Preventing Corrosion on the Interior Surface of Metal Jacketing

By Jim Young

Insulation is used on the exterior surface of pipes, tanks, ducts, vessels, and equipment for the same reason it is used on building envelopes: to reduce the flow of heat. The insulation is part of a complex construction called a mechanical insulation system, which can include one or more layers of insulation, adhesive at the insulation joints, vapor retarder, and metal jacketing. These systems are often more complicated than building envelope insulation because of their complex geometry, the unidirectional heat/moisture flow, the extreme temperatures of the equipment being insulated, and the often outdoor-exposed location.

Mechanical insulation systems for hot applications are applied to pipes and equipment that can be hotter than 1,200°F (>649°C). The main purposes of the insulation system are to improve energy efficiency, prevent contact burns, and maintain process control, with secondary effects including improved fire resistance and sound deadening.
Examples of applications for hot mechanical insulation systems are hot service water in a commercial building, power plants, oil/gas refining and cracking, petrochemical manufacture, and food production.

Mechanical insulation systems for cold applications are applied to pipes and equipment that range from just below ambient temperature to near absolute zero. The main purposes of the insulation system are to improve energy efficiency, minimize condensation on the system surface, prevent contact burns, and maintain process control.

Examples of applications for cold mechanical insulation systems are food/beverage refrigeration, commercial building chilled water air conditioning, liquid natural gas handling and shipping, and petrochemical manufacture.

Due to the complex nature of mechanical insulation systems, they are typically designed by engineers hired directly by the facility owner or architect, or by a subcontracted engineering design firm. Specifications for mechanical insulation can range from simple, short documents to more than 100 pages depending on the complexity of the job and insulation system.

Metal Jacketing and Interior Surface Corrosion

Very few insulation materials can be left exposed in outdoor applications, so metal jacketing is widely used to protect the insulation system from damage due to UV exposure, physical abuse, and environmental water. Many types of
metal have been used as jacketing, including aluminum, stainless steel, aluzinc, aluminized steel, and even galvanized steel. Of these, the most commonly used in North America are aluminum and stainless steel; the use of both these materials is growing outside North America as well. Outside North America, aluzinc and aluminized steel are very popular. All metal types have benefits and disadvantages, but all have two key weaknesses: 1) joints that are impossible to perfectly seal against water penetration and 2) the possibility of interior surface corrosion.

Water intrusion into the insulation system through the joints in the metal jacketing is inevitable because these joints cannot be made watertight using adhesives/sealants. In addition, damage to metal jacketing is common due to factors such as hail, wind, foot traffic, and having ladders leaned against it. The water can come from rain, condensation, dew, mist, fog, snow, cooling tower spray, and even ocean spray.

When water enters the insulation system, its specific location will depend on various factors, including whether a vapor retarder is present on the surface of the insulation and the integrity of this vapor retarder. Hot applications typically do not have a vapor retarder present, and water can migrate throughout the insulation system subject to the influence of gravity, temperature, and other factors. Cold applications usually have a vapor retarder that should be fully intact on the outer surface of the insulation. Any water that penetrates through the joints in the metal jacketing will collect in the small space between the vapor retarder and the metal jacketing (see Figure 1).
When water penetrates through the joints in the metal jacketing, it can cause corrosion of the jacketing’s interior surface. This type of corrosion is not the classic corrosion under insulation (CUI), which is caused by water in direct contact with the pipe or equipment. This is also not corrosion of the edges of the metal jacketing, which can occur with the coated steel type jacketing (aluzinc, galvanized, and aluminized steel). Because this corrosion occurs on the interior surface of the jacketing, it is very hard to detect until it penetrates completely through the metal and leaves visible holes in the jacketing. Often, this type of corrosion is assumed to have begun on the exterior of the jacket because it is not observed until a hole has formed, at which point the two loci of corrosion (interior and exterior) are difficult to distinguish.

This interior corrosion can start almost immediately after installation and form holes in as quickly as 6 months. This corrosion cannot be repaired; all that can be done is to replace the metal jacketing, which is very expensive. It is far better to protect the metal initially to prevent corrosion than attempt to fix the problem after it has occurred. Figure 2 shows interior jacket corrosion that has formed holes through the jacketing.

Types of Corrosion on Jacket Interior Surfaces

Galvanic or dissimilar metal corrosion occurs when two different metals are coupled in the presence of an electrolyte.
V. Mitchell Liss describes the source of galvanic corrosion in mechanical insulation systems: “Galvanic corrosion ngenerally results from wet insulation with an electrolyte or salt present that allows a current flow between dissimilar
metals (i.e., the insulated metal surface and the outer jacket or accessories).”1

Galvanic corrosion can occur with all types of metal jacket and is most prevalent in hot applications where wet insulation can touch both the jacket and the pipe or equipment, forming a bridge between the dissimilar metals.

When this occurs, the more active metal corrodes. This is usually the jacket but can be the pipe or equipment when stainless steel jacketing is used with carbon steel pipe or equipment. Water in the insulation system is necessary for this type of corrosion since it is both the electrolyte and an excellent source for the ions that give the water its electrical conductivity. A convenient way to describe galvanic corrosion is that it occurs when two dissimilar metals are coupled in a single environment.

Crevice and pitting corrosion are similar in both chemistry and result. Crevice/pitting corrosion is a localized form of corrosion associated with a stagnant solution in contact with metal. This corrosion can occur when a small droplet of
water is trapped between the interior surface of the metal jacketing and either the insulation or a vapor retarder on the outside of the insulation. This thin space between the jacket and the underlying surface acts like a crevice, and moisture trapped in this crevice can lead to crevice/pitting corrosion, especially when chlorides are present, as they are in most environmental sources of water. To function as a corrosion site, this crevice must be large enough to
permit entry of water but small enough to keep the water stagnant. This can readily occur in the thin gap between metal jacket and vapor retarder.

Crevice/pitting corrosion can occur with aluminum, coated steel, and even stainless steel jacketing. All stainless steels are susceptible to crevice corrosion. The commonly used type S304 stainless is susceptible to crevice/pitting in the presence of salty water above about 50°F (10°C), and the less commonly used type S316 stainless is more resistant but can be attacked if the temperature increases even slightly above 50°F (10°C).2,3 Pitting and crevice corrosion together account for perhaps 25 percent of all corrosion failures in stainless steel.2,3

Most mechanical insulation systems are designed so the jacket temperature is fairly close to the ambient temperature and NOT to the pipe or equipment temperature, so it is easy to get above 50°F (10°C). A convenient way to describe crevice/pitting corrosion is that it occurs when one metal type is in the presence of two connected micro-environments.

Lab Testing of Resistance to Interior Surface Jacket Corrosion

Lab corrosion tests were conducted to examine the potential for galvanic or pitting/crevice corrosion of various metal jacketing and to determine how effective polysurlyn moisture barrier (PSMB) was at preventing this type of corrosion.

For the first test, a mock-up of a common mechanical insulation system was constructed. Standard carbon steel pipe was covered with mineral wool insulation, which was then covered with various types of metal jacketing both with and without PSMB lining. When PSMB-lined metal jacketing was used, an X was scribed through the PSMB to mimic damage that might occur during handling and installation. The fibrous insulation used in this test was a way to keep the pipe and jacket separated while also allowing the added salt water to form a bridge between these two metals due to the open-cell/fibrous nature of the mineral wool. The mineral wool insulation was wetted with salty water, and an induced electrical potential was applied between the pipe and jacket to accelerate galvanic corrosion. Each test lasted 75 minutes. The underside of the metal jacketing was then examined for evidence of corrosion.

Four different types of metal jacketing were examined in this test: 3105 aluminum alloy, aluzinc coated steel, galvanized steel, and aluminized steel. For all four bare metal types, there was significant corrosion visible on the surface in contact with the insulation. In the tests where a PSMB was applied to the metal jacketing, there was no corrosion present. Figure 3 shows pictures of the jacketing after this test was complete.

In a second lab test, the jacketing was stainless steel and the pipe was carbon steel. In this case the corrosion would be expected to occur on the pipe. Four 75-minute-long voltage applications were made and the pipe examined. When type S304 or S316 bare jacketing was used, the pipe exhibited significant corrosion. When both types of stainless jacketing were lined with PSMB, no pipe corrosion occurred. Figure 4 shows the pipes after this test was completed.

In addition to visual observation of corrosion, the mass loss due to corrosion for each pipe was determined. It was found that when the stainless steel jacketing had a PSMB, there was no mass lost from the underlying pipe due to corrosion. When bare stainless steel jacketing was used, the mass lost from the pipe was 3.5 percent for type S304 stainless jacket and 2.5 percent for type S316 stainless jacket. Note that this large amount of mass loss occurred after only four 75-minute exposures to the corrosion conditions. Figure 5 shows this result graphically.

In this case, the corrosion science and the lab testing are in agreement. Bare metal jacketing can lead to interior surface jacket (or pipe) corrosion, and the use of PSMB prevents this corrosion. The use of PSMB on metal jacketing protects all types of metal jacketing from this corrosion and protects the pipe under the insulation from this corrosion when the jacket is stainless steel.

While this lab testing was a simulation, the difference between these accelerated lab tests and actual field experience is only the time required for failure.

Real-World Results

Figure 6 describes six real-world cases where interior surface metal jacket corrosion was encountered. No specific company or facility names are used. In all these examples, the metal jacketing was aluminum with a polykraft
moisture barrier.

Figure 7 shows the poor condition of the polykraft moisture barrier and the interior surface corrosion holes in the metal jacketing from the first example in Figure 6.

In addition to these examples of corrosion occurring when polykraft moisture barrier is used, there is an important related observation. This author’s employer, one of the largest global providers of metal jacketing for mechanical insulation systems, sells primarily jacketing with PSMB and has never had a claim or any knowledge of a case of interior surface corrosion occurring when a PSMB was used on the metal jacketing.

Corrosion science, lab test results, and real-world experience are all in agreement. Interior surface jacket corrosion can steal longevity from insulation systems in all industries, all applications, using all insulation types, with all metal
jacketing types, and in all climates. Using an effective moisture barrier like PSMB on the interior surface of the metal jacketing is an effective way to prevent this type of corrosion.

Moisture Barriers

There are three general types of moisture barriers used on metal jacketing in mechanical insulation systems: paint, polykraft, and polysurlyn. Painted moisture barrier is a thin (0.7 mil, ~18 μm) layer of lightly pigmented paint that is typically applied in the mill that produces the metal coils. This type of moisture barrier is common on pre-formed two-piece elbows, where it is probably acceptable due to the ultrapure corrosion-resistant aluminum alloy used.
Polykraft is a layer of kraft paper laminated to a single thin layer of polyethylene film by a metal jacketing company.
PSMB is a thick, three-layer film applied by a jacketing company and represents the current state of the art for moisture barriers.

The real-world examples above indicate that polykraft is ineffective and PSMB is effective at preventing this corrosion, but why? To answer this, the properties of the various moisture barriers must be considered in light of their main purpose: to keep water from contacting the underside of metal jacketing to reduce corrosion potential.

The key properties of a moisture barrier are shown below, and the performance of each moisture barrier type in these key properties is listed in Figure 8.

  • Pinholes: Each pinhole is a place where corrosion can start
    • Fewer is better and zero pinholes is most desirable
  • Water resistance: Keep the corrosive water from touching the interior metal surface
    • Low water absorption and low water vapor transmission rate are desirable
  • Toughness/durability: Damaged or decayed moisture barrier from the inevitable rough handling and installation is a locus for possible corrosion
    • Strong, tough, scratch resistant, and durable film is desirable
  • Flammability: Lower flammability is preferred

Author Recommendations for Contractors and Facility Owners

Insulation contractors should minimize damage to the moisture barrier during cutting, field fabrication, and installation and educate workers on moisture barriers and their importance. For aluminum jacketing, use the new
ASTM standard for this type of jacketing and specify that it comply with ASTM C1729, Type I, Grade 1 or 2, Class A to ensure a PSMB is present. Lastly, discuss the use of PSMB-lined metal jacketing with specifiers, engineers, and owners.

Owners and engineers should consider metal jacketing with PSMB. For aluminum jacketing, use the new ASTM standard for this type of jacketing and specify that it comply with ASTM C1729, Type I, Grade 1 or 2, Class A to ensure a PSMB is present. Lastly, ensure that contractors know to minimize damage to PSMB during handling and installation.

Metal insulation jacketing with 3 mil (76 μm) thick PSMB factory heat laminated on the inside surface protects against jacket corrosion when aluminum and aluminum-coated steel jacket is used and against jacket and pipe corrosion when stainless steel jacket is used. Use of PSMB is cheap insurance to prevent the costly alternative of jacket corrosion.


1. V. Mitchell Liss, corrosion engineering consultant, “Preventing Corrosion Under Insulation,” Bulletin of the National Board of Boiler and Pressure Vessel Inspectors, January 1988.

This paper was originally presented at the 2011 NIA Convention in Tucson, Arizona.