At road, rail and waterway crossings the carrier pipe is usually run inside a separate steel or concrete casing pipe. The casing pipe takes the traffic load and ground settlement, and the carrier pipe runs freely inside it. A ring-shaped space, the annulus, is left between the two pipes. This empty space is the weakest corrosion point of the crossing.
The problem is straightforward. Moisture and oxygen reach the empty annulus, and their combination corrodes the surface of the carrier pipe at a point that cannot be inspected or maintained without opening up the whole crossing. This article goes through why empty annular spaces corrode and how Stopaq Casing Filler solves the matter by filling the space with a compound that displaces water and oxygen.
Kampek is the importer and stockist of Stopaq products in Finland. We provide operator-level guidance on selecting the site and the product by phone. Detailed installation instructions always come from the manufacturer, Seal for Life Industries.
Why casing annuli corrode
Corrosion requires three things: metal, water and oxygen. Inside the casing pipe, metal is always present, because the carrier pipe is steel. Water and oxygen come from the air and the soil. The groundwater level varies, the ends of the crossing leak slowly, and temperature variation brings moisture into the annular space through condensation.
What makes it particularly difficult is that the annulus acts like a condensation chamber. When the warm carrier pipe cools at night, moisture condenses on its surface. The water sits at the bottom of the annular space and cannot evaporate out. Standing water together with the oxygen in the air keeps the corrosion process going year after year.
Crossings are also critical locations. Damage to the carrier pipe under a road or rail line means excavation in the middle of infrastructure, traffic interruptions and expensive repairs. That is why, at these sites in particular, it is worth making sure no conditions for corrosion are left in the annular space.
Stopaq Casing Filler is type-tested to ISO 16440 and withstands the ASTM B117 salt spray test for 720 hours.ISO 16440 · ASTM B117 720 h (Stopaq Casing Filler, PDS V9)
The problem of the empty annular space: hidden surface corrosion
In an empty annulus the surface of the carrier pipe corrodes from the outside. The damage progresses locally where water sits and oxygen is available, that is, usually at the bottom of the annular space and near the ends. Coating defects, knocks during installation and the edges of weld seams are typical starting points.
The worst part is that this corrosion cannot be seen. The casing pipe covers the carrier pipe completely, so visual inspection reveals nothing. Corrosion can progress a long way before it shows up as a leak or wall thinning. By then the repair is already far more expensive than the cost of filling would have been.
Plugging the pipe ends alone is not enough. Plugs slow the airflow but leave the moisture that has already entered the annular space in place and do not prevent condensation. To remove the conditions for corrosion, both water and air must be displaced from the entire annular space. That is exactly what the filler compound is meant for.
Stopaq Casing Filler: a filler compound that displaces water and oxygen
Stopaq Casing Filler is a polyisobutylene-based (PIB) filler compound that is pumped into the annulus between the casing pipe and the carrier pipe. The compound fills the entire annular space and displaces water and air from it. When two of the three conditions for corrosion, water and oxygen, are removed from the surface of the carrier pipe, surface corrosion stops.
Polyisobutylene is the same material family as in Stopaq's viscoelastic corrosion protection wraps. It does not react with steel and contains no water, so it remains neutral with respect to corrosion. The compound adheres to the surface of the carrier pipe and fills irregularities, so no space for moisture is left between the steel and the fill.
Casing Filler is type-tested to ISO 16440, which covers organic filler compounds for underground pipelines in exactly this kind of casing application. In addition it has passed the ASTM B117 salt spray test for 720 hours, which indicates the corrosion protection capability of the fill. Both values come from the manufacturer's datasheet (PDS V9).
A hot-applied compound: filling the annulus
Casing Filler is installed hot. The compound is heated to the installation temperature, at which it becomes fluid, and it is pumped into the space between the casing pipe and the carrier pipe so that the entire annulus is filled and no air pockets are left in the annular space. This is the operating principle; the exact installation method and equipment are defined in the manufacturer's installation instructions.
The application service temperature range is −20 °C to +35 °C according to the manufacturer's datasheet. The temperature value applies to the annular space and the compound. The ambient weather is a separate matter. In Finnish conditions this means the installation must be planned so that the compound stays workable throughout the pumping and the fill reaches both ends in time.
This is a point where it is worth staying in touch with the manufacturer's instructions. The amount of compound required depends on the diameters of the casing pipe and the carrier pipe as well as the length of the crossing. Kampek helps with the sizing and advises what equipment the site requires. The pumping and heating itself are done in accordance with the manufacturer's installation instructions.
Setting after cooling: flexible and water-tight
Once the fill has cooled, the compound sets in place. It stays soft and flexible and does not cure brittle. This is the advantage of polyisobutylene: the compound withstands movement of the ground and the pipe without cracks forming in the fill through which water could get back in.
The set compound keeps water out and stays bonded to the surface of the carrier pipe. If the carrier pipe moves due to thermal expansion or ground settlement, the flexible fill follows the movement and stays intact. This way the annular space remains filled and dry for a long time.
In practice this means the fill protects the carrier pipe in the annular space over the long term. We do not state a numerical service life, because there is none on the datasheet. The advantage of a polyisobutylene-based fill is that it retains its flexibility where curing compounds become brittle over time. The exact suitability limits for the site are worth checking with the manufacturer.
ISO 16440 and the ASTM B117 720 h salt spray test
The two key references for Casing Filler are ISO 16440 and ASTM B117. ISO 16440 is the standard for organic filler compounds for underground pipelines, that is, for exactly this purpose. It defines how such an annulus filler compound is tested and what requirements it must meet.
ASTM B117 is a salt spray test in which a sample is exposed to salt spray for a set time and the durability of the corrosion protection is assessed. Casing Filler has passed this test for 720 hours. Salt spray is an aggressive test environment, so the 720-hour result indicates the corrosion protection capability of the compound.
It is important to keep the standards within their correct product families. ISO 16440 and ASTM B117 apply to Casing Filler. Stopaq's corrosion protection wraps fall under ISO 21809-3 and EN 12068, and joints on pre-insulated district heating pipes fall under EN 489:2009. Each product has its own standard, and the value of one family should not be applied to another.
Service temperature and compatibility with cathodic protection
The service temperature range of Casing Filler is −20 °C to +35 °C according to the manufacturer's datasheet. This needs to be taken into account at sites where the carrier pipe is warm, such as district heating transmission lines. The temperature profile of the site is worth reviewing in advance so that the right solution is chosen.
On underground gas and oil lines the carrier pipe often has cathodic protection. Stopaq's polyisobutylene-based products are designed so that they do not interfere with the cathodic protection of the steel structure, which means the two protection methods can work side by side. This is essential at crossings where the annulus fill and the cathodic protection of the line meet.
Casing Filler belongs to the same Stopaq system as the Wrappingband corrosion protection wrap and the mechanical Outerwrap. A typical crossing is a whole in which the outer surface of the carrier pipe is protected with a Stopaq wrap and the casing annulus is filled with Casing Filler. Tell us the site type, the pipe sizes and the length of the crossing, and we will propose the right combination.
