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

Graph 1 : PREN Chart .

12 Stainless Steel World Americas - December 2023 | www . ssw-americas . com