January 2021

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Corrosion: Address damage caused by galvanic corrosion

At petrochemical and refining facilities, it is common for U-clamps over piping runs to become corroded, sometimes to the extent where they are completely thinned out.

Anwer, A., Qureshi, N., Contributing Writer

At petrochemical and refining facilities, it is common for U-clamps over piping runs to become corroded, sometimes to the extent where they are completely thinned out. Localized thinning of supports can also happen, which may remain unnoticed. Fasteners lose their coating and a thin layer of rust appears that can potentially turn into something more significant that requires action.

These forms of galvanic corrosion can be observed externally with visual aids only. Slowly, these damages lead to the implementation of refurbishment projects to eliminate this form of corrosion. While notable cases exist where galvanic corrosion happens within process equipment, the focus here is the monitoring and elimination of those areas where galvanic corrosion happens externally and is often noticeable with visual aids only.

Galvanic corrosion can start where dissimilar metals are in contact without any precautions taken. One of the metals must be a noble-type; the other must be an active metal. While this contact can sometimes be avoided during the design stage, it is not always possible and designers may have many places onsite where a noble metal is in contact with an active metal. Another required element for galvanic corrosion is the presence of an electrolyte. Because this article is focused on the external areas that are vulnerable to galvanic corrosion, atmospheric conditions play an important role in defining the electrolyte present in the galvanic triangle: electrolyte, noble metal and active metal. Any missing element eliminates the chances of galvanic corrosion.

Vulnerable areas

Galvanic corrosion can become evident in numerous places, and some are more critical to plant integrity than others. Supports over or under austenitic stainless-steel piping are critical areas that should be inspected closely for any signs of damage by galvanic corrosion. Vessel internals constructed from dissimilar materials can cause serious damages. In heat exchangers in which the baffles or tube sheet are of different material than the tube, galvanic corrosion can aggressively attack the active metal.

Areas that are less critical but still experience unnoticed and unreported galvanic corrosion include instrument tubing supported through U-bolts over the length. Instrument tubing is normally of austenitic stainless-steel metallurgy and the installed U-bolts are of plain carbon steel material. Also, non-pressure parts can be attached together without considering galvanic corrosion damages. This is sometimes done inadvertently when a superior material is used just in case the required is unavailable, and that better material becomes the noble and the other active. A classic case is when austenitic stainless-steel nuts and bolts are placed over carbon steel flanges, leading to the inevitable and noticeable galvanic corrosion.

Knowledge of active and noble materials is important, and information tables are available where the significance of galvanic corrosion is shown when any two materials come in contact. Based upon this significance, galvanic corrosion should be monitored across the site. This galvanic series is useful during the design stage as well, especially when selecting piping support materials for different piping networks.

Prevention

Prevention of galvanic corrosion is normally done by applying coating systems. Essentially, coating should be applied over the noble metal if austenitic stainless steel is in contact with carbon steel—the austenitic stainless steel must be coated, but what normally is found onsite is that the coating has been applied on carbon steel material, which is the active metal. Installation of structural steel supports like U-bolts over austenitic stainless-steel piping with rubber pads or other foam material in between is a common practice that is incorrect. Over time, these rubber pads degrade, and close inspection of piping at all levels to ensure that these insulating consumable pads still exist is rare.

To preventively address damages by galvanic corrosion, a 5-yr survey is recommended across the site to identify those areas where this damage already exists or can potentially exist and to take remedial action. This survey should essentially remove all existing and potential threats due to galvanic corrosion found externally. At the same time, there must be a record of equipment in which dissimilar materials have been installed as internals. That record can be kept as a list and for all such equipment, a check for galvanic corrosion should be made part of the written scheme of inspection.

For those areas where galvanic corrosion is seen or can potentially exist, corrective action must be taken.

To be identified, galvanic corrosion does not demand state-of-the-art inspection techniques. Visual aids can easily identify problematic areas and corrective actions can be taken accordingly based upon the situation. Once galvanic corrosion is eliminated at sites, major replacement or refurbishment costs can be significantly lowered, especially for those locations where operating assets are installed in a marine environment. HP

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