Sigma Phase

There is always a concern about duplex stainless steels and the presence of sigma phase. This is an intermetallic phase that can precipitate due to poor heat treatment or to excessive heat inputs during welding. Sigma phase causes a loss in both toughness and corrosion resistance.

Provided care is taken during manufacture and fabrication, sigma phase should not be a problem. However, when it occurs, the question is often asked, how much is acceptable. There is not a simple answer to this question, because the effect of sigma phase depends on its size. The larger the diameter of the sigma particle, the greater is the area around it that is denuded in chromium and molybdenum, and generally, the more detrimental is its effect on properties. However, a wide range of particle sizes is possible and it is better to conduct fitness for purpose tests if sigma is suspected.

The most common tests are a Charpy impact toughness test at the desired temperature (often -46 or -50°C) and an ASTM G48 method A corrosion test. The minimum Charpy energy has been suggested as 70J for parent metal, as it will decrease further on welding, and 35J for welds. In the corrosion test, the following temperatures have been recommended, with the pass criteria being no pitting and a weight loss < 4g/m2.

Recommended ASTM G48 test temperatures for duplex stainless steels.

Posted on: 21st Dec 2016

Start-Up

In seawater cooling systems a leak of the tubing or piping costs money and loses production. However, it has been demonstrated that the use of a “soft” start-up is of great benefit to many alloys and reduces the risk of a failure.

Copper alloys are widely used for heat exchangers and seawater piping, but in the presence of pollutants, such as sulphide and ammonia, they can suffer from accelerated localised corrosion, leading to failure. Once protective films form in clean, natural seawater, they are much more resistant to corrosion by pollutants. As pollution is often intermittent, a long life can be achieved by a start-up when no pollution is present. Simple monitoring of the seawater at the inlet can determine the presence of both sulphide and ammonia.

High alloy stainless steels, such as the 6% Mo austenitic and 25% Cr superduplex alloys, are limited by the maximum temperature of seawater that they can tolerate without suffering crevice corrosion or pitting at welds. This is typically 30 to 40°C, depending on the alloy. However, where discharge temperatures from heat exchangers are higher, it has been shown that running the system with cooler water first, strengthens the passive film and enables operation at 10 to 15°C higher than normal. One recommended start up regime is:

2 days minimum with cold, untreated seawater
5 days minimum with cold chlorinated seawater
Thereafter, turn on heat exchangers.

A superduplex seawater piping system was given a soft start-up and the piping after the heat exchangers ran at 55°C without any problems.
 

Posted on: 7th Dec 2016

Change of Use

It is quite common to re-use components from one system in a new one, to save money. However, this may cause premature failure unless attention is paid to the conditions of use, compared with the previous service.

The photograph below shows an aluminium alloy shell, naval brass tube plates and aluminium brass tubes from a cooler that originally had seawater as the coolant in the tubes and lubricating oil on the shell side. Then the unit was used as a water heater with brackish water on both sides of the unit. The aluminium alloy shell suffered rapid galvanic corrosion coupled to the large area of copper alloy and leaked very quickly. The use of an aluminium alloy shell with copper alloy tubes is only acceptable with a non-conducting process-side fluid, such as lubricating oil.
 

Corrosion of aluminium alloy shell in a copper alloy heat exchanger.

Posted on: 21st Nov 2016

Transpassive Corrosion

Nickel alloys are often chosen for aggressive environments because of their high corrosion resistance. However, nickel alloys can sometimes be the wrong choice, because they go transpassive.

A mineral processing plant was heating a raffinate with steam at about 80°C and the raffinate contained:

                                    Sulphuric Acid    66.5g/L
                                    Chloride                0.2g/L
                                    Copper                 13g/L
                                     Iron                       50g/L

Alloy C-276 (Ni/15Cr/15Mo/3W) was chosen for the heat exchanger tubes and it would have resisted the acid/chloride mixture at 80°C. However, both cupric and ferric ions are strong oxidizers, and the presence of substantial quantities of both in the raffinate meant that the electrochemical potential of the alloy was in the transpassive zone. In this region molybdenum salts are soluble and pitting is likely. The photo shows pitting and severe corrosion of both the parent tube and the seam weld.

The solution is to use an alloy with a wider passive zone, such as occurs at higher chromium contents. Although there are nickel alloys with higher chromium contents, the use of Z100 superduplex stainless steel (Fe/25Cr/7Ni/3.5Mo/0.7Cu/0.7W) offered a lower cost solution. The alloy has performed well in this environment.

                                  Pitting of seam-welded alloy C-276 tube handling oxidizing raffinate.
 

Posted on: 7th Nov 2016

Sulphide Polluted Seawater

Copper alloys are widely used in seawater cooled heat exchangers because of their good thermal conductivity and resistance to corrosion. It is well documented that the presence of small amounts of sulphide, as a pollutant, can cause rapid failure of aluminium brass and copper-nickel alloys. Concentrations as low as 0.01mg/L have been observed to cause corrosion.

Chlorine or hypochlorite is often added to seawater to reduce fouling, particularly of carbon steel, stainless steels and non-metallic piping. Dosing levels are usually small and heat exchanger inlet values may vary from 0.25 to 1.0mg/L free chlorine. What is less well known is that when chlorine is also present, the corrosion can be greatly increased. When both chlorine and sulphide are present together, the 66/30/2/2 Cu-Ni-Fe-Mn alloy is the most susceptible to this type of attack, followed by aluminium brass and 90/10 copper nickel.

The photograph below shows pitting of a 66/30/2/2 Cu-Ni-Fe-Mn heat exchanger tube that perforated just a few months after start-up. Chlorine concentrations were high initially (~1mg/L at the heat exchanger). The sulphide concentration is not known, but sulphide was clearly detected in the corrosion products.

 

                                  Pitting in a 66/30/2/2 Cu-Ni-Fe-Mn tube due to sulphide and chlorine.
 

Posted on: 21st Oct 2016

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Image (top left) by Agnieszka