PropertiesWet-corrosion characteristics of different types of tube and bar material

Ferritic chromium steelsFerritic stainless steels are practically immune to stress corrosion cracking (SCC). They have almost the same resistance to pitting and crevice corrosion as austenitic grades with the same content of Cr and Mo. They are susceptible to intergranular corrosion (IGC) after heat treatment and welding.

The only ferritic stainless steel on the tube programme, Sandvik 4C54, is used for high temperature applications.

Martensitic chromium steelsThe corrosion properties of martensitic stainless steels are moderate, esp. in the annealed condition. They can, however, resist detergents and other household chemicals as well as weak acids at low temperatures.

There are some martensitic chromium steels on the product programme mainly for strip, wire and billets, but none for tube and pipe.

Austenitic-ferritic (duplex) gradesThe duplex stainless steels have excellent resistance to SCC and good to very high resistance to pitting and crevice corrosion. Their resistance to general corrosion is as good – or better – than the corresponding austenitic grade. With the exception of the hollow-bar steel Sandvik 10RE51, they are all ELC steels and thus their resistance to IGC is good.

Our duplex grades are mainly those belonging to the SAF family, i.e. Sandvik SAF 2304, SAF 2205 and SAF 2507. Out of them Sandvik SAF 2304 is intended to be a both technically and economically superior alternative to the standard austenitic steels – AISI 304/304L, 316/316L, 321 and 347 – and to ferritic and martensitic stainless chromium steels. Sandvik SAF 2205 is an all-round stock-standard steel for use under more corrosive conditions than Sandvik SAF 2304. Sandvik SAF 2507 is a superduplex grade, which means that its PRE value is >40. This stock-standard grade is the choice where the corrosion conditions are very severe. The main advantage of SAF 2507 is found in chloride containing environments and acids, but in e.g. hot nitric acid SAF 2304 performs better.

There is also a specialized duplex steel, developed by Sandvik for use in urea – SAFUREXTM. SAFUREX has, owing to its high content of important alloying elements, a very high resistance to IGC.

Standard austenitic steelsWith varying contents of above all Cr and Mo the general, pitting and crevice corrosion properties of standard stainless steels vary. They are all characterised by susceptibility to SCC. The ELC and stabilised grades have good resistance to intergranular attacks.

High-performance austenitic gradesThe high-performance grades have been developed to serve under severe corrosive conditions. They have been designed to make the most efficient use of the specific alloying elements to combat a particular form of corrosion. Some of them are so-called multi-purpose grades, i.e. they can be used in a great variety of applications. 2RK65, Sanicro 28 and 254 SMO are examples of

such stock-standard grades. Originally Sanicro 28 was developed for use in phosphoric acid, but its excellent corrosion properties are now taken advantage of in a large number of applications.

There are also "single-purpose" grades, e.g. 2RE10 for strongly oxidising acids (e.g. HNO3), 2RE69 for urea. Sanicro 30 is primarily used for steam-generator tubing in the nuclear industry.

TitaniumTitanium is used in applications where stainless steels suffer from rapid corrosion. As with stainless steels the corrosion properties of titanium are based on the formation of a stable, protective oxide film. It follows that the material is significantly more resistant to corrosion in oxidising than in reducing media, where high corrosion rates can occur. Examples of media where titanium should not be used are: hot strongly reducing acids, red fuming nitric acid, water-free methanol and hydrofluoric acid.

There is no evidence of pitting or SCC in aqueous solutions of solutions of inorganic metal chlorides. Titanium also has exceptional resistance to seawater even under high velocity conditions or in polluted water.

Besides the chemically pure (CP) grades of titanium, grades 1-4, we also offer grade 7, with palladium, grade 26, with ruthenium, and grade 12, with nickel and molybdenum. Grade 7 and 26 are similar in corrosion resistance and may be used in reducing acids of higher concentrations and temperatures than what is possible with CP titanium. They are also more resistant to crevice corrosion attack, which can occur on CP titanium in hot brines in tight crevices. Grade 12 is harder and more erosion resistant than the other titanium grades. It also has an improved resistance to crevice corrosion and reducing acids compared with CP titanium, however, not in the order of grades 7 and 26.

ZirconiumWhat is said above about the protective film and the good corrosion properties of titanium is valid also for zirconium, but to an even higher degree. The material can therefore be used in mineral acids, strong alkalis and most organic acids. There are a few media that will attack zirconium, among them are hydrofluoric acid, ferric chloride, concentrated sulphuric acid and wet chlorine gas.

Zirconium is immune to SCC, with the exception of a few hot and highly oxidising environments such as FeCl3, and exhibits high resistance to pitting and crevice corrosion.

Summary of wet-corrosion properties for some steelsThe table below is a comparison of properties for some of our important grades. Together with the corrosion resistance we show strength and thermal expansion as well.

0 = Reference value + = Better than - = Inferior toSteel type General corrosion Pitting/

crevicecorr.

SCC Erosioncorr.

Strength Therm.expan-sion a

Sulph.acid

Nitricacid

Formicacid

Aceticacid

3R12 b - + - - - - 0 0 03R60/3R65 c 0 0 0 0 0 0 0 0 03R64 0 - 0 0 + 0 0 0 03R68 d + - 0 0 ++ 0 0 + 02RK65 +++ 0 + + +++ +++ + 0 0Sanicro 28 +++++ ++ + ++ ++++ ++++ + 0 0254 SMO +++ - ++ + +++++ ++++ ++ ++ 02RE10 + ++++ 0 + + + 0 0 02RE69 +++ +++ + ++ +++ + + + 0

Sanicro 41 +++ + + + ++ ++++ + + 0Alloy 400 +++ - 0 0 +++ +++ +++ 0/+++e 0SAF 2304 + ++ 0 0 0 ++ ++ +++ +SAF 2205 + 0 ++ + +++ +++ +++ ++++ +SAF 2507 ++ + +++ ++ +++++ +++++ ++++ +++++ +Ti Grade 1 0 + ++ ++++ +++++ +++++ + 0 ++Ti Grade 2 0 + ++ ++++ +++++ +++++ + + ++Ti Grade 3 0 + ++ ++++ +++++ +++++ + ++ ++Ti Grade 7 + + ++ ++++ ++++++ +++++ + + ++Ti Grade 12 + ++ ++ ++++ +++++ +++++ ++ ++ ++Ti Grade 26 + + ++ ++++ ++++++ +++++ + + ++Zr 702 ++++ +++++ ++++ +++++ +++++++ +++++ + 0 +++________________________________a) + denotes lower thermal expansion.b) Also valid for type 321.c) Also valid for type 316Ti.d) Also valid for 3R66 and 2R68.e) With special heat treatment.

Do you have any comments or suggestions? Please send a mail to Martin Crowther.

A document from the Sandvik Steel web-site.

http://www.smt.sandvik.com/sandvik/0140/SM/se01240.nsf/0/9bcb0bb8e9c4e118ca256713001a95ff?OpenDocument