Nickel Alloys

Nickel alloys offer a combination of excellent corrosion resistance, strength, toughness, metallurgical stability, fabricability and weldability. Many Nickel alloys additionally posses outstanding heat resistance, making them ideal choice for applications requiring chemical resistance and strength at elevated temperatures.

Nickel alloys represent a step up from conventional stainless steels and superaustenitic iron-based alloys in resisting corrosion by a wide spectrum of acids, alkalis and salts. An outstanding attribute of nickel alloys is exceptional resistance to aqueous solutions containing halide ions. In that regard, nickel alloys are far superior to austenitic stainless steels, which are notoriously prone to attack by wet chlorides and fluorides.

This superior corrosion behavior of nickel alloys manifests itself not only in term of lower metal loss, but in the ability to better withstand localized attack, notably pitting/crevice corrosion, intergranular attack and stress corrosion cracking. These forms of localized attack, more so than general thinning, account for the majority of corrosion-induced failures in the chemical industry.

Nickel alloys owe their corrosion resistance parity to the inherent lower reactivity of nickel relative to iron, as measured by its more noble oxidation potential in the EMF series. Similar to stainless steels, chromium-containing nickel alloys have the capability to passivate.

An added advantage of nickel over iron is the ability to accept fractions of alloying elements without forming bridle phases. Common alloying additions for enhanced corrosion resistance are chromium, molybdenum and copper.

A number of other applications for nickel alloys involve the unique physical properties of special-purpose nickel base or high-nickel alloys. These include:

1. Heat-resistant applications: Nickel-base alloys are used in many applications where they are subjected to harsh environments at high temperatures. Nickel-chromium alloys or alloys that contain more than about 15% Cr are used to provide both oxidation and carburization resistance at temperatures exceeding 760ºC

2. Corrosion resistance: Nickel-base alloys offer excellent corrosion resistance to a wide range of corrosive media. However, as with all types of corrosion, many factors influence the rate of attack. The corrosive media itself is the most important factor governing corrosion of a particular metal.

3. Low-expansion: Nickel was found to have a profound effect in the thermal expansion of iron. Alloys can be designed to have a very low thermal expansion or display uniform and predictable expansion over certain temperature ranges.

4. Electrical resistance: Several alloys systems based on nickel or containing high nickel contents are used in instruments and control equipment to measure and regulate electrical characteristics (resistance alloys) or are used in furnaces and appliances to generate heat (heating alloys)

5. Soft Magnetic: The high-nickel alloys (about 79% Ni with 4 to 5% Mo; bal Fe) have high initial permeability and low saturation induction.

6. Shape memory: Metallic materials that demonstrate to return to their previously shape when subjected to the appropriate heating schedule are referred to as shape memory alloys. Nickel-titanium alloys (50% Ni – 50% Ti) are one of the few commercially important shape memory alloys.

Commercial forms:

• Pure Nickel: Nickel 200/201:

• Nickel – Copper Alloys: Alloy 400, Alloy K500

• Nickel-Chromium and Nickel-Chromium-Iron Alloys: Alloy 600, X750, Alloy 625, Alloy C22, Alloy C276

• Iron-Nickel-Chromium Alloys: Alloy 800, 800HT,

• Controlled-expansion alloys: Alloy 902…

Nickel alloys are more expensive than stainless steels. However, economic comparisons on a first-cost, rather tan on a life-cycle basis, can be deceiving. For instance, Ni-Cr-Mo alloys cost roughly five times as much as 18Cr-8Ni stainless steels and about twice as much as super-austenitic stainless steels.

Stainless austenitic and superaustenitic

Stainless steels are iron base alloys that include chromium, carbon and other elements, mainly nickel, molybdenum, manganese, silicon and titanium. Chromium, present in no less than 10%, provides higher corrosion resistance than a simple iron alloy. This feature is due to the passivation of the alloys in an oxidizing environment.

They are very used on a wide range of industrial applications due to their excellent mechanical properties and also their corrosion resistance. There are five different families; four of them are classified by their particular crystal structure formed in the alloy: Austenitic, ferritic, martensitic and duplex (austenitic and ferritic); while the fifth correspond to precipitation-hardened alloys, based more on the type of heat treatment used than the crystal structure.

Heat treatments were performed in stainless steels to produce changes on their physical and mechanical properties, their residual stress level and restore their maximum corrosion resistance. Very often with the same treatment achieves a satisfactory corrosion resistance and excellent mechanical properties.

Austenitic and superaustenitic are more resistance to corrosion than ferritic and martensitic, because chromium carbides decompose and Chromium and Carbon remain in solid solution by rapid cooling from high temperature. However, if it is cooled slowly, as in welding, between 870 and 600ºC, chromium carbides precipitated in grain boundary leaving Chromium poor the area beside the edge, which makes the phenomenon called “intergranular corrosion”. This can be fixed downing to the lowest the Carbon content (0.03%), or adding Niobium or Titanium; these elements have a higher tendency to form carbides than Carbon, allowing it remain in solid solution on the iron and thus maintain its ability to resist corrosion.

They are stainless steel with high nickel content (4 to 37%) to stabilize the austenite. They could also content molybdenum, copper, silicon, aluminum, titanium and niobium, elements that are used to obtain certain characteristics.

Main properties of Austenitic and Superaustenitic stainless due to FCC structure, which provides high ductility, formability, toughness and excellent impact resistance. They can be hardened by cold working, but not by heat treatment, cause nickel stabilizes the austenite at room temperature.

Oxidation resistance is far superior to other types of stainless steels due to above reasons, which favors welding procedures that can be done perfectly; they are very used to manufacture tubes for the chemical and petrochemical industry, where corrosion is a crucial service condition.

They tend to be non-magnetic and sometimes, when they have been cold-worked they can be. Cold forming is a way to improve their mechanical properties, specifically yield strength that is relatively low compared to other materials. Cold-working and the section reduction increase the yield strength and the tensile strength limits, while decrease the steel elongation.

Duplex-Superduplex

They are two-phase austenitic-ferritic alloys, with yield strength twice than austenitic steels and a similar corrosion resistance. This allows them support greater efforts at work or decrease size of components, which means significant savings. They have excellent toughness than ferritic steels.

Titanium and exotic alloys

The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes Titanium the best material choice for many critical applications.

Today, Titanium alloys are used for demanding applications such as static and rotating gas turbine engine components. Some of the most and highly-stressed civilian and military airframe parts are made of these alloys.

The primary reasons for using titanium-base products are its outstanding corrosion resistance of titanium and its useful combination of low density (4.5g/cm3) and high strength. The strengths vary from 480MPa for some grades of commercial titanium to about 1100MPa for structural titanium alloy products and over 1725MPa for special such as wire and springs.

Another important characteristic of titanium-base materials is the reversible transformation of the crystal structure alpha (hexagonal close-packed) structure to beta (body-centered cubic) structure when the temperature exceed certain level. This allotropic behavior, which depends in the type and amount of alloys contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as coppers or aluminum.

Titanium has the following advantages:

• Good strength

• Resistance to erosion and erosion-corrosion

• Very thin, conductive oxide surface film.

• Hard, smooth surface that limits adhesion of foreign materials.

• Surface promotes drop wise condensation.