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

Galvanic Corrosion 2

Seawater is a very corrosive liquid and there is an increasing trend to use high alloy stainless steels, nickel alloys or titanium, because of their high resistance to corrosion and the consequent reduction in maintenance costs, particularly offshore. The most common high alloy stainless steels are superduplex (Fe/25Cr/7Ni/3.5Mo/0.25N/0.7Cu/0.7W) and 6%Mo austenitic (Fe/20Cr/18-25Ni/6Mo/0.2N/0.7Cu). Commonly used nickel alloys include alloys 625 (Ni/20Cr/9Mo/3.5Nb) and alloy C-276 (Ni/15Cr/15Mo/3W). Titanium is usually used as commercially pure grades 1 or 2 and also grade 5 (Ti/6Al/4V), where higher strength is required.

All of these alloys have a high resistance to pitting and crevice corrosion in seawater. Some complex components require parts to be made of different alloys and the question then arises as to the risk of galvanic corrosion between these alloys. Research in a number of laboratories has shown that all of these alloys can be safely connected together in seawater, provided none of the alloys is working outside its limits of use.

In fact the work extended to cover all nickel-chromium alloys, where the molybdenum content exceeded 7wt%, and also all titanium alloys. Superduplex stainless steel piping has been safely connected to titanium plate heat exchangers on offshore platforms for many years. Piping in 6%Mo alloys has been safely connected to flanges overlaid with a range of nickel alloys, including alloys C-276 and alloy 686 (Ni/21Cr/15Mo/3.5W).
 

Posted on: 7th Oct 2016

Hot-Spot Corrosion

It is fairly well known that copper alloy heat exchanger tubes can suffer hot-spot corrosion in seawater. This usually occurs when seawater flows are low, < 0.5m/s, and process side temperatures are high. This typically occurs in clean seawater when local tube wall temperatures are ~130°C. When the seawater is polluted with sulphides, even at low concentrations, (<1mg/L), the threshold temperature decreases to 75 to 100°C.

What is less well known is that small amounts of ammonia in the seawater (1 to 2mg/L) can also cause hot spot attack. The ammonia is present as a pollutant and the seawater flow in the heat exchanger tubes does not need to be that low. Failures have been seen in heat exchangers with a nominal flow rate of 2m/s. In addition, the threshold temperature is very low and attack has been seen in tubes with very small heat transfer rates. The attack takes the appearance of rather broad pitting, which is deep in places. The pits are filled with copper oxide and there is metallic, re-deposited copper either at the base of the pit or mixed in with the copper oxide.

The alloy most susceptible to this form of attack is 90/10 copper-nickel, while the alloy most resistant (but not immune) is aluminium brass. The photo below shows pitting of 90/10 copper-nickel in the as-received condition and after acid cleaning. Once the tubes form a protective film in clean seawater, it is much harder to initiate hot-spot attack. Ferrous sulphate dosing at start-up can assist in the formation of protective films.

Acid Cleaned

                             As-Received                                                  Acid Cleaned
 

Posted on: 21st Sept 2016

Austenitic and Duplex Stainless Steels

Austenitic stainless steels are widely used in the process industries because of their corrosion resistance, ductility and weldability. As corrosion processes become more aggressive it has been usual to switch from a low alloy, such as 304L or 316L, to other austenitic alloys with more molybdenum and, sometimes, nitrogen.

Austenitic alloys contain significant quantities of nickel and molybdenum, which are both expensive. There is now a full range of duplex stainless steels, with lower nickel and molybdenum contents, and, hence, a lower cost. A cost comparison in summer 2012 for hot rolled 10mm plate showed that if 316L is 1.0, 2101 duplex was 0.8. Similarly, 6%Mo austenitic was 3.1, while superduplex was 1.5.

For every austenitic stainless steel there is a duplex stainless steel with similar corrosion resistance and much higher strength, offering further potential cost savings. The modern duplex stainless steels are fully weldable with good toughness and ready availability in a wide range of product forms. Below are some common austenitic alloys with their duplex equivalents.


            AUSTENITIC                               DUPLEX
                 316L                                   2101/ 2003

            317L/ 904L                                  2205

                6%Mo                                   Z100/ 2507

Posted on: 7th Sept 2016

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Image (top left) by Agnieszka