Galvanic Corrosion And Stainless Steel

GALVANIC CORROSION AND STAINLESS STEEL

“What you need to know” (Probably more than you’ll ever want to know!)

The following is a discussion of the various types of stainless steel. For other terms and their definitions you will encounter when dealing with stainless steel see below.

18-8: 300 series stainless steel having approximately (not exactly) 18% chromium and 8% nickel. The term "18-8" is used interchangeably to characterize fittings made of 302, 302HQ, 303, 304, 305, 384, XM7, and other variables of these grades with close chemical compositions. There is little overall difference in corrosion resistance among the "18-8" types, but slight differences in chemical composition do make certain grades more resistant than others do against particular chemicals or atmospheres. "18-8" has superior corrosion resistance to 400 series stainless, is generally nonmagnetic, and is hardenable only by cold working.

304: The basic alloy. Type 304 (18-8) is an austenitic steel possessing a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon. 304 is a nonmagnetic steel which cannot be hardened by heat treatment, but instead, must be cold worked to obtain higher tensile strengths. The 18% minimum chromium content provides corrosion and oxidation resistance. The alloy's metallurgical characteristics are established primarily by the nickel content (8% mm.), which also extends resistance to corrosion caused by reducing chemicals. Carbon, a necessity of mixed benefit, is held at a level (0.08% max.) that is satisfactory for most service applications. The stainless alloy resists most oxidizing acids and can withstand all ordinary rusting. HOWEVER, IT WILL TARNISH. It is immune to foodstuffs, sterilizing solutions, most of the organic chemicals and dyestuffs, and a wide variety of inorganic chemicals. Type 304, or one of its modifications, is the material specified more than 50% of the time whenever a stainless steel is used. Because of its ability to withstand the corrosive action of various acids found in fruits, meats, milk, and vegetables, Type 304 is used for sinks, tabletops, coffee urns, stoves, refrigerators, milk and cream dispensers, and steam tables. It is also used in numerous other utensils such as cooking appliances, pots, pans, and flatware. Type 304 is especially suited for all types of dairy equipment - milking machines, containers, homogenizers, sterilizers, and storage and hauling tanks, including piping, valves, milk trucks and railroad cars. This 18-8 alloy is equally serviceable in the brewing industry where it is used in pipelines, yeast pans, fermentation vats, storage and railway cars, etc. The citrus and fruit juice industry also uses Type 304 for all their handling,

Thermal Resource Sales, Inc. | Confidential

of 9 GALVANIC CORROSION AND STAINLESS STEEL

crushing, preparation, storage and hauling equipment. In those food processing applications such as in mills, bakeries, and slaughter and packing houses, all metal equipment exposed to animal and vegetable oils, fats, and acids is manufactured from Type 304. Type 304 is also used for the dye tanks, pipelines buckets, dippers, etc. that come in contact with the lormic, acetic, and other organic acids used in the dyeing industry. In the marine environment, because of it slightly higher strength and wear resistance than type 316 it is also used for nuts, bolts, screws, and other fasteners. It is also used for springs, cogs, and other components where both wear and corrosion resistance is needed.

Type Analysis of Stainless Type 304 Carbon 0.08% max. Silicon 1.00% max. Manganese 2.00% max. Chromium 18.00-20.00% Phosphorus 0.045% max. Nickel 8.00-10.50% Sulfur 0.030% max.

316: For severe environments. Of course, there are many industrial processes that require a higher level of resistance to corrosion than Type 304 can offer. For these applications, Type 316 is the answer. Type 316 is also austenitic, non-magnetic, and thermally non-hardenable stainless steel like Type 304. The carbon content is held to 0.08% maximum, while the nickel content is increased slightly. What distinguishes Type 316 from Type 304 is the addition of molybdenum up to a maximum of 3%. Molybdenum increases the corrosion resistance of this chromium-nickel alloy to withstand attack by many industrial chemicals and solvents, and, in particular, inhibits pitting caused by chlorides. As such, molybdenum is one of the single most useful alloying additives in the fight against corrosion. By virtue of the molybdenum addition, Type 316 can withstand corrosive attack by sodium and calcium brines, hypochlorite solutions, phosphoric acid; and the sulfite liquors and sulfurous acids used in the paper pulp industry. This alloy, therefore, is specified for industrial equipment that handles the corrosive process chemicals used to produce inks, rayons, photographic chemicals, paper, textiles, bleaches, and rubber. Type 316 is also used extensively for surgical implants within the hostile environment of the body. Type 316 is the main stainless used in the marine environment, with the exception of fasteners and other items where strength and wear resistance are needed, then Type

304 (18-8) is typically used.

Type Analysis of Stainless Type 316: Carbon 0.08% max. Silicon 1.00% max. Manganese 2.00% max. Chromium 16.00-18.00% Phosphorus 0.045% max. Nickel 10.00-14.00% Sulfur 0.030% max. Molybdenum 2.00-3.00%

STAINLESS STEEL TERMS AN: Stands for Air Force-Navy.

ANSI: Stands for American National Standards Institute.

ASME: Stands for American Society of Mechanical Engineers.

ASTM: Stands for American Society of Testing and Materials.

AIRCRAFT QUALITY: Fittings made with a particularly high level of attention in manufacture and inspection.

ANNEAL: To heat metal in order to lower its hardness. The term anneal refers to the heat treatment given all 300 series stainless and most 400 series stainless by the steel mill after the raw material has been completed but before fittings are manufactured. Anneal also refers to the heat treatment given 400 series stainless fittings after their manufacture (also called hardening and tempering) to lower hardness and increase toughness. For example, fittings of 410 stainless may score over 200,000 psi after manufacture and be too brittle. By annealing at 1000 degrees F., tensile strength would reduce to 125,000-150,000 psi, while annealing the same material to 500 degrees F. would bring tensile to 160,000-190,000 psi.

BL: Stands for "Break Load", meaning the weight at which a product will break. Usually about 4 to 5 time the WLL (working load).

ELONGATION: Stretching a fitting to the point that it breaks. The percent of elongation at rupture (same as measure of ductility) is determined by dividing to total length after stretching to the original length. Elongation decreases as strength and hardness increase.

GALVANIC CORROSION: An accelerated degree of corrosion occurring when two different metals are in contact with moisture, particularly seawater or Condensate in a drain pan. All metals have what is termed a specific electric potential, so that low-level electric current flows from one metal to another. A metal with a higher position in the galvanic series (see below) will corrode sacrificially rather than one with a lower position, meaning stainless, for example, will corrode before gold. The further apart the metals on the chart, the more electric current will flow and the more corrosion will occur. No serious galvanic action will occur by combining the same metals only dissimilar ones. To prevent galvanic corrosion, use insulation, paint or coatings when separating dissimilar metals; or put the metal to be protected next to a metal which is not important in the assembly, so it can corrode sacrificially.

The metals listed at the top of the table below will corrode first due to galvanic reaction before those at bottom of the table.

Galvanic Corrosion Table

CORRODED END Anodic or less noble (ELECTRONEGATIVE)

Magnesium Zinc Aluminum 1100 (Common) Cadmium Aluminum 2024 Steel and Iron Lead Tin Nickel Brass Bronzes Copper Bronze Monel 304 Stainless Steel 316 Stainless Steel Silver Titanium Graphite Gold Platinum

PROTECTED END Cathodic or more noble Thermal Resource Sales, Inc. | Confidential

(ELECTROPOSITIVE)

ISO: Stands for International Organization for Standardization.

MS: Stands for Military Standards. The overriding characteristic of MS fittings compared to commercial products is the extensive inspection and lot traceability for MS, guaranteeing the chemical, physical and dimensional qualities. While commercial fittings may look similar and happen to pass many tests given MS products, the commercial fittings lack the pedigree of guaranteed quality or chemical, physical and dimensional aspects that users who order MS fittings rely on.

MAGNETIC PERMEABILITY: Test simply determines the level of magnetism.

MARINE QUALITY: This term differs depending on the application. Fittings and hardware are usually made of Type 316 stainless to resist corrosion. They are usually highly polished. Nuts bolts, screws, and other fasteners and items where strength and wear are required are usually made of Type 304 (18-8).

MOLYBDENUM: Nicknamed Moly, molybdenum is a metal added to 316 stainless steel, sharply increasing its corrosion resistance to chlorides and sulfates especially various sulfurous acids in the pulp industry. Molybdenum helps reduce hardness and increase tensile strength at higher temperatures.

MONEL: Invented by the International Nickel Co., and composed basically to two-thirds nickel, one- third copper. Monel has good strength, excellent corrosion resistance against salt water and in high temperatures, and is very expensive. Monel is also known as Cooper-nickel.

PASSIVATING: A very confusing term, since the common usage has taken on a different meaning that the technical definition. Users (including engineers) of commercial fittings seldom mean the old technical terminology. Technically, passivation is not cleaning but is a process of dipping fittings into a nitric acid solution to rapidly form a chromium oxide on the surface of the material, creating a passive film that protects stainless from further oxidation (see PASSIVE FILM). The purpose of passivation is to remove both grease left from manufacturing and traces of steel particles that may have rubbed off manufacturing tools onto the fitting. In common commercial parlance (meaning non-military and aerospace), passivating means cleaning to users, and the terms "passivating" and "cleaning" are used interchangeably. A wide range of cleaning methods using different mixtures containing nitric, phosphoric and other acids or simply exposing cleaned stainless fittings to air for a period of time will result in a "passivated" condition. For fittings that have been properly cleaned, it is impossible to determine the method of cleaning or passivation that was used. Thermal Resource Sales, Inc. | Confidential

PASSIVE FILM: The major characteristic of stainless is its ability to form a thin layer of protection called a "passive film" on its outside surface. This film results from a continual process of low-level oxidation, so oxygen from the atmosphere is needed for the passive film to exist. Once formed, it prevents further oxidation or corrosion from occurring. Even if chipped or scratched, a new passive film on stainless will form.

PICKLING: Removing surface impurities by using chemicals.

PITTING CORROSION: Pitting indicates deep corrosion in localized spots on a fastener. Dirt or grease on certain portions of a fastener may block oxygen from that surface, thus impeding the passsive film that protects stainless from corrosion.

PROOF LOAD: A test load that a fitting must undergo without showing significant deformation. It is usually 90% that of yield strength.

SOLUTION ANNEALED (same as CARBIDE SOLUTION ANNEALED) : A process of heating and removing carbide precipitants (carbon that has broken loose from its stainless steel solution) by heating a finished fitting to over 1,850 degrees F. and cooling it quickly, usually in water, so carbon content goes back into the stainless solution.

THREAD: Class 1 threads are a loose tolerance. Class 2 threads comprise 90% of stainless fittings and are normal commercial tolerance. Class 3 threads have a stricter tolerance and tighter fit such as socket cap and set screws. No definite relationship exists between tensile strength and tightness or looseness of fit. The symbol "A" added to threads, such as 2A, means external threads (screws), and "B" means internal (nuts).

WLL: Stands for "Work Load Limit", this is the recommended weight limit for safe use of a product.

Dissimilar Metals: is defined as 2 or more metals with different base origins. "304 and 316 SS are not considered to be dissimilar.

What the TRS Sales Staff needs to Know!

GALVANIC CORROSION: An accelerated degree of corrosion occurring when two different metals are in contact with moisture, particularly seawater or Condensate in a drain pan.

Galvanic corrosion does not happen with out moisture.

All metals have what is termed a specific electric potential, so that low-level electric current flows from one metal to another. A metal with a higher position in the galvanic series (see below) will corrode sacrificially rather than one with a lower position, meaning stainless, for example, will corrode before gold.

The further apart the metals on the chart, the more electric current will flow and the more corrosion will occur.

No serious galvanic action will occur by combining the same metals only dissimilar ones.

To prevent galvanic corrosion, use insulation, paint or coatings when separating dissimilar metals; or put the metal to be protected next to a metal which is not important in the assembly, so it can corrode sacrificially.

You have heard the term dielectric union for piping to water heaters it’s basically the same thing.

DO NOT UNDER ANY CIRCUMSTANCES ALLOW ONE OF YOUR MANUFACTURES TO SET A GALVANIZED COIL CASING IN A STAINLESS STEEL DRAIN PAN, PERIOD. IT CANNOT TOUCH IT, EVEN IF IT IS COATED. IT CAN RUB THROUGH DURING SHIPPING AND THEN IT WILL CORRODE UNDER THE COATING.

The metals listed at the top of the table below will corrode first due to galvanic reaction before those at bottom of the table.

Galvanic Corrosion Table CORRODED END Anodic or less noble (ELECTRONEGATIVE)

Magnesium Zinc Aluminum 1100 (Common) Cadmium Aluminum 2024 Steel and Iron Lead Tin Nickel Brass Bronzes Copper Bronze Monel 304 Stainless Steel 316 Stainless Steel Silver Titanium Graphite Gold Platinum

PROTECTED END Cathodic or more noble (ELECTROPOSITIVE)

What is missing in the table? Correct Galvanized Steel !

The reason it is missing is simple, galvanized metal is not a base steel alloy it is common steel with a zinc coating.

And in DH’s opinion, is not worth a damn. As soon as you cut or scratch it its back common steel.

G60 & G90 is the most common (the G number is the ounces of zinc per SF both sides G60 = 0.60 ozs/sf, and G110 = 1.10 ozs/sf all both sides)

So if this is correct why is zinc more vulnerable to corrosion than steel in the table above?

Cathodic protections can occur when two metals are electrically connected. Any one of these metals and alloys will theoretically corrode while offering protection to any other which is lower in the series, so long as both are electrically connected. However, in actual practice, zinc is by far the most effective in this respect.

DH terms for the TRS guys

Cathodic protections can occur when two metals are electrically connected (hot dipped electroplated zinc to steel). Any one of these metals and alloys will theoretically corrode while offering protection to any other which is lower in the series, (zinc is lower and in a degrading environment it will “White Rust” and corrode to protect the steel) so long as both are electrically connected. However, in actual practice, zinc is by far the most effective in this respect.

“White Rust” is a common term used with aluminum, common uncoated aluminum will “White Rust” as a natural characteristic and corrode to protect itself.

From the table above do you see why we have first “White Rust” on the coil fins and then the aluminum starts to crumble. This is also why a fin stock coating is not good the edge is raw aluminum and exposed so the coating is useless for true corrosion protection.

Another way to define Galvanic Corrosion (sometimes called dissimilar metal corrosion) is the process by which the materials in contact with each other oxidizes or corrodes. There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimilar metals present. Second, there must be an electrically conductive path between the two metals. And third, there must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal. If any one of these three conditions does not exist, galvanic corrosion will not occur.