Corrosion Resistance

Q What material is recommended for condenser tubes cooled by seawater?

A 90/10 (or 70/30) copper nickel alloys.

Q What material is recommended for underground pipe fittings?

A Gunmetal or dezincification resistant brass.

Q Is brass resistant to stress corrosion cracking?

A Yes if cold worked components are stress-relieved at 250-350° for ½ to 1 hour.

Q Which copper alloy has the best corrosion resistance in seawater?

A Various copper alloys have particular strengths under specific sea water conditions but on balance 90-10 copper-nickel has excellent corrosion resistance in many applications and is perhaps the most versatile. There is a detailed description of the properties and applications of this and other popular copper nickel alloys on the www.coppernickel.org website. Of course, aluminium bronze alloys, particularly nickel aluminium bronze, are also recognised for their corrosion resistance in severe seawater conditions and for that reason also have established applications as propellers and pumps and valves.

Q What method of protection may be used for the transport/shipping of brass finished products?

A It is common to use a tarnish inhibitor prior to transporting brass components. These are based on benzotriazole. One such product is Armagard produced by a UK company, Armack Chemicals. Hot water is added to the Armagard and the component is dipped into the solution, then flash dried. Components are then wrapped in acid free paper. No standards relate to this process.

Q What are the critical pitting and crevice corrosion temperatures for copper nickels in chloride environments?

A Copper nickels do not behave in the same way as stainless steels do towards corrosion by chlorides and these parameters are not appropriate to them. They do not have a critical temperature limit.

Q Are 90-10 and 70-30 copper nickels susceptible to stress corrosion cracking?

A They do not experience chloride or sulphide stress corrosion. They have a high resistance to ammonia stress corrosion compared to other copper alloys and do not require a stress relief anneal for seawater service.

Q What types of corrosion are copper nickels sensitive to?

A It is important that maximum velocity guidelines are adhered to for piping and heat exchanger/condenser service, as high velocities can cause impingement attack. Exposure to sulphides and ammonia in polluted seawater can lead to pitting or high corrosion rates and it is important to avoid these conditions, particularly during commissioning, start up and standby.

Q What is the safe limit for ammonia in contact with brass to prevent stress corrosion cracking?

A There is no acceptable limit - it is essential that internal stresses are removed as far as possible by a stress relief anneal at 250-350^oC for 1/2 to 1 hour.

Q We manufacture an aluminium component which we clean in a tank which is also used to clean copper and brass. How can we avoid contaminating the aluminium with copper?

A The best solution is to use separate tanks since copper ions will always migrate to the aluminium. Failing this, immerse the aluminium component in a 5% solution of nitric acid. This will dissolve any traces of copper.

Q I have a number of machined aluminium bronze components which are to be packed in wooden boxes and shipped to Australia. How do I avoid corrosion/staining?

A The key is to clean thoroughly, remove water, protect and keep dry:

1. After cleaning dry thoroughly using a de watering fluid such as WD40.
2. Remove displaced water with forced warm air.
3. Coat with protective coating such as benzotriazole inhibitor or a block copolymer.
4. Pack into benzotriazole treated paper lined wooden boxes. Moisture absorbent granules may also be used to keep the air inside the boxes dry.
5.Remove the coatings with a phosphoric based solution.
6. Dry thoroughly.

Q Is it possible to heat treat the aluminium bronze CA 104(CW307G) to improve its corrosion resistance?

A Yes, this is covered in NES 833 Part 2, Issue 2 1991. (NES= Naval Engineering Standards, now Def Stan. NES 833 is Def Stan 02-833). For extruded rods and sections of size 40 mm and below an anneal at 740^oC plus or minus 20^oC followed by air cooling is carried out. This mandatory heat treatment is to eliminate phases which are likely to give rise to selective corrosion in a seawater environment.

Q We have supplied copper nickel tubes to the UK from overseas. The wooden boxes containing the tubes became saturated with rain water at the port of exit and on arrival in the UK were heavily tarnished (green). Is there any way of cleaning the tubes?

A If the oxide is tenacious grit blasting may be required. This should be followed by pickling in a hot 5-10% sulphuric acid solution containing 0.35g/litre potassium dichromate. The pickled tubes should be rinsed thoroughly in hot fresh water and finally dried in hot air. To avoid future problems see answer to Question 11 Corrosion Resistance.