Galvanic Corrosion: The Galvanic Series, Area Ratio and Prevention

Galvanic corrosion, also called bimetallic or dissimilar metal corrosion, is the accelerated corrosion of a metal caused by electrical contact with a more noble metal in a shared electrolyte. The two metals form a galvanic cell. The less noble metal becomes the anode and corrodes faster than it would on its own, and the more noble metal becomes the cathode and corrodes more slowly, or not at all. Three conditions must be present together: two metals with different corrosion potentials, an electrical path between them, and an electrolyte such as seawater, rain or condensation that bridges the joint. The galvanic series in seawater shows which metal of a pair will suffer. Magnesium and zinc lie at the active end, followed by aluminium, carbon steel and cast iron; copper alloys and lead lie in the middle; passive stainless steel, titanium, platinum and graphite lie at the noble end. The rate of attack then depends on how far apart the two metals are in the series, on the conductivity of the electrolyte and, above all, on the ratio of cathode area to anode area.

Conditions for galvanic corrosion

Each of the three conditions is necessary, and removing any one of them stops the cell. Dry metals do not corrode galvanically, however far apart they sit in the series, which is why mixed-metal assemblies kept indoors and dry seldom show attack. Almost any water can serve as the electrolyte. Distilled water is the exception, and rainwater becomes conductive once it has collected contaminants from the air and from the surfaces it runs over. The conductivity of the electrolyte decides where the damage falls. In seawater the current spreads, and the attack on the anode extends some distance from the joint. In a weak electrolyte, such as a film of condensation, the attack concentrates on the anode close to the line of contact. Chloride ions, as in seawater and sea spray, make any electrolyte more aggressive.

The galvanic series in seawater

The galvanic series ranks metals and alloys by their corrosion potential, measured in a stated electrolyte against a reference electrode. The most widely reproduced series was measured in flowing seawater at ambient temperature, with potentials given against the saturated calomel electrode. It is not the same list as the electrochemical series of standard electrode potentials, which ranks pure metals in solutions of their own ions and ignores the oxide films that govern how real alloys behave. Aluminium shows the difference. In the electrochemical series it lies far below iron, at minus 1.66 volts against minus 0.44, but in seawater its oxide film places it only 0.1 to 0.3 volt below steel. Stainless steel appears in the series twice. With its chromium oxide film intact it is passive and near the noble end. In a crevice, under marine growth or in stagnant, poorly aerated water the film can break down, and the potential then falls by several tenths of a volt into the active range.

The potentials are rounded from the flowing seawater series. A different electrolyte, temperature or flow rate moves them, and can change the order of metals that sit close together.

How far apart is too far

The series shows which metal will corrode, not how fast. A difference of a few tens of millivolts between two metals rarely causes trouble, while a difference of several hundred millivolts in seawater usually does. Corrosion guides use about 0.2 volt as a rule of thumb for the point at which a couple needs attention, with a lower limit for outdoor and marine service and a higher one for dry, controlled interiors. Passive stainless steel and aluminium alloys differ by several tenths of a volt in seawater, so that pair is isolated in marine work. Passive 304 and 316 differ by a few hundredths of a volt and can be combined freely.

Anode and cathode area

The current consumed at the cathode must be supplied by the anode. When the cathode is large and the anode small, the whole current concentrates on a small area of metal, and the anode corrodes rapidly. When the anode is large and the cathode small, the same current spreads thinly, and the attack may be negligible. For this reason the fastener, which is almost always the smaller part of a joint, should be the more noble metal. Carbon steel bolts holding a stainless steel handrail corrode quickly. Stainless steel fasteners in carbon steel plate or in aluminium sheet are common practice and usually give no trouble. Aluminium screws in stainless steel sheet would corrode rapidly. One qualification applies when a joint is only partly wet: the area that counts is the wetted area. A stainless steel bolt in an aluminium plate can still attack the aluminium around its head if water collects there while the rest of the plate stays dry. The same reasoning governs coatings. Painting only the anode of a couple is risky, because a scratch in the paint exposes a small anode to a large cathode, so where only one metal can be coated the coating goes on the more noble one.

Common metal pairs and their risk

Examples in service

In 1761 the Royal Navy ordered the bottom of the frigate HMS Alarm to be sheathed in copper to protect the hull from shipworm. The copper bolts that held the sheathing reacted with the iron bolts of the hull, and many of the iron bolts were left nearly useless. The copper was removed in 1766. The Statue of Liberty was built with a copper skin over an armature of puddled iron bars, separated by an insulating layer of shellac. In time the insulation failed, and the iron corroded where it touched the copper. The restoration of the 1980s replaced the whole armature with stainless steel bars and the insulation with PTFE. In 2011 a heavy light fixture fell from the ceiling of a road tunnel in Boston after corrosion weakened its support, where aluminium had been placed in contact with stainless steel and wetted by salt water.

Galvanic corrosion and stainless steel

Passive stainless steel lies near the noble end of the series, so in most couples it is the cathode and the damage falls on the other metal. The corrosion of stainless steel article sets galvanic attack beside the other five forms that affect the family. Two cases reverse this. The first is contact with a still more noble material, above all graphite: graphite-filled gaskets, packing and lubricants, and rubber loaded with carbon black, can attack stainless steel in seawater. The second is loss of the passive film inside a crevice or under deposits, which drops the steel to its active potential and can make it the anode next to its own passive surface. Grades with more chromium, molybdenum and nitrogen keep their film under harsher conditions. The duplex stainless steels stay passive in seawater where 304 would not, which keeps them at the noble end of the series in service.

Prevention

The methods of prevention each remove one of the three conditions or reduce the rate. Material selection comes first: one metal throughout, or metals close together in the series, with fasteners of the most noble metal in the joint. Electrical insulation breaks the circuit, using non-conducting washers, sleeves around bolt shanks, gaskets or spacers; a small gap between galvanised roofing and a stainless steel downpipe does the same. Keeping the electrolyte away is effective where the joint can be sealed, painted over or drained so that no water stands at the contact. Coatings help when they are applied to the cathode, or to both metals. Cathodic protection makes the whole structure the cathode. On ships, jetties and pipelines, sacrificial anodes of zinc, aluminium or magnesium are attached and allowed to corrode in place of the structure, and on large structures an impressed current from a rectifier does the same job. Galvanising is the same principle applied as a coating: the zinc corrodes first and protects the steel at scratches and cut edges.

Stainless bar and fasteners from Laxcon Steels

Laxcon Steels makes stainless steel threaded bars in 304 (A2 and B8) and 316 (A4 and B8M), grades that lie at the passive end of the seawater series. Duplex steel is made for chloride and seawater service, and bright bar from 5 to 115 mm is made in the stainless grades listed in the grade reference.

Frequently asked questions

Can stainless steel be used with aluminium?

Yes, in most conditions, when the stainless steel is the smaller part. Stainless steel fasteners in aluminium sheet are common practice in air, because the large aluminium area spreads the current. In seawater, or where water is trapped at the joint, the aluminium around the fastener can pit, and insulating washers and sleeves are used. Aluminium fasteners in stainless steel should be avoided.

Does galvanic corrosion happen without water?

No. An electrolyte must bridge the two metals to complete the circuit, so dry couples do not corrode galvanically. Rain, condensation, sea spray, wet soil and trapped moisture are all sufficient, and chloride from sea air speeds the attack.

What is the difference between the galvanic series and the electrochemical series?

The electrochemical series lists the standard electrode potentials of pure metals in solutions of their own ions under standard conditions. The galvanic series lists the measured corrosion potentials of real metals and alloys in a real environment, usually seawater, and so includes the effect of oxide films. The galvanic series is the one used to judge a pair of metals.

Can 304 and 316 stainless steel be used together?

Yes. Both are passive in most waters, and their potentials differ by a few hundredths of a volt, too little to drive significant galvanic corrosion. Where the two are combined, 316 is the usual choice for the fasteners because it is the slightly more noble of the pair.

Sources

• ASTM G82, Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance.
• ASTM G71, Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes.
• MIL-STD-889, Dissimilar Metals.
• ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing, and Protection: galvanic corrosion.
• Roberge, P. R., Handbook of Corrosion Engineering, McGraw Hill: the anodic index.
• British Stainless Steel Association and Specialty Steel Industry of North America, guidance on bimetallic corrosion of stainless steel.