Stainless Steel World Americas Journal December 2023 | Page 12

Technical Article

Introduction of Chrome Nickel Moly Alloy UNS N06059 as Choice of Stainless Steel Seamless Heat Exchanger Tubes in Between Alloys UNS N10276 and UNS N06022

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 .
By 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 . dustry 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 of Tungsten , or N10276 / N06022 , leading to better thermal stability .
Corrosion Properties
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 / NiCr- 23Mo16AI , N 06059 , Standard ASTM B 622 .
The high Chromium content contains good anti-corrosion properties in oxidizing media , while the Molybdenum content protects from a corrosive attack in reducing media . The high Nickel content makes it insensitive to stress corrosion cracking . The purest form of Cr- Ni-Mo with minimal Fe and absence of other alloys such as Cu and W ( which are present in other C-Alloys ), has excellent resistance against a multitude of corrosive media under oxidizing and reducing conditions , and outstanding resistance against chloride-induced pitting and
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 increvice corrosion , as well as resistance against stress corrosion cracking .
It also has excellent resistance to mineral acids such as nitric , phosphoric , sulfuric , and salt acids , but especially against sulfur / salt acid mixtures . There is excellent resistance in contaminated mineral acids and good resistance to hydrochloric acid over the entire concentration range up to 40˚ C .
Applications R e fi n e r y
Crude Distillation Units overhead condensers are prone to hydrochloric acid condensation ( dew point corrosion )
Table 2 — PRE Number ( Pitting Resistance Equivalent ) Using Cr + 3.3 Mo + 30 N
Grade PREN Range
Alloy 31
Alloy 625
Graph 1 : PREN Chart .
12 Stainless Steel World Americas - December 2023 | www . ssw-americas . com