Galvanic Series Chart: Metal Compatibility Table (Free PDF)

Metals is listed on the table in order from least cathodic (most anodic) to most cathodic. When you connect two different metals in contact with each other in seawater, the one closer to top of the table will be the anode, the corroding member, while the other, lower in the list, will be the cathode, the protected member.

As you can see, the greater the distance between them in terms of their possible range, the greater will be the driving force for corrosion. Usually, if the couple differs by less than ~0.1 V, the difference is considered insignificant, while values of about 0.2 V or more need design consideration.

Looking at the corrosion potential column, you find the freely corroding potential referenced to a saturated calomel electrode in flowing seawater. Because this varys depending on the water conditions and alloy composition, there’s a range rather than a single number listed in each entry.

Because the two references differ by only about 0.01 V, you can interchange boat-oriented tables from ABYC; they quote just about identical numbers referenced to a silver/silver-chloride electrode. That leaves out the copper/copper-sulfate electrode whose reference is for buried structures, which you don’t want to mix in unless corrected by roughly +0.07 V.

For nickel-chromium alloys and stainless steels, you note the condition column. They is passive and noble in clean water due to their chromium oxide film, but their film breaks down in stagnant water or crevices. This causes the identical grade to drop several hundred millivolts down the series from being noble to active, hence two listings for 304 and 316.

Where crevices are possible, you design pairs to work against the active value, the film breaks down easy in still water.

Three classic traps to look out for: 1) Graphite (including those fancy new carbon-fiber laminated) are on the noble side, meaning they will aggressively corrode steel or aluminum. 2) Copper has a broad enough range that even a moderately sized gap between copper and steel is treated like a real danger. And 3) Galvanized steel acts like Zinc until the coating wears off, then it starts acting more like bare steel, which accelerates corrosion, so you factor that into your design.

Then you apply the series to sacrificial anodes. For example, aluminum anode alloys range from about -1.08 to -1.10V and zinc anodes is around -1.01V. Both remain anodic relative to all common hull metals.

Magnesium is the most anodic material used in the system, but it is only used in freshwater because its driving voltage is higher than what zinc can provides in that medium.

For near neutral, aerated water such as seawater, you take care to apply the table. In hot deaerated systems, acids, or alkalis, the ranking may shift, limiting its use to suitable environments.

You mention that MIL-STD-889D cautions that the galvanic series tells you only which member is the anode; it doesn't tell you how quickly it'll corrode. For that judgment, you should of use the design rules and limits of the anodic index because the series by itself isn't enough to give you the rate of corrosion.

From MIL-STD-889 we gets a fast engineering workaround called anodic index. This gives each metal/finish a single number that indicates its electrochemical voltage relative to gold.

To determine if a pair is good, you simply subtracts one value from another. That’s the voltage potential of the pair. The item with the higher number become the anode, the one that gets eaten away.

Say you have an 18-8 stainless screw (index=0.50). And you’ve got a 6061 aluminum housing (index = 0.90). So what? The difference is 0.40 V. In this scenario, aluminum is anode.

A column group together metals and platings that is similar from an electrochemical standpoint. Remember that a plated piece is considered by what's on top, not underneath. So until it wears through to reveal metal beneath, a zinc-plated steel screw is zinc (index = 1.25). Once it does, it's steel (index = 0.85).

The typical hardware examples column is just for translating purposes. Always refer to the actual finish you're dealing with. For example, a chromated cadmium plate is 0.95, while a tin plate is 0.65 and a zinc plate is 1.25. It is the same steel screw, but it appears in three locations along the scale depending on the finish.

Environment determine what is acceptable. See the table of limitations below.

Harsh: Here you get 0.15 V outdoors, salty, humid environments. Normal: Here you get 0.25 V in storage-type environments. In controlled temperature and humidity, you gets 0.50 V.

In the stainless-on-aluminum example above, the 0.40 V difference fails both the harsh and normal limits. This combination relies on finishing and the small-cathode area rule rather than raw compatibility.

There are two traps to be aware of here: The index reduces all categories down to one number. What it obscures is the active/passive split of stainless steels (see the table of seawater series above). Do not take stainless index arithmetic too serious. Wherever there are stagnant wet conditions or crevices, assume the worst.

The second trap is the sign convention. In some references, these numbers is listed negatively. In those tables, beryllium has a value of -1.85. Gold is -0.00. Only the difference between two entry ever matters. Both conventions yield same answer.

That said, you’re using this table for enclosure design and atmospheric considerations, sheet-metal assemblies, fasteners, brackets, etc. For submerged seawater service, then you’ll be working based off the table of measured seawater series. Immersion amplifies little differences. The index rounds them away.

The table tell you how big a voltage differential you should accept. The anodic index raises this question.

In the first column, you locate where your service environment lie. You subtract the index values for your two metal. Then, in the third column, you see if the values remains equal to or less than the volts listed there. Assuming that they do, you can usually join them together bare. Otherwise, you follow one of the mitigation measure in the final table.

This is the type of electrolyte environment that the joint will actualy experience. Conductive liquid film are required for galvanic corrosion, so the cleaner/drier the environment, the higher voltage it can withstands.

Very conductive film are formed from salt fog and high humidity (like coastal areas). Sea salt has magnesium chloride which begin to absorb water at 34% relative humidity. That is why the limit is so tight on the harsh side, at only 0.15 V.

An interior with controlled air and no condensation can tolerate up to 0.50 V because a continuous electrolyte film is unlikely to form.

Harsh: even indoors in an unconditioned coastal garage; salt-sprayed roadway underbody (salt on the road is bad). Controlled: equipment kept dry within a sealed, conditioned cabinet, even on an oil rig, including enclosed cabinets containing connectors and switches that is protected from weather.

Immersion is beyond even the harsh class; metal permanently under seawater gets designed from the seawater series with insulation and sacrificial anodes, not these atmospheric limits.

By definition, per MIL-STD-889 if you have a necessary combo that’s over the limit, then you don’t scrap the design; instead, you deal with it.

Plating one side towards the other (e.g., zinc or cad plating on steel screws mating to aluminum) is common practice. Interposing a gasket or washer that won’t corrode is another option. And sealing and insulating the joint to prevent a bridge of electrolyte formation is yet another technique.

The mitigation table covers those techniques, row by row.

Find the metal of the part you are fastening in the left column, then read across to the fastener material you plan to use. It’s assuming normal fastening shapes, i.e., that the base metal is large and the fastener is small with respect to wetted areas.

Good indicates the pair is suitable for outdoor practice. Uncertain means it depends on whether dirt, salt, and wetness are kept; the area should of be isolated. Avoid indicates one member or right nearby will be damaged, typicaly either the fastener or the immediate surrounding metal.

One of the physical rules applies to the entire matrix, known as the area ratio effect. It states that corrosion current generated on the cathode will concentrate on the anode.

The worst type of design is having an anodic fastener in a noble structure. For example, a zinc plated screw can be in copper flashing, or an aluminum bolt can be in stainless plate. On the other hand, a small noble fastener in a large anodic structure spreads the attack over such a great area that it's usually tolerable. Standard practice is to use stainless screws in aluminum sheet, even though those two metals is far apart in the series.

The small details of this are marked by starred entries. Galvanic is fine for a stainless screw in carbon steel or a carbon steel bolt in carbon steel. But either one require plating or paint anyway because the steel will rust at its naturaly rate.

On the other hand, don’t forget that if you use a galvanized fastener in a copper or stainless structure, the zinc comes off much faster then when alone. As the zinc-iron alloy layer is approached, the corrosion product shifts from white to orange; beyond that point, the steel core will corrode faster.

Upgrade, don’t gamble: For all coastal and marine structures, use A4/316 stainless fasteners on aluminum rails, masts and fittings with an isolating washer or paste between head and surface. Use silicon bronze fasteners on bronze or copper hardware below the waterline; never use plain or galvanized steel against copper alloys in salt spray.

The Australians give a strong warning for coated roofing. Stainless uninsulated fixings aren't allowed on aluminum zinc-coated sheet because galvanic current cause blisters in the coating at each screw.

For immersion service, seawater is its own deal (sacrificial anodes and the rest of the seawater series), as are magnesium castings, which are not covered in the matrix. Magnesium-compatible coatings can be used on fasteners for magnesium castings; otherwise treat all non-magnesium-compatible fasteners as magnesium risks and segregate accordingy.

Passive stainless and titanium is noble and generally OK as fasteners in most bases, but neither is OK as the base material for anodic fasteners. For immersion service, see sacrificial anodes and the seawater series.

Fastener threads deserve consideration, as well. Any plating is cut through at threads, so specify formed then plated fasteners. Apply anti-seize that’s compatible with the pair to prevent crevice attack in the threaded connection.

When one of the above tables indicate that a pair of metals is potentially dangerous but there’s no way around using them in your design, consult this one instead. The rules is listed in approximately the order a designer encounters them: Row 1 to 3 specify the geometry and materials; row 4-6 break the corrosion cell; and row 7 and 8 manage whatever current is left over.

Finally, the why it works column links each rule to one or more of the three necessary conditions for every galvanic cell: two dissimilar conductor, an electrical connection between them, and a common electrolyte. If you eliminate any of these conditions, the corrosion cease.

One rule that gets particular care is the area ratio. In both directions, area ratio overrides the series tables: a terrible-looking pair on paper (say, SS and Al, with 0.40 V apart on the anodic index) is routine if one partner's anode is huge; whereas one that looks mild fail promptly where the anode is tiny.

Caution: after a rain, a wetted-area ratio of 50 to 1 can quickly reduce to 1 to 1 as surfaces dry out unevenly. Judge the worst credible wetting, not the sunny-day case.

Most insulation failures is mechanical. Examples include leaving a sleeve out of one of several hole, re-establishing a conductive path through another piece of cable tray, pipe, or earth strap elsewhere in the assembly, or crushing paint films with a torqued bolt head.

Don’t forget: Anodized aluminum and stainless steel do not use their oxide film as electrical insulation for these purposes. Gaskets made with carbon loading and even graphite is conductors at the high end of the series. They’ve eaten away stainless flange faces in seawater service.

Many folks is surprised by the coat the cathode rule (it goes against your gut, which tells you to shield the stuff that’s getting eaten away). Painting just the anode is risky business. At each pinhole or scratch, all the galvanic current will focus on a few square millimeters of bare anode and drill down deep local pit.

It works better to starve the cell by coating both members or just the more noble one. Because the galvanic interaction occur only a short distance across a thin layer of electrolyte film, make sure the coating extends far past where the two meet; standard advice is 30-50 mm beyond the joint.

The rules address the issue of galvanic corrosion only.  This does nothing to eliminate the rate at which any metal corrodes on its own.  Even if you have a perfect couple with a carbon steel bracket, that bracket will still rust.

The rules don’t address pitting and other types of corrosion such as stress corrosion cracking or crevice corrosion since those require their own countermeasures.  Treat coatings and sacrificial anodes like consumable items and develop an inspection schedule for any structure they’re installed on if it is immersed.  It's not a one time thing and you should of checked it.