Deicing salts are used worldwide to remove snow and ice on public roads and bridges, and they are sometimes used by large process plants as well. However, melted ice and salts may inadvertently cause metallic corrosion if they contact valves, pressure regulators, or other related equipment.
Aluminum alloys are especially susceptible to galvanic corrosion, which may lead to exfoliation corrosion. While equipment purchasers may be tempted to focus primarily on initial cost, careful material selection goes far in preventing costly damage down the line.
My article in the April 2024 issue of Processing, titled “Selecting the right materials to prevent exfoliation corrosion,” presents and explains an instance in which improper material selection resulted in serious corrosion damage.
A North American municipality installed two Emerson Fisher EZL Series regulators on a 3-inch natural gas piping system, just below roads that were deiced frequently during winter months. The regulator actuator casings were made of anodized aluminum alloy with brass bolts.
During routine inspection and servicing, the municipality found localized corrosion at the contact point between the brass bolts and aluminum casing.
Casing 1 with actuator top removed, and its casing showed localized corrosion at outlet pressure tube (left) and the diaphragm level indicator (right)
Metallographic evaluation revealed identical corrosion morphology in both casings. Exposure to moisture and a heavy concentration of aggressive anions led to accumulation of salt and, ultimately, corrosion propagation along elongated grains.
Casing 2, cross section of affected area in the removed top casing of the actuator shows corrosion layer buildup (top left). Also shown are scanning electron micrograph of exfoliation corrosion (top right), micrograph of cross section morphology showing layers separation at 100x magnification (bottom left), and intergranular cracking at 500x magnification (bottom right)
Corrosion initiated due to galvanic contact between the brass bolts and the anodized aluminum body of the pressure regulator.
A combination of environmental debris, moisture, and deicing salts on the brass-aluminum contact area created low-pH conditions conducive to galvanic corrosion.
Once initiated, corrosion progressed into the aluminum body. Aluminum can form a galvanic couple with its intermetallic compounds, particularly copper and copper alloys, causing grain boundaries to preferentially corrode.
Furthermore, studies indicate that aluminum alloys containing copper within grain boundaries exhibit stronger electromagnetic force and increased corrosion rates as compared to solid solutions of aluminum alloy and copper. This phenomenon explains how galvanic corrosion can spread without contact between two different metals.
While there is no agreement regarding the underlying mechanism, research shows that exfoliation corrosion tends to progress more quickly through elongated grains than coarse grains. Additionally, deformed elongated grains lead to stacking faults during recrystallization, which in turn affect surface properties and reactivity.
There are several types of stacking faults, but aluminum and its alloys have a propensity towards intrinsic stacking. Intrinsic stacking occurs when vacancies (missing atoms) in a crystal lattice cluster together along a plane.
Depiction of stacking faults sequences
Aluminum and their alloys tend to have high stacking fault energies, which favors dissociation of dislocation into partial dislocations, with minimal separation…For this reason, aged-hardened aluminum alloys tend to be prone to exfoliation corrosion.
Stacking faults are inevitable, but proper manufacturing and postprocessing can alleviate undesirable mechanical and corrosion properties.
Improper material selection, site geography, dissimilar metal contact, and application of deicing salts all contributed to exfoliation corrosion in these regulators. In this case, early detection and application of an external coating covering the entire surface would have prevented propagation.
Generally, appropriate materials and coatings are the most important safeguards against corrosion. End users should carefully consider environmental conditions during material selection and preparation because good decisions in this stage will increase service life, uptime, and safety, while reducing costs.
Visit Fisher EZL Series on Emerson.com to know more about our accurate pilot-operated, pressure balanced, soft-seated regulators. They are designed for use in natural gas distribution applications such as district regulating stations and commercial/industrial meter sets. They provide low differential, smooth, reliable operation, tight shutoff and long life.
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