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Corrosion Prevention and Control

Galvanic Table — The Galvanic Table lists metals in the order of their relative activity in sea water environment. The list begins with the more active (anodic) metal and proceeds down the to the least active (cathodic) metal of the galvanic series.

The obvious means of prevention is therefore to avoid mixed metal fabrications. Frequently this is not practical, but prevention can also be by removing the electrical contact - this can be achieved by the use of plastic or rubber washers or sleeves, or by ensuring the absence of the electrolyte such as by improvement to draining or by the use of protective hoods. This effect is also dependent upon the relative areas of the dissimilar metals. If the area of the less noble material (the anodic material, further towards the right in Figure 2) is large compared to that of the more noble (cathodic) the corrosive effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in contact with a small area of less noble will accelerate the galvanic corrosion rate. For example it is common practice to fasten aluminium sheets with stainless steel screws, but aluminium screws in a large area of stainless steel are likely to rapidly corrode. Galvanic series for metals in flowing sea water. Source: Stainless Steel - Corrosion Resistance, Azom. Additional reference: Galvanic Series, Corrosion Source

Design — Some preventive measures are generic to most forms of corrosion. These are most applicable at the design stage, probably the most important phase in corrosion control. It cannot be overemphasized that corrosion control must start at the "drawing board" and that design details are critical for ensuring adequate long term corrosion protection. It is generally good practice to (reference):

• Provide adequate ventilation and drainage to minimize the accumulation of condensation
• Avoid depressed areas where drainage is inadequate
• Avoid the use of absorptive materials (such as felt, asbestos and fabrics) in contact with metallic surfaces)
• Prepare surfaces adequately prior to the application of any protective coating system
• Use wet assembly techniques to create an effective sealant barrier against the ingress of moisture or fluids (widely used effectively in the aerospace industry)
• Provide easy access for the purposes of corrosion inspection and maintenance work

Galvanic corrosion can be minimized in design. Corrosion engineers have found the following practical rules invaluable in this respect.

• Select combinations of metals which will be in electrical contact from groups as close together as possible in the galvanic series
• Electrically insulate from each other metals from different groups, wherever practical. If complete insulation cannot be achieved, paint or plastic coating at joints will help
• If you must use dissimilar materials well apart in the series, avoid joining them by threaded connections as the threads will probably deteriorate excessively. Brazed or thermal joints are preferred, using a brazing alloy more noble than at least one of the metals to be joined
• Avoid making combinations where the area of the less noble, anodic metal is relatively small compared with the area of the more noble metal.
• Apply coatings with judgment. Example: Do not paint the less noble metal without also painting the more noble; otherwise, greatly accelerated attack may be concentrated at imperfections in coatings on the less noble metal. Keep such coatings in good repair.
• Consider use of cathodic protection.

Materials selection — Without adequate corrosion resistance, or corrosion allowance, components often fall short of the expected design life. Unlike mechanical properties, there are no codes governing corrosion properties.

• Galvanic series relationships are useful as a guide for selecting metals to be joined, will help the selection of metals having minimal tendency to interact galvanically, or will indicate the need or degree of protection to be applied to lessen the expected potential interactions.
• Generally, the closer one metal is to another in the series, the more compatible they will be, i.e., the galvanic effects will be minimal. Conversely, the farther one metal is from another, the greater the corrosion will be.
• To avoid corrosion, avoid a small anodic area relative to the cathodic area.
• Metals widely separated in the galvanic series must be protected if they are to be joined. Appropriate measures should be taken to avoid contact. This can be accomplished by several methods.

Inhibitor — A corrosion inhibitor may be defined, in general terms as a substance which, when added in a small concentration to an environment, effectively reduces the corrosion rate of a metal exposed to that environment. Inhibition is used internally with carbon steel pipes and vessels as an economic corrosion control alternative to stainless steels and alloys, coatings, or non-metallic composites.

Passivating inhibitors — Passivating inhibitors (passivators) cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range. There are two types of passivating inhibitors:

• Oxidizing anions, such as chromate, nitrite and nitrate, that can passivate steel in the absence of oxygen
• Non oxidizing ions such as phosphate, tungstate and molybdate that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used.

Cathodic inhibitors — Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc or magnesium, may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulfite (Na₂SO₃).

Cathodic Protection Against Corrosion

Underground steel pipes offer the strength to transport fluids at high pressures, but they are vulnerable to corrosion driven by electrochemical processes. A measure of protection can be offered by driving a magnesium rod into the ground near the pipe and providing an electrical connection to the pipe. Since the magnesium has a standard potential of -2.38 volts compared to -.41 volts for iron, it can act as a anode of a voltaic cell with the steel pipe acting as the cathode. With damp soil serving as the electrolyte, a small current can flow in the wire connected to the pipe. The magnesium rod will be eventually consumed by the reaction Mg(s) -> + Mg2+(aq) + 2e- while the steel pipe as the cathode will be protected by the reaction O2(g) + 2H2O(l) + 4e- -> 4OH-(aq). Source: Corrosion, HyperPhysics.

Sacrificial — by applying to the cathodic member a sacrificial coating having a potential similar to or near that of the anodic member. The sacrificial element should be on the anodic side and smaller. (Plating)

Alloys — Corrosion-resistant alloys are used where corrosive conditions prohibit the use of carbon steels and protective coatings provide insufficient protection or are economically not feasible. Examples of these alloys include stainless steels, nickel-base alloys and titanium alloys.

Sealing - by sealing to insure that faying surfaces are water-tight. Seal all faying edges to preclude the entrance of liquids. Apply corrosion-inhibiting pastes or compounds under heads of screws or bolts inserted into dissimilar metal surfaces whether or not the fasteners had been previously plated or otherwise treated. In some instances, it may be feasible to apply an organic coating to the faying surfaces prior to assembly.

Sealant Joint Design At the design stage, it is important to know the total movement at the joint and the percentage movement of the sealant. Most joint movements result from expyearsion and contraction due to seasonal temperature changes (which can reach 38°C (100°F)), windloading, and moisture changes. The joint shape also affects the movement of sealants. Butt joints stress sealants in tension and compression while lap joints stress in shear only, permitting greater movement.

Dimensional relationship of joint

Forgues, Y.E. Properly Sealed Construction Joints, IRC, NRCC

Surface Preparation — Surface pre-treatment by chemical or mechanical means is also important in painting, and the methods used are designed to ensure good adhesion of the paint to the alloy surface. Surface engineering for increased material performance is one important element of the world of metal finishing. Most metal surface treatment and plating operations have three basic steps: (reference)

• Surface cleaning or preparation, which involves the use of solvents, alkaline cleaners, acid cleaners, abrasive materials, and/or water
• Surface modification, which involves some change in surface properties, such as application of a metal layer or hardening
• Rinsing or other work-piece finishing operations to produce the final product

Steel Surface Preparation — Preparation of the surface consists of a first phase of eliminating calamine, oxide and extraneous materials from the steel surface before applying the primer. The second phase is to eliminate oxide and extraneous materials, if there are any, from the primed iron surface before applying the complete painting system. A steel surface can be deoxidised in the following ways:

• Mechanical brushing
• Descaling
• Pin Hammer
• Flame Cleansing
• Sanding with Grinding Discs
• Brushing
• Pellets
• Powder
• Wet abrasion, Jet cleaning

Resistance — by painting or coating all surfaces to increase the resistance of the electrical circuit.

Coating

Metallic — Metallic coatings provide a layer that changes the surface properties of the workpiece to those of the metal being applied. The workpiece becomes a composite material exhibiting properties generally not achievable by either material if used alone. The coatings provide a durable, corrosion resistant layer, and the core material provides the load bearing capability. The deposition of metal coatings, such as chromium, nickel, copper, and cadmium, is usually achieved by wet chemical processes that have inherent pollution control problems.

The most widely used metallic coating for corrosion protection is galvanizing, which involves the application of metallic zinc to carbon steel for corrosion control purposes. Hot dip galvanizing is the most common process, and as the name implies, it consists of dipping the steel member into a bath of molten zinc. These include:

Inorganic coatings — Inorganic coatings can be produced by chemical action, with or without electrical assistance. The treatments change the immediate surface layer of metal into a film of metallic oxide or compound which has better corrosion resistance than the natural oxide film and provides an effective base or key for supplementary protection such as paints. In some instances, these treatments can also be a preparatory step prior to painting.

Anodizing — Anodizing involves the electrolytic oxidation of a surface to produce a tightly adherent oxide scale which is thicker than the naturally occurring film. Anodizing is an electrochemical process during which aluminum is the anode. The electric current passing through an electrolyte converts the metal surface to a durable aluminum oxide. The difference between plating and anodizing is that the oxide coating is integral with the metal substrate as opposed to being a metallic coating deposition. The oxidized surface is hard and abrasion resistant, and it provides some degree of corrosion resistance.

Chromate filming — A number of proprietary chromate filming treatments are available for aluminum, magnesium, cadmium and zinc alloys. The treatments usually involve short time immersion in strongly acid chromate solutions, but spraying or application by brushing or swabbing can also be used for touch-up of parts. The resulting films are usually about 5 mm thick, and are colored depending on the base alloy, being golden yellow on aluminum, dull gold on cadmium and zinc, and brown or black on magnesium. The films contain soluble chromates which act as corrosion inhibitors, and they provide a modest improvement in corrosion resistance of the base metal. However their main purpose is to provide a suitable surface for sealing resins or paints. Epoxy primer, for example, which does not adhere well to bare aluminum, adheres very well to chemical conversion coatings.

Phophatizing (phosphating or phosphate conversion coating) — A number of proprietary treatments such as 'Parkerizing' or 'Bonderizing' are available for use on steel. They are applied by brushing, spraying or prolonged immersion in an acid orthophosphate solution containing iron, zinc or manganese. The coatings consist of a thick porous layer of fine phosphate crystals, tightly bonded to the steel.

Nitriding — Steels containing nitride forming elements such as chromium, molybdenum, aluminum and vanadium can be treated to produce hard surface layers providing improved wear resistance.

Organic — Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors but, as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic inhibitors usually designated as 'film-forming', protect the metal by forming a hydrophobic film on the metal surface. In some instances, it may be feasible to apply an organic coating to the faying surfaces prior to assembly. Stock should be factory primed.

Jointing compounds and sealants — Various synthetic resins are used for this purpose. The compounds harden sufficiently at edges to take paint, but they remain tacky within the joint so that flexure does not cause cracking. Sealants of the type now being specified are also elastomeric, and the most popular are polysulphide sealants containing corrosion inhibitors. The inhibitive sealants are very effective when used in faying surfaces and butt joints, for wet installation of fasteners and over fastener patterns. They are also effective in insulating dissimilar metals.

Dehumidification — Corrosion may be prevented or controlled by modification of the corrosive environment. One way of achieving this is by preventing the ionic charge flow within the environment. This is readily achieved by reducing the water content of the environment by dehumidification. At relative humidity below a certain level, a continuous film of moisture will not form on metal surfaces, and this film is required to complete the corrosion cell.

Iron and steel does not rust if the air over the surface has a relative humidity below 50%RH. Source: Humidity Control	A surface will not have condensation on it if the air in contact with it has a dewpoint lower than the surface temperature.

 

When drying out moisture from buildings, new buildings or buildings with water damage, the most effective way is to use sorption dehumidifying. Heating will only move the moisture to another part of the building. Heating in combination with outdoor ventilation will create high energy costs. With sorption dehumidifying the moisture is taken out from the building in a very energy efficient way. Source: Humidity Control

Monitoring — Decisions regarding the future integrity of a structure or its components depend entirely upon an accurate assessment of the conditions affecting its corrosion and rate of deterioration. With this information, an owner can make an informed decision as to the type, cost and urgency of remedial measures. Monitoring corrosion characteristics of a proposed or existing structure can lead to the proper selection of longer life materials, durable and protective coatings and corrosion control measures.

Indoor corrosion/degradation monitoring techniques include:

• careful visual inspection, especially in areas vulnerable to (preferential) corrosion damage such as edges, crevices, dissimilar metal interfaces, joints, seams, areas of moisture accumulation/wicking etc.
• coupon corrosion sensors in the form of polished metal strips, with the option of subsequent laboratory surface analysis techniques
• corrosion sensors based on the quartz crystal microbalance
• chemical sampling (for example acid indicator detection strips)

After a corrosion problem has been identified and the "baseline" problem conditions characterized by monitoring, the ongoing monitoring of corrosion can be helpful to confirm that corrective action has indeed solved the problem. Furthermore, early warning of future problems can be provided from highly sensitive monitoring techniques. Corrosion monitoring may be useful, as it is difficult to anticipate the influence of numerous variables that could lead to corrosion problems, such as humidity, temperature, light, historical exposure to corrosive environments (surface contamination), historical surface/preservation treatments, materials of construction and their historical processing, etc. 

References

• Corrosion Club
• Corrosion Doctors
• Protective Coatings