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Materials
Introducing Cr-Ni-Mo alloy UNSNO6059 for seamless heat exchanger tubes
Stainless steels alloyed with a high percentage of Chromium , Nickel , and
Molybdenum have been widely popular as a material in the construction of heat exchanger tubing ( as well as pipes , fittings , plates , and more ) for demanding and highly corrosive process conditions such as acids and salts , as well as operating in high temperatures . The two most used alloys in this segment are UNS N10276 and UNS N06022 . Another alloy , UNS N06059 , is gaining popularity as a material used in severe corrosive environments .
Venkat Ramesh – Global Product Manager ( Heat Exchanger tubing ) and Shabareesh Nair – Director ( Technical Marketing Asia ), Tubacex Group , Spain
UNS N06059 , also referred to as Alloy 59 , is a suitable alternative within Chromium , Nickel , and Molybdenum alloys depending on process conditions . This gives flexibility to material selection engineers on the choice of material depending on technical and commercial aspects . This material has been used for many years , mostly in the form of plates or welded tubes . This article will discuss its suitability for seamless tubular applications . UNS N06059 has good resistance against a multitude of corrosive media under oxidizing and reducing conditions , against chlorine-induced pitting and crevice corrosion , as well as resistance against stress cracking . This alloy was already used in the fertilizer industry for melamine processes and in the power generation industry for Flue Gas Desulphurization applications ( plates and welded tubes ). It was then explored for other applications within the mid- and downstream hydrocarbon industry , working with formic , acetic , hydrochloric , and sulfuric acid , and more . This article highlights the corrosion properties and applications of Alloy 59 in the stainless steel seamless tube category in more detail .
Critical alloying elements Extremely low carbon and silicon mean there is no tendency for grain boundary corrosion . A high percentage of Chromium , Molybdenum , and Nickel lead to a high resistance to chlorides . Higher Molybdenum in Alloy 59 compensates for the absence
Heat exchanger assembly .
Sample tubes for testing .
of Tungsten , or N10276 / N06022 , leading to better thermal stability .
Corrosion properties The high chromium content gives them good anticorrosion properties in oxidizing media , while the molybdenum content also protects them from corrosive attack in reducing media . The high nickel content makes them insensitive to stress corrosion cracking . Purest form of Cr-Ni-Mo with minimal Fe and absence of other alloys such as Cu and W ( which are resent in other C-Alloys ). Excellent resistance
Table 1 - Chemical composition ( key elements ) |
C Max |
Cr |
Ni |
Mo |
Co |
Al |
Others |
N 06059 |
0.01 |
22-24 |
56-63 |
15-16.5 |
< 0.3 |
0.1-0.4 |
Si < 0.1 |
N 10276 |
0.01 |
15-16.5 |
51-59 |
15-17 |
< 2.5 |
|
W < 4.5 |
N 06022 |
0.015 |
20-22 |
51-60 |
12.5-14.5 |
< 2.5 |
|
W 2.5-3.5 |
Alloy 59 approved with both European and ASTM , Designations 2.4605 / NiCr23Mo16AI , N 06059 , Standard ASTM B 622 . |
40 Heat Exchanger World February 2024 www . heat-exchanger-world . com