Years ago, fluid transport in a sanitary process meant moving chilled milk in a dairy, and 18-8 300 series stainless steels were adequate for these applications. However, many sanitary processes have become more demanding and complex, and the corrosion resistance of conventional alloys is often inadequate.

To meet this need Hygeniks provides the technical expertise and resources to supply, fabricate and deliver corrosion resistant process materials. Hygeniks is the leading supplier of super-austenitic AL-6XN alloy and Hastelloy C-22 fittings and tubing. We also design and fabricate custom items in our state of the art facility to accommodate unique process demands, from the very simple to the most complex.

The AL-6XN^® alloy carries the Unified Numbering System (UNS) designation of N08367. AL-6XN alloy is an austenitic stainless steel alloy with high nickel and molybdenum contents plus nitrogen. The alloy was a further development of the Allegheny Ludlum alloy AL-6X. The addition of nitrogen provided the alloy with improved resistance to pitting and crevice corrosion, a greater resistance to localized corrosion in oxidizing chlorides and reducing solutions plus higher strength. Nitrogen also helps to significantly reduce the potential formation of harmful secondary phases during welding.

AL-6XN alloy has a face-centered cubic crystal structure similar to other austenitic stainless steels. The alloy is non-magnetic and its magnetic permeability remains low even after severe cold forming.

Alloy Development
Over the years corrosion resistance in chloride environments have forced the change of the more popular 18%-8% Ni austenitic stainless steels. Corrosion resistance, weld ability and strength of the austenitic alloys were improved by changing the basic chemical composition to meet the more demanding applications.

• Molybdenum was added to improve corrosion resistance in chloride environments.
• Carbon was reduced to minimize sensitization during welding.
• Nitrogen was added to compensate for the reduced strength of the "L" grades, improve phase stability and together with chromium and molybdenum, improve pitting resistance.
• Chromium was increased to improve oxidation resistance.
• Nickel was added to stabilize the austenitic crystal structure and, at higher contents, improve stress-corrosion cracking resistance and general corrosion resistance in reducing environments.

Chemical Composition of AL-6XN Alloy

The high strength and corrosion resistance of the AL-6XN alloy make it a better choice than the conventional duplex stainless steels and a cost effective alternative to more expensive nickel-based alloys in applications where excellent formability, weldability, strength and corrosion resistance are essential. It is a cost effective alternative to less expensive stainless steels that do not have the strength or corrosion resistance to maximize life cycles.

AL-6XN alloy has exceptional resistance to chloride corrosion, pitting, crevice corrosion, stress corrosion cracking, and excellent general corrosion resistance to various acid, alkalis and salt solutions.

As new processes are developed and cleaning procedures become increasingly harsh, demands on materials are greater than ever. To compound the problem, for several years steel producers have been employing a method known as “alloy shaving.” This technique limits the concentrations of the steel alloying elements to the low end of the allowed ASTM range. The resulting steel meets the ASTM specification, but the overall mean corrosion resistance is lower than that of steels produced to the same specification 25 to 30 years ago.

 Austenitic stainless steels are made using up to 80% scrap iron. Not only is stainless steel an alloy of iron, but depending on the fabrication process, it can be loaded with tramp elements and inclusions detrimental to finishing requirements and corrosion resistance. Austenitic stainless steels have been the workhorse of the sanitary food and pharmaceutical industry for years. However, for reasons mentioned above, processors are beginning to use more corrosion resistant alloys to achieve the required life-cycles for their systems. One such material is Hastelloy^® C-22^®.

C-22 alloy is one of the most versatile nickel-chromium-molybdenum-tungsten alloys available today. This alloy demonstrates improved corrosion resistance to both uniform and localized corrosion and will exceed the performance of C-276, C-4 and alloy 625 in a variety of corrosive environments.

C-22 Nominal Chemical Composition, wt%

The high chromium (Cr) content contributes to the alloy's outstanding resistance to pitting, crevice corrosion and stress corrosion cracking. C-22 has excellent resistance to oxidizing aqueous media, including wet chlorine and nitric acid mixtures or oxidizing acids with chloride ions. Molybdenum (Mo) and tungsten (W) supply the resistance to wet reducing media (e.g. hydrochloric and sulfuric acids). The high level of nickel (Ni) is responsible for the alloy's resistance to attack by chlorides, fluorides and other halide compounds at ambient and elevated temperatures.

Because of such versatility, it can be used in applications where “upset” conditions are likely to occur.