Materials of Construction

Raw Materials Bar Stock
All Parr raw materials are carefully identified throughout the manufacturing process for trace ability – as required by ASME and other codes.

Parr reactors are normally made of Type 316 Stainless Steel, but they can also be made of other alloys as well. The list of available construction materials include:

  • Type 316/316L Stainless Steel
  • Alloy A-286
  • Alloy 20
  • Alloy 230
  • Alloy 400
  • Alloy 600
  • Alloy 625
  • Alloy B-2/B-3
  • Alloy C-276
  • Nickel 200
  • Titanium Grades 2, 3, 4, & 7
  • Zirconium 702 & 705

Additional materials available upon request.

When special alloy construction is specified for a Parr reactor, the head, cylinder and all internal wetted parts of the vessel will be made of the prescribed material, with the external valves and fittings constructed of stainless steel. Usually the external parts are not subject to the same corrosive conditions which exist inside the vessel, therefore, the higher cost of external parts made of special alloys is seldom justified. If external parts made of special alloys are required, Parr will attempt to meet this requirement, although there may be exceptions where such parts cannot be made.

Alloy Designation

Parr uses alloy designation numbers to identify the various corrosion resistant alloys available for use in Parr reactors and pressure vessels. These alloys can also be identified by trade names and by ASTM, ASME, DIN and other specification numbers. Many of the high nickel alloys were originally patented and sold under trade names, such as Monel1, Inconel1, Incoloy1, Carpenter Alloy 202, Hastelloy3, etc. Most of the original patents have expired and these alloys are now materials of construction available from other reputable suppliers, as well as from the owners of the original trade names.

Among the many corrosion resistant alloys now available, there may be two or three with very similar compositions and intended for use in the same corrosive environment. In these cases, Parr will select and offer the most widely used alloy in each of the basic corrosion resistance categories, rather than catalog and stock all three.

Each of these alloys has its own physical strength and temperature characteristics as well as its own unique resistance to certain corrosive materials. All of these factors must be considered when making a selection, with cost and availability also becoming factors in the final choice.

1 MONEL, INCONEL, and INCOLOY are Registered Trademarks of Special Metals Corp.

2 CARPENTER 20 is a Registered Trademark of Carpenter Technology Corporation.

3 HASTELLOY is a Registered Trademark of Haynes International, Inc.

Parr Instrument Company will attempt to answer questions regarding corrosion resistance and may suggest materials that might be suitable for specific applications, but no guarantee can be made that any particular alloy will be fully resistant to a prescribed set of corrosive conditions.

The basic composition of these alloys is listed in the table below. Corrosion resistance information can be obtained from various corrosion handbooks and metallurgical publications. Helpful information can also be obtained from the individual alloy manufacturers.

Any abridged listing of corrosion resistance of various metals and alloys can be potentially misleading since it cannot possibly deal with all of the effects of concentration, temperature, pressure and the presence of additional ions, all of which have a significant effect upon the ability of a reactor to withstand corrosion. In addition, the vulnerability of any material to stress corrosion cracking, intergranular corrosion and pitting must also be considered when judging the suitability of a material for a particular application.

The principal characteristics of the several construction materials offered by Parr are summarized below. These listings are intended to serve only as a starting point for any study of comparative corrosion resistance and physical properties. Material manufacturer’s booklets on each alloy are available on our website. Additional details may also be obtained from other sources.

Nominal Chemical Composition of Pressure Vessel Materials
Major Elements (Percent)
Material Typical Trade Name Fe Ni Cr Mo Mn Other
T316 Stainless Steel 65 12 17 2.5 2.0 Si 1.0
Alloy 20 Carpenter 20 35 34 20 2.5 2.0 Cu 3.5, Cb 1.0 max
Alloy 230 Haynes 230 3 52 22 2 0.7 Co-5, W-14, Si-0.5
Alloy 400 Monel 400 2.5 max 63 min  2 max Cu 31.5
Alloy 600 Inconel 600 8 76 15.5
Alloy 625 Gr1 Inconel 625 Gr1 5 58 22 9 0.5 Cb+Ta 3.7
Alloy A-286 Alloy 286 53 25 15 1.2 2 Si-1, Ti-2.1, Al-0.35
Alloy B-2/B-3 Hastelloy B-2/B-3 2 66 1 28 1 Co 1.0
Alloy C-276 Hastelloy C-276 6.5 53 15.5 16 1 W4.0, Co 2.5
Nickel 200 99
Titanium Grade 2, 3, 4 Commercially pure titanium Ti 99 min
Titanium Grade 7 Ti 98.8 min, 0.15 Pd
Zirconium Grade 702 Zr + Hf 99.2 min, Hf 4.5 max
Zirconium Grade 705 Zr + Hf 95.5 min, Hf 4.5 max, Nb 2.5


Type 316/316L Stainless Steel

Type 316 Stainless Steel is an excellent material for use with most organic systems. A few organic acids and organic halides can, under certain conditions, hydrolyze to form inorganic halogen acids which will attack T316SS. Acetic, formic and other organic acids are routinely handled in T316SS.

T316SS is not normally the material of choice for inorganic acid systems. At ambient temperatures it does offer useful resistance to dilute sulfuric, sulfurous, phosphoric and nitric acids, but sulfuric, phosphoric and nitric acids readily attack T316SS at elevated temperatures and pressures. Halogen acids attack all forms of stainless steel rapidly, even at low temperatures and in dilute solutions.

Although T316SS offers excellent resistance to surface corrosion by caustics, they can cause stress corrosion cracking in stainless pressure vessels. This phenomenon begins to appear at temperatures just above 100 °C and has been the most common cause of corrosion failure in stainless laboratory vessels. T316SS does offer good resistance to ammonia and to most ammonia compounds.

Halogen salts can cause severe pitting in all stainless steels. Chlorides can cause stress corrosion cracking, but many other salt solutions can be handled in stainless vessels, particularly neutral or alkaline salts.

At moderate temperatures and pressures, T316SS can be used with most commercial gases. In scrupulously anhydrous systems, even hydrogen chloride, hydrogen fluoride and chlorine can be used in stainless steel.

Essentially all of the T316SS produced today also meet the specifications for T316L, low carbon stainless steel.

Alloy 20

Alloy 20 is an enriched grade of stainless steel, designed specifically for use with dilute (up to 30 percent by weight) sulfuric acid at elevated temperatures. It can also be used for nitric and phosphoric acid systems as well as for all systems for which T316SS is suitable.

Alloy 400

Alloy 400 is an alloy comprised essentially of two-thirds nickel and one-third copper. For many applications it offers about the same corrosion resistance as nickel, but with higher maximum working pressures and temperatures and at a lower cost because of its greatly improved machinability.

Alloy 400 is widely used for caustic solutions because it is not subject to stress corrosion cracking in most applications. Chloride salts do not cause stress corrosion cracking in Alloy 400. It is also an excellent material for fluorine, hydrogen fluoride and hydrofluoric acid systems. Alloy 400 offers some resistance to hydrochloric and sulfuric acids at modest temperatures and concentrations, but it is seldom the material of choice for these acids. As would be expected from its high copper content, Alloy 400 is rapidly attacked by nitric acid and ammonia systems.

Alloy 600

Alloy 600 is a high nickel alloy offering excellent resistance to caustics and chlorides at high temperatures and high pressures when sulfur compounds are present. In caustic environments, Alloy 600 is unexcelled. It also is often chosen for its high strength at elevated temperatures. Although it can be recommended for a broad range of corrosive conditions, its cost often limits its use to only those applications where its exceptional characteristics are required.

Alloy B-2/B-3

Alloy B-2/B-3 is an alloy, rich in nickel and molybdenum, which has been developed primarily for resistance to reducing acid environments, particularly hydrochloric, sulfuric and phosphoric. Its resistance to these acids in pure forms is unsurpassed, but the presence of ferric and other oxidizing ions in quantities as low as 50 ppm can dramatically degrade the resistance of this alloy.

Alloy C-276

Alloy C-276 is a nickel chromium-molybdenum alloy having perhaps the broadest general corrosion resistance of all commonly used alloys. It was developed initially for use with wet chlorine, but it also offers excellent resistance to strong oxidizers such as cupric and ferric chlorides, and to a variety of chlorine compounds and chlorine contaminated materials. Because of its broad chemical resistance, Alloy C-276 is the second most popular alloy, following T316SS, for vessels used in research and development work.

Nickel 200

Nickel 200 is one of the designations of commercially pure nickel. It offers the ultimate in corrosion resistance to hot caustic environments, but its applications are severely restricted because of its poor machinability and resultant high fabrication costs.


Titanium is an excellent material for use with oxidizing agents, such as nitric acid, aqua regia and other mixed acids. It also offers very good resistance to chloride ions. Reducing acids, such as sulfuric and hydrochloric, which have unacceptably high corrosion rates in their pure form can have their corrosion rates in titanium reduced to acceptable levels if relatively small quantities of oxidizing ions, such as cupric, ferric, nickel or even nitric acid are present to act as corrosion inhibitors.

This phenomenon leads to many successful applications for titanium in the hydrometallurgy field where acids, particularly sulfuric acid, are used to leach ores. In these operations, the extracted ions act as corrosion inhibitors.

Prospective users must remember that titanium will burn vigorously in the presence of oxygen at elevated temperatures and pressures. While there have been many successful applications in hydrometallurgy where oxygen and sulfuric acid are handled in titanium equipment, the danger of ignition is always present and must be protected against whenever titanium and oxygen are used together.

Commercially pure titanium is available in several grades. Grade 2 is the material most commonly used for industrial equipment since it can be fabricated by welding and can be used to make vessels compliant to the PED & ASME Codes. Grade 4, which has slightly higher trace levels of iron and oxygen, has higher strength than Grade 2 but it is not suitable for welding and it is not covered by the PED or ASME Codes.

Since most Parr vessels are not welded, Grade 4 can be used to obtain higher working pressures than can be obtained with Grade 2. Grade 7, containing small amounts of palladium, and Grade 12 containing small amounts of nickel and molybdenum, offer enhanced resistance to certain environments and can be used for Parr reactors and pressure vessels if suitable billets can be obtained.


Zirconium offers excellent resistance to hydrochloric and sulfuric acids however, as with Alloy B-2/B-3, oxidizing ions such as ferric, cupric and fluorides must be avoided. Zirconium also offers good resistance to phosphoric and nitric acids, and to alkaline solutions as well. Two different grades are available: Grade 702 which contains hafnium is the standard commercial grade offering the best resistance to most corrosive agents. Grade 705 contains small amounts of both hafnium and niobium which increases the strength characteristics and allows for higher maximum working pressures for a vessel. Grade 702 typically offers better corrosion resistance than Grade 705. Grade 702 is also more widely available from commercial stocks of raw materials.

High Temperature / High Strength Alloys

In addition to the metals chosen for their corrosion resistance Parr also offers some alloys that are selected for their outstanding strength values, their high temperature strengths, or both.

Alloy 230

Alloy 230 is an alloy approved for ASME pressure vessel design for temperatures up to 980 °C. It is an alloy high in nickel, chromium, tungsten, and cobalt. While it has resistance similar to Alloy 600, it is normally selected for its high strengths at very high temperatures. It is sometime selected as a bolting material.

Alloy 625

Alloy 625 is an alloy with chemical resistance similar to Alloy C-276, but with much greater strength. We use this alloy to obtain additional pressure ratings for high temperature applications.

Alloy A-286

Alloy A-286 is an alloy of the Stainless Steel family with very high strengths up to 371 °C. It is commonly used as a bolting material.