Get in touch today to discuss the right materials for your next project.

Heat Treatments for Austenitic Stainless Steels

Introduction

Unlike martensitic steels, the austenitic stainless steels are not hardenable by heat treatment as no phase changes occur on heating or cooling.
Softening is done by heating in the 1050/ 11200°C range, ideally followed by rapid cooling.
This is of course the complete opposite to martensitic steels, where this sort of treatment would harden the steel.

Apart from inter-stage annealing during complex or severe forming operations, for many applications, final stress relieving austenitic stainless steel products is not normally needed.

Effect of residual stresses

Cold worked austenitic stainless steels will contain some ‘strain induced’ martensite, which, as well as making the steel partially ‘ferro-magnetic’, can also reduce the corrosion resistance.
A highly stressed cold worked structure may also have lower general corrosion resistance than a fully softened austenitic structure.

The main hazard is stress corrosion cracking (SCC), which relies on tensile stresses as part of the failure mechanism.
Stress relieving removes such residual tensile stresses and so improves the SCC resistance.

The other main reason for stress relieving is to provide dimensional or shape stability. The risk of distortion can be reduced during forming or machining operations by stress relieving.

The approach to heat treatment selection

A full solution anneal stress-relieving heat treatment will re-transform any martensite formed back to austenite. (This will also give the lowest magnetic permeability possible for any particular grade.)
Slow cooling is advisable to avoid introducing distortion problems or residual thermal tensile stresses and so the risk of sensitisation during a slow cool may have to be accepted.

The temperature ranges used in stress relieving must avoid sensitising the steel to corrosion or the formation of embrittling precipitates.
As a general guideline, it is advisable that the range 480-900°C is avoided.
The low carbon (304L or 316L) or the stabilised (321 or 347) types should not be at risk from corrosion sensitisation during stress relieving treatments.

Stress relieving treatments for austenitic stainless steels

The table shows alternative treatments in order of preference.

Process or Corrosion Hazard Steel Grade Types
Standard Carbon 304, 316 Low Carbon 304L, 316L Stabilised 321, 347
Annealing following severe forming C A,C A,C
Forming interstage annealing C(A,B) A,B,C B,A,C
Post welding heavy sections and/or high service loading applications C A,C,B A,C,B
Dimensional stability D D D
Severe SCC risk in service Note 1 A,B B,A
Some risk of SCC in service C A,B,C B,A,C

Note 1
Standard carbon grades are susceptible to intergranular corrosion (ICC) on slow cooling treatments. Fast cooling treatments are not advisable as residual tensile stresses could result in SCC.

Note 2
Treatment B is also intended to reduce the risk of “knife-line” attack in the stabilised grades. This form of attack is due to the solution of titanium or niobium carbides at higher annealing temperatures.

Heat Treatment Codes

Code Treatment Cycle
A 1050 / 1120°C, slow cool
B 900°C, slow cool
C 1050 / 1120°C, fast cool
D 210 / 475°C slow cool (approx. 4 hours per 25mm of section)

Selecting welding consumables for stainless steels

Matching the consumables to the parent material

The composition of stainless steel welding consumables is matched with the base or parent material. The chemical analyses (composition) of the consumables used are usually balanced to optimise the welding process and avoid hot cracking.

Austenitic stainless steels

Low carbon levels are normally used to reduce the risks of intergranular (intercrystalline) corrosion following cooling through temperatures from around 850 down to 450 C after weld solidification. See Corrosion mechanisms in stainless steel.
Consumables such as 19 9 and 19 12 2 with higher carbon levels should give higher strength welds, more suited for high service temperature applications.

The titanium stabilized steels, 1.4541 (321) and 1.4571 (316Ti) are welded with consumables containing niobium, rather than titanium. The very high melting point titanium carbides that would be present in the consumable would be unlikely to melt during the welding process, whereas the niobium carbo-nitrides in the niobium type consumables have lower melting points and are a better choice.

Ferrite levels of austenitic consumables are normally balanced between 4 and 12 %, to reduce the risk of hot cracking at temperatures just below the solidification point of the weld metal. For welding the special low / zero ferrite grades, intended for special corrosion resistant, cryogenic temperature or low magnetic permeability service conditions, matching low /zero ferrite consumables, such as 18 15 3 L, should be used.

Ferritic, martensitic and precipitation hardening stainless steels

Generally, either matching consumables, or an austenitic filler with matching chromium and molybdenum contents, can be used. Austenitic fillers are used where good weld toughness is essential, but these are not a good idea where the weld appearance (colour), mechanical strength (in the case of welds between martensitic and precipitation hardening parent material) and physical properties (thermal expansion) need to be matched with the parent material.

Duplex stainless steels

In contrast to the austenitic consumables, duplex fillers, such as 22 9 3 N L are balanced to produce more austenite in the weld than in the parent metal. This is done to optimise weld mechanical properties and corrosion resistance and is achieved by adding more nickel and usually nitrogen to the consumable than is present in the matched base metal.

Welding dissimilar grades of stainless steel

There is a useful table downloadable from here. This shows the correct consumable for a wide range of stainless steels. It also includes recommendations for welding stainless steels to carbon steel.

Compositions of consumables

The consumable alloy symbols are common in the European standards. The compositions can vary, however, for the various consumable types between BS EN 1600, BS EN 12072 and BS EN 12073 for the same ‘Alloy symbol’ used in each standard. For each specific consumable type the particular standard should be consulted.

As a guide the table below gives the compositions in BS EN 1600. For these coated electrode types, the type of covering determines to a large extent the usability characteristics of the electrode and properties of the weld metal.

Two symbols are used to describe the type of covering: R for Rutile covering and B for Basic covering. A description of the characteristics of each of the types of covering is given in Annex A of BS EN 1600. (See also paragraph 4.3 of the standard)

Alloy symbols Chemical composition (% by mass – max unless stated)
. C Si Mn P S Cr Ni Mo Others
13 0.12 1.0 1.5 0.030 0.025 11.0/14.0
13 4 0.06 1.0 1.5 0.030 0.025 11.0/14.5 3.0/5.0 0.4/1.0
17 0.12 1.0 1.5 0.030 0.025 16.0/18.0
19 9 0.08 1.2 2.0 0.030 0.025 18.0/21.0 9.0/11.0
19 9 L 0.04 1.2 2.0 0.030 0.025 18.0/21.0 9.0/11.0
19 9 Nb 0.08 1.2 2.0 0.030 0.025 18.0/21.0 9.0/11.0 Nb-8x%C min, 1.1%max
19 12 2 0.08 1.2 2.0 0.030 0.025 17.0/20.0 10.0/13.0 2.0/3.0
19 12 3 L 0.04 1.2 2.0 0.030 0.025 17.0/20.0 10.0/13.0 2.5/3.0
19 12 3 Nb 0.08 1.2 2.0 0.030 0.025 17.0/20.0 10.0/13.0 2.5/3.0 Nb-8x%C min, 1.1%max
19 13 4 N L 0.04 1.2 1.0/5.0 0.030 0.025 17.0/20.0 12.0/15.0 3.0/4.5 N 0.20
22 9 3 N L 0.04 1.2 2.5 0.030 0.025 21.0/24.0 7.5/10.5 2.5/4.0 N 0.08/0.20
25 7 2 N L 0.04 1.2 2.0 0.035 0.025 24.0/29.0 6.0/9.0 1.0/3.0 N .020
25 9 3 Cu N L 0.04 1.2 2.5 0.030 0.025 24.0/27.0 7.5/10.5 2.5/4.0 N 0.10/0.25 Cu 1.5/3.5
25 9 4 N L 0.04 1.2 2.5 0.030 0.025 24.0/27.0 8.0/10.5 2.5/4.5 N 0.20/0.30 Cu 1.5 W 1.0
18 15 3 L 0.04 1.2 1.0/4.0 0.030 0.025 16.5/19.5 14.0/17.0 2.5/3.5
18 16 5 N L 0.04 1.2 1.0/4.0 0.035 0.025 17.0/20.0 15.5/19.0 3.5/5.0 N 0.20
20 25 5 Cu N L 0.04 1.2 1.0/4.0 0.030 0.025 19.0/22.0 24.0/27.0 4.0/7.0 Cu 1.0/2.0 N 0.25
20 16 3 Mn N L 0.04 1.2 5.0/8.0 0.035 0.025 18.0/21.0 15.0/18.0 2.5/3.5 N 0.20
25 22 2 N L 0.04 1.2 1.0/5.0 0.030 0.025 24.0/27.0 20.0/23.0 2.0/3.0 N 0.20
27 31 4 Cu L 0.04 1.2 2.5 0.030 0.025 26.0/29.0 30.0/33.0 3.0/4.5 Cu 0.6/1.5
18 8 Mn 0.20 1.2 4.5/7.5 0.035 0.025 17.0/20.0 7.0/10.0
18 9 Mn Mo 0.04/0.14 1.2 3.0/5.0 0.035 0.025 18.0/21.5 9.0/11.0 0.5/1.5
20 10 3 0.10 1.2 2.5 0.030 0.025 18.0/21.0 9.0/12.0 1.5/3.5
23 12 L 0.04 1.2 2.5 0.030 0.025 22.0/25.0 11.0/14.0
23 12 Nb 0.10 1.2 2.5 0.030 0.025 22.0/25.0 11.0/14.0 Nb-8x%C min, 1.1%max
23 12 2 L 0.04 1.2 2.5 0.030 0.025 22.0/25.0 11.0/14.0 2.0/3.0
29 9 0.15 1.2 2.5 0.035 0.025 27.0/31.0 9.0/12.0
16 8 2 0.08 1.0 2.5 0.030 0.025 14.5/16.5 7.5/9.5 1.5/2.5
19 9 H 0.04/0.08 1.2 2.0 0.030 0.025 18.0/21.0 9.0/11.0
25 4 0.15 1.2 2.5 0.030 0.025 24.0/27.0 4.0/6.0
22 12 0.15 1.2 2.5 0.030 0.025 20.0/23.0 10.0/13.0
25 20 0.06/0.20 1.2 1.0/5.0 0.030 0.025 23.0/27.0 18.0/22.0
25 20 M 0.35/0.45 1.2 2.5 0.030 0.025 23.0/27.0 18.0/22.0
18 36 0.25 1.2 2.5 0.030 0.025 14.0/18.0 33.0/37.0

Suggested Welding Consumables for stainless steel

Euro Inox publication ‘Welding of Stainless Steels’ tabulates suggested consumables for welding a range of stainless steel base or parent grades. This publication can be viewed or down loaded in pdf format from the Euro Inox web site, www.euro-inox.org by selecting the Materials and Applications sub menu.

This table is shown below.

 

Base Material Welding Consumables
EN10088 Number AISI Grade EN 1600 EN 12072 EN 12073
1.4301 304 E 19 9 G 19 9 L T 19 9 L
1.4306/1.4307 304L E 19 9 L G 19 9 L T 19 9 L
1.4541 321 E 19 9 Nb G 19 9 Nb T 19 9 Nb
1.4401 316 E 19 12 2 G 19 12 2 L T 19 12 2 L
1.4404 316L E 19 12 3 L G 19 12 3 L T 19 12 3 L
1.4571 316Ti E 19 12 3 Nb G 19 12 3 Nb T 19 12 3 Nb
1.4438 317L E 19 13 4 N L G 19 13 4 L T 19 13 4 N L
1.4310 301 E 19 9 G 19 9 L T 19 9 L
1.4318 301L E 19 9 L G 19 9 L T 19 9 L
1.4833 309S E 22 12 G 22 12 H T 22 12 H
1.4845 310S E 25 20 G 25 20 T 25 20
1.4438 317L E 19 13 4 N L G 19 13 4 N L T 19 13 4 N L
1.4512 409 E 19 9 L G 19 9 L T 13 Ti
1.4016 430 E 17 or 19 9 L G 17 or 19 9 L T 17 or 19 9 L
1.4510 430Ti (439) E 23 12 L G 23 12 L T 23 12 L
1.4521 444 E 19 12 3 L G 19 12 3 L T 19 12 3 L
1.4509 441 E 23 12 L G 23 12 L T 23 12 L
1.4113 434 E 19 12 3 L G 19 12 3 L T 19 12 3 Nb
1.4362 (2304) E 25 7 2 N L G 25 7 2 N L T 22 9 3 N L
1.4462 (2205) E 25 7 2 N L G 25 7 2 L T 22 9 3 N L
1.4006 410 E 13 or 19 9 L G 13 or 19 9 L T 13 or 19 9 L
1.4021 420 E 13 or 19 9 L G 13 or 19 9 L T 13 or 19 9 L
1.4028 420 E 13 or 19 9 L G 13 or 19 9 L T 13 or 19 9 L

Notes

Only the steel number is shown. The original Euro Inox table also has the steel name.

AISI is the American Iron and Steel Institute

Wire electrodes covered by EN 12072 may use the following prefixes G for GMAW (MIG), W for GTAW (TIG) P for PAW (plasma arc), or S for SAW (submerged arc).

Tubular cored electrodes are sometimes referred to as flux cored electrodes.

Related Articles

  1. The Schaeffler and Delong diagrams for predicting ferrite levels in austenitic stainless steel welds

Related References

  1. Welding of Stainless Steels
  2. Euro Inox, Materials and Applications Series, vol.3
  3. Welding of Stainless Steels and Other Joining Methods
  4. The Nickel Institute, AISI Designers Handbook Series No 9002

 

Source : 

British Stainless Steel Association | Regus | Blades Enterprise Centre | John Street | Sheffield | S2 4SW 

+44 (0) 114 292 2636 | 7 +44 (0) 114 292 2633 | ™ [email protected] | www.bssa.org.uk

Pitting and Crevice Corrosion – Offshore

Pitting and Crevice Corrosion of Offshore Stainless Steel Tubing

Oil and gas platforms regularly use stainless steel tubing in process instrumentation and sensing, as well as in chemical inhibition, hydraulic lines, impulse lines, and utility applications, over a wide range of temperatures, flows, and pressures. Corrosion of 316 stainless steel tubing has been observed in offshore applications around the world. Corrosion is a serious development that can lead to perforations of the tubing wall and the escape, under pressure, of highly flammable chemicals.

The two prevalent forms of localized corrosion are pitting, often readily recognizable, and crevice, which can be more difficult to see. Many factors contribute to the onset of localized corrosion. Inadequate tubing alloy and suboptimal installation practices can lead to deterioration of tubing surfaces in a matter of months. It is speculated that today’s minimally alloyed 316 stainless steel tubing, with about 10% nickel, 2% molybdenum, and 16% chromium, may more readily corrode than the more generously alloyed 316 tubing products produced decades ago.

Contamination is another leading cause for surface degradation. Such contamination may be caused by iron particles from welding and grinding operations; surface deposits from handling, drilling, and blasting; and from sulfur-rich diesel exhaust. Periodic testing of seawater deluge systems, especially in combination with insufficient freshwater cleansing, may leave undesirable chloride-laden deposits behind.

Pitting and crevice corrosion

Pitting corrosion of tubing usually is readily recognized. Individual shallow pits, and in later stages, deep and sometimes connected pits can be seen with the unaided eye. Pitting corrosion starts when the chromium-rich passive oxide film on 316 tubing breaks down in a chloride-rich environment. The higher the chloride concentration and the more elevated the temperature, the more likely the breakdown of this passive film.

Corrosion of 316 stainless steel tubing.
(Above) Corrosion of 316 stainless steel tubing. (Below) Pitting often can be seen with the unaided eye.
Pitting often can be seen with the unaided eye.

Once the passive film is breached, an electrochemical cell becomes active. Iron goes into solution in the more anodic bottom of the pit, diffuses toward the top, and oxidizes to iron oxide. The concentration of the iron chloride solution in a pit can increase as the pit deepens. The consequences are accelerated pitting, perforation of tubing walls and leaks. Pitting can penetrate deep into the tubing walls, creating a situation where tubing could fail.

Crevices are difficult, or even impossible, to avoid in tubing installations. They exist between tubing and tube supports, in tubing clamps, between adjacent tubing runs, and underneath contamination and deposits that may accumulate on tubing surfaces. Relatively tight crevices pose the greatest danger. General corrosion of tubing in a tight crevice causes the oxygen concentration in the fluid that is contained within a crevice to drop. A lower oxygen concentration increases the likelihood for breakdown of the passive surface oxide film. The result is a shallow pit.

 Iron goes into solution in the more anodic bottom of a pit, diffuses toward the top, and oxidizes to iron oxide (rust).
Iron goes into solution in the more anodic bottom of a pit, diffuses toward the top, and oxidizes to iron oxide (rust).

Unlike in pitting corrosion, formation of a pit on tubing surrounded by a crevice leads to an increase of the Fe++ concentration in the fluid contained in the gap. Because of the strong interaction of the Fe++ ions with the OH ions, the pH value drops. Chloride ions also will diffuse into the gap, being attracted by the Fe++ ions. The result is an acidic ferric chloride solution that can accelerate corrosion of tubing within the crevice.

Ideally, tubing should resist all forms of corrosion, including general, localized (pitting and crevice), galvanic, microbiological, chloride-induced stress corrosion cracking, and sour gas cracking. The tubing also should have adequate mechanical properties, especially when fluid pressures are high. Resistance to erosion comes into play when fluids contain potentially erosive particles. The environmental impact of the tubing also should be a concern; aquatic life can be harmed by small concentrations of copper ions that can be released by copper-zinc alloys.

The resistance of an alloy to localized tubing corrosion can be estimated from its chemical composition by calculating the alloy’s pitting resistance equivalent number (PREN). The most frequently used relationship is: PREN = %Cr + 3.3 %Mo + 16 %N. The higher the PREN value of an alloy, the higher its resistance to localized corrosion, i.e., the higher its critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT). These critical temperatures can be determined by common testing procedures such as ASTM G48 and ASTM G150.

Alloy selection

The importance of selecting the optimal alloy is demonstrated when austenitic 316 stainless steel tubing shows heavy corrosion while no signs of corrosion were detected on alloy 2507 superduplex tubing installed side by side. In a Gulf of Mexico installation of alloy 2507 tubing, only a few instances of external chloride crevice corrosion damage were identified. No perforations leading to the loss of containment of system fluids were observed. The only instances where crevice corrosion damage occurred involved the use of plastic support strips and neoprene gaskets.

Austenitic 316 stainless tubing shows corrosion while 2507 does not in side-by-side tests.
Austenitic 316 stainless tubing shows corrosion while 2507 does not in side-by-side tests.

Numerous alloys have been used or have presented themselves as candidates for use in installations that require resistance to seawater corrosion. The most frequently used alloys have been the 300-series austenitic stainless steels, mainly 316 and in some cases 317. Alloys with at least 6% molybdenum, the so-called “6-moly” alloys, perform well offshore. Typical 6-moly alloys include 254SMO, AL6XN, and 25-6Mo.

More recently, alloys with slightly more than 6% molybdenum have been introduced: 654SMO, AL6XN Plus, 27-7Mo, and 31. The published properties of these alloys suggest that they would perform well in chloride environments.

Nickel alloys such as 825, 625, and C-276 are more frequently used in sour gas applications. Of these alloys, 625 and C-276 demonstrate excellent resistance to localized corrosion. Ferritic alloys like Sea-Cure and AL29-4C resist attack by aqueous chloride solutions and are primarily used as heat exchanger tubing.

Mechanical properties for duplex alloys.
Mechanical properties for duplex alloys.

Tubing alloys are available that offer a combination of attractive properties for even unique applications in global construction projects. It is good practice to select an alloy with a critical pitting temperature above operating temperature. Depending on the application, it may be just as important to select an alloy with a critical crevice corrosion temperature above operating temperature.

Even highly corrosion-resistant tubing can be sacrificed when tubing surfaces are not kept clean. If possible, tubing should be installed following heavy construction activities that would otherwise allow weld splatter and grinding debris to accumulate on tubing. Where adjustments of construction sequences are not possible, tubing should be shielded from contamination, and if contaminated, should be thoroughly cleaned.

The growing number of duplex alloys reflects the increasing use of this promising class of materials. The workhorse 2205 duplex alloy was introduced decades ago. Now there is superduplex alloy 2507, which has performed very well in recent years in more demanding applications that require PREN values of 40 and above. More recently, the hyperduplex alloy 3207 was introduced with an even higher PREN value.

At the low end of alloy content, several lean duplex alloys such as 2101, 2304, and 2003 are candidates for less demanding applications.

Tubing is sandwiched between two half-round rods of thermoplastic.
Tubing is sandwiched between two half-round rods of thermoplastic.

A graph plot of the critical pitting temperature and critical crevice temperature shows the increase in chromium, molybdenum, and nitrogen leads to an increase in the CPT and CCT values of austenitic and duplex stainless steels. That also illustrates the economic advantage of duplex alloys. Despite an overall lower content of costly nickel and molybdenum, they offer performance similar to that of highly alloyed austenitic stainless steels.

Not only do duplex alloys offer satisfactory resistance to localized corrosion, they also have high mechanical properties, which make them prime candidates for high-pressure applications. Note that 2507 has a yield strength more than three times that of 316L.

Jacketed tubing

For applications in seawater, a tubing alloy that is highly resistant to localized corrosion is not the only option. Alternatively, one may select a less resistant alloy and then shield or protect the tubing.

Adequate protection appears to be offered by a thermoplastic polyurethane jacket that can be cost-effectively extruded onto continuous tubing. Recent installations in the Gulf of Mexico combine this clamping concept with superduplex tubing, and will generate valuable performance data.

An alternate approach uses jacketed tubing. The extrusion of a thermoplastic coating onto tubing is an economically attractive solution. Tubing is typically 316 or 317 stainless steel, and the preferred coating is polyurethane. Limited installations that use urethane jacketed 316 tubing report satisfactory results.

While the jacket must offer reliable protection from corrosive fluids, it must fulfill additional requirements. The jacket must resist impact, abrasion, and degradation by UV-radiation. It must allow tubing to bend, and must allow for cost-effective tubing installation, i.e., removal of the jacket and make-up of tubing connections. Once made up, the connections typically have to be protected from the environment using shrink tubing or tape. Without this protection, seawater access could cause pitting corrosion of exposed tubing or crevice corrosion in the gap between the tubing and the jacket.

Appropriate tubing clamps must be selected and care taken to prevent them from cutting into jackets and sacrificing their protective character. Jacketed tubing also can insulate, or heat and insulate, tubing when system fluids must be kept above ambient temperature.

Tubing supports and clamps

Many types of tubing supports and clamps have been used. Some have led to significant crevice corrosion, especially when tight crevices with large crevice surface areas result in depletion of oxygen so the alloy cannot reform the passive oxide layer. In particular, plastic tubing clamps are prone to inducing crevice corrosion because the plastic deforms around the tubing to create tighter crevices that limit oxygen ingress.

One early approach to preventing or mitigating crevice corrosion was the use of marine aluminum alloys in tubing supports and clamps. The tubing rests on a thin strip of aluminum alloy contained within a fiber-reinforced plastic tray. The tubing is held in place with an aluminum alloy bar.

Tubing support structures that use aluminum alloys appear to perform well. Galvanic corrosion between aluminum alloy and stainless steel may occur, but the aluminum alloy is more anodic than stainless steel, which means aluminum will corrode preferentially. Once sufficient corrosion has taken place over a number of years, affected aluminum supports and clamps can be replaced while the stainless steel tubing remains in place.

An alternate design originally developed for piping supports has recently been adopted for the installation of stainless steel tubing. The tubing is sandwiched between two half-round rods of a thermoplastic material. With the round tubing running perpendicular to the round support rod surface, the crevice contact area is minimized. Theoretically, there should be only one point of contact; however, some plastic deformation of the support rod takes place that results in a finite contact (crevice) area. A benefit of this design is that the supports/clamps allow for differential expansion of tubing and support structure.

Industry standards

The recently published industry standard, NACE SP0108-2008 “Corrosion Control of Offshore Structures by Protective Coatings,” provides guidance for more effective corrosion protection for offshore structures. TAnother industry standard, API RP 552 “Transmission Systems,” contains a section on installation practices. Those described practices do not address the avoidance of crevice corrosion.

Chloride Stress Corrosion Cracking & Relative resistance of Various Stainless Steel Grades

CORROSION: Chloride Stress Corrosion Cracking

Definition:

The combination of tensile stress and a specific corrosive environment can crack stainless steels.  This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides.  Although no stainless steel grade is totally immune to chloride SCC, the relative resistance of stainless steels varies substantially.

Influence of Alloy Composition:

The relative resistance to chloride SCC is dependant on the stainless steel family.  The austenitic family of stainless steels is the most susceptible.  The resistance of austenitic stainless steels to SCC is related to the nickel content of the steel. 

The most susceptible austenitic grades have nickel contents in the range of 8 to 10 wt%.  Therefore, standard grades such as 304/304L and 316/316L are very susceptible to this mode of attack.  Austenitic grades with relatively high nickel and molybdenum contents such as alloy 20, 904L, and the 6% molybdenum super austenitic grades have substantially better chloride SCC resistance.

The ferritic family of stainless steels, which includes grades such as type 430 and 444 are very resistant to chloride SCC.  The duplex stainless steel with their dual austenite/ferrite microstructures has a resistance that is in between that of the austenite and ferrite grades.

Corrosion Testing

The relative resistance of a stainless steel to chloride SCC is often quantified by the use of standard boiling salt solutions.  The following table summarizes the results of testing in boiling salt solutions of 26% NaCl (sodium chloride), 33% LiCl (lithium chloride), and 42% MgCl2 (magnesium chloride).  The boiling LiCl and MgCl2 test solutions are very aggressive relative to practical applications and only austenitic alloys with compositions that approach those of nickel-base alloys will routinely resist cracking in these test solutions.

Table 1: Relative chloride SCC resistance measured using fully immersed U-bend specimens in standard boiling salt solutions. (Taken from producer data)

Alloy

42% MgCl2

33% LiCl

26% NaCl

Austenitic SST

Type 304L (S30403)

SCC

SCC

SCC

Type 316L (S31603)

SCC

SCC

SCC

904L (N08904)

SCC

SCC

No Cracking

6% Mo SST

SCC

SCC

No Cracking

Alloy 20 (N08020)

SCC

No Cracking

No Cracking

Duplex SST

2205 (S32205)

SCC

No Cracking

No Cracking

255 (S32550)

SCC

No Cracking

2507 (32750)

SCC

No Cracking

Ferritic SST

439 (S43035)

No Cracking

No Cracking

No Cracking

444 (S44400)

No Cracking

No Cracking

No Cracking

 

Crack Appearance

The typical crack morphology for chloride stress corrosion cracking consists of branched transgranular cracks.  Figure 1 shows the cracking that occurred on a 6Mo super austenitic stainless steel  (N08367) exposed to 0.2% chlorides at 500 °F (260 °C)

Figure 1: Typical appearance of chloride stress corrosion cracking

Photo courtesy of TMR Stainless

Environmental Factors:

The environmental factors that increase the cracking susceptibility include higher temperatures, increased chloride content, lower pH, and higher levels of tensile stress.  Temperature is an important variable.  When stainless steels are fully immersed, it is rare to see chloride stress corrosion cracking at temperatures below 60 °C (150 °F). 

There is a synergistic relationship between dissolved oxygen and the chloride level.  If the oxygen level is reduced to the 0.01 to 0.1 ppm range, aqueous solutions containing low to moderate chloride levels are not likely to crack austenitic alloys, such as 304L and 316L.  The normal solubility of O2 in water at room to moderate temperatures (e.g. up to 140°F/60°C) is 4.5 to 8 ppm at atmospheric pressure.

In actual service environments, evaporation can produce local build-up of aggressive corrosive substances, such as chlorides and the H+ ions, resulting in conditions that are substantially more aggressive.  Under severe evaporative conditions, stainless steels can crack at temperatures well below the thresholds measured under conditions where there is full immersion.  Because of this, one must use caution when specifying materials for applications that involve the evaporation of chloride-bearing solutions on hot stainless steel surfaces.

The Materials Technology Institute (MTI) of the Chemical Process Industry has reviewed literature and collected case histories to define guidelines for the chloride SCC susceptibility of types 304L and 316L stainless steel in neutral water environments. 

Figure 2 shows the cracking threshold for 304L and 316L stainless steel as a function of temperature and chloride content.  The level of chlorides required to produce cracking is relatively low.  Failures have been reported in environments with as little as 10 ppm chlorides.  This is particularly true for environments having concentrating (evaporating) mechanisms such as wet/dry interfaces or a film of solution in immediate contact with a heat-rejecting surface.  In these situations, a few ppm of chlorides in the bulk solution can concentrate to hundreds of ppm in the area of evaporation.

Figure 2: Cracking threshold for 304 and 316 alloys exposed to near neutral chloride-bearing waters

The cracking threshold of a 6Mo super austenitic stainless steel (UNS N08367) immersed in oxygen-bearing neutral chloride solutions is shown in Figure 3.  The temperature thresholds are well above the 212°F (100°C) range, indicating that exposures to atmospheric boiling in neutral chloride solutions are very unlikely to produce cracking.

Figure 3: Cracking threshold for a 6Mo super austenitic steel ( UNS N08367) immersed in neutral NaCl solutions.

Courtesy of TMR Stainless

Swimming Pools

As was noted above, it is rare to see chloride stress corrosion cracking at temperatures below 60 °C (150 °F). Elevated load bearing applications in interior swimming pools are an exception to this rule and have a unique set of conditions.  For more information, please read, Successful Stainless Swimming Pool Design, Stainless steels for swimming pool building applications – selection, use and avoidance of stress corrosion cracking,  and Stainless Steel in Swimming Pool Buildings.

Additional References

There are numerous NACE papers and stainless steel producer brochures on this topic. Additionally, the following industry association brochures are suggested as general references.

Nickel Institute brochure No. 11 021 High Performance Stainless Steels

Nickel Institute brochure 16001 Practical Guidelines for the Fabrication of High Performance Austenitic Stainless Steels

International Molybdenum Association brochure Practical Guidelines for the Fabrication of Duplex Stainless Steelsh

 Source : SSINA per David A. Hartquist, counsel to SSINA.

Titanium & Titanium Alloy Pipe Dimensions

Titanium Pipe Dimensions

Titanium Pipe Dimensions

Reproduced, with permission, from B861-14 Standard Specification for Titanium and Titanium Alloy Seamless Pipe, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA   19428.  A copy of the complete standard may be purchased from ASTM International, phone:  610-832-9585, fax:  610-832-9555, e-mail: [email protected], website: www.astm.org.

Stainless Steel Pipe Dimensions

A312/A312M - 15

Reproduced, with permission, from B861-14 Standard Specification for Titanium and Titanium Alloy Seamless Pipe, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA   19428.  A copy of the complete standard may be purchased from ASTM International, phone:  610-832-9585, fax:  610-832-9555, e-mail: [email protected], website: www.astm.org.

An Understanding for Pressure Class for Flanges

 

Pressure Temperature Rating (PT-Rating)


All Pipes and various pipe fittings are most commonly classified based on their pressure temperature rating or commonly known as P-T Rating. The most common way of specifying pipe pressure temperature rating is given by ASME 16.5, using pound ratings (or lb ratings) – #150, #300, #400, #600, #900, #1500 and #2500. The pressure rating or pound rating for a pipe is determined using the design pressure and design temperature for the pipe.

Among other methods of classification by using pressure temperature ratings is ‘Pression Nominal’ or ‘Pressure Nominal’ or PN number method. This PN number is a rough indicator of pressure rating in bar.

Pressure rating or pound rating of a pipe is dependent on the pipe material and design temperature. The pipe pressure rating for the same material changes at different temperatures. For the same material and constant design pressure, different pressure ratings are applicable over different ranges of design temperatures. As the design temperature increases the pressure rating requirement for the pipe also increases for the same design pressure.

It should be noted that the pound rating for the whole piping system is equal to the pound rating of the weakest part, having the lowest pressure rating in the system. The weakest part may be any piping component or fitting which contains the pressure in the system and has the lowest pound rating due to any possible reasons.

All the piping components are not designated by Pressure class, only flanges and items related to flanges like gaskets(not bolts) are designated as class.Also socket welded components are designated by Pressure class eg 3000, 6000 & 9000 AND SCREWED COMPONENTS Eg 2000, 3000 & 6000 based on the thickness as per ASME 16.11.Rest all components are designated by thickness(either sch or thickness)

So we can broadly divide the piping components in two groups
1- Flanges,Gaskets,Valves,Socket Welded fittings,Screwed fittings designated by CLASS.
2-Piping components designated by THICKNESS and SCHEDULE.

The basic principle of any design is to make the weakest component strong.A flange joint is the weakest in piping system. (Note: joint is weak not the flange.) this is the reason why we establish rating first for flanges(group-1). And then based on the corresponding pressure and temperature .Allowable stresses are noted down and pipe thickness are computed.

Rating of a piping component is designated by class followed by a dimensionless number(for example Class150 ). This is the very important term used to differentiate piping components in a specifications.

Understanding of pressure temperature rating is very essential and mandatory for the study of piping engineering.
Pressure temperature ratings are defined on the basis of definitions of design pressure,design temperature stated in code ASME B31.3 and & Material group (ref B16.5)

We get design condition from process and base material from metallurgist, we need to choose relevant material group based on base material. The material group in B16.5 is only flange material not pipe/fitting material.

Design Pressure as per code ASME B31.3-2008(Revision of ASME B31.3-2006)states that

as per para 301.2.1

(a)The design pressure of each component in a piping system shall be not less than the pressure at the most severe condition of coincident internal or external pressure and temperature (minimum or maximum) expected during service, except as provided in para. 302.2.4.

(b)The most severe condition is that which results in the greatest required component thickness and the highest component rating.

(refer ASME B31.3 for full definition,points (c) and (d) will complete the definition of code)

There are two methods of calculating pipe thickness.

1. Based on exact design conditions provided by process.

2. Flange rated method.(sometimes called P/S ratio method)

Based on exact design conditions provided by process.

The maximum withstanding pressure of a piping component below its respective allowable stress depends on temperature and the greatest thickness computed.

Co-incidental maximum pressure and temperature computed for the greatest thickness required, below its respective allowable stresses are selected(interpolated) below the range of pre-defined piping class of flange for the respective material in the standard.This range of series of co-incidental pressure and temperature are tabulated in specification document for every material sheet for reference.

Based on design condition pipe thickness calculations are straight and simple. Put the design pressure and corresponding allowable for temp and calculates as per 31.3. this calculation is normally not used unless governed by economical reasons. Eg exotic/costly materials.

Flange rated method.(sometimes called P/S ratio method)

Flange rated method is to calculate the thickness based temp-pressure combination which will give you the max thickness for the particular rating and material. This is the most conservative approach and most of the time used.We get more then required thickness here.one more important reason to follow this method is to generalize a piping class(spec) for the various design temperature-pressure with same rating and material. This will not lead to various thickness and difficult to procure handle and maintain inventory.

Notes:

To calculate PT-Rating of any piping component, predefined class(predefined means ,values present in the standard ASME B16.5 were derived theoretically and substantiated by investigative work) of flange is considered because flange joints are considered to be the weakest joint assembly in a piping system. If any failure of piping system should occur unfortunately, then it’s supposed to occur at flange joint assembly.

By knowing PT-Rating or class of a component it’s impossible to identify its design pressure or temperature.Hence PT-Rating for a particular material are therefore specified on each specification sheet to identify the range of co-incidental pressure and temperatures that the particular material and class can withstand.

Again one more thing we need to take care while selecting the allowable stress based on exact design conditions.
One Question. Which component material’s allowable stress to be considered for thickness calculation. 1 pipe, 2. Flange, 3. Fitting. Which one???

We need to see all the three material’s allowable and select the one which will give you max thickness. And apply this thickness to all the components.

Source : www.pipingstudy.com