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CROSS Safety Report

Concerns over corrosion of dissimilar metals in fixing cavity wall tie systems

Report ID: 1186 Published: 24 May 2023 Region: CROSS-UK


Overview

A reporter is concerned about corrosion that may occur between dissimilar metals that can be used in cavity wall tie systems. The reporter has noted that stainless steel and carbon steel may be fixed together in wall cavities, which are damp environments, and that corrosion of dissimilar metals in contact may occur.

Key Learning Outcomes

For architects, engineers and other specifiers:

  • Corrosion resulting from dissimilar metals in contact in damp environments, can lead to structural failures
  • Select the correct fixings for wall tie systems to achieve at least the required design life
  • Consult manufacturers regarding the life of wall tie systems and their fixings, taking into account the site exposure conditions
  • The whole wall tie system, including fixings, should be shown on the construction drawings
  • During site visits, check that the whole wall tie system is being installed adequately

For manufacturers:

  • Ensure that the design life for all systems and materials, for varying exposure conditions, is readily available to specifiers

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A reporter is concerned about corrosion that may occur between dissimilar metals that can be used in cavity wall tie systems. In particular, the reporter has identified that stainless steel and carbon steel may be fixed together in wall cavities that could be considered damp environments. The reporter cites that, as modern methods of construction evolve, this situation could occur more frequently with metal components, which are likely to be stainless steel, positioned in the cavity and fixed back to framing or other structural elements forming part of the ‘inner skin’. The reporter is concerned that the fixings holding the stainless steel components could be more susceptible to corrosion if they are made of carbon steel and are positioned in the cavity. The issue is illustrated in Figure 1 below.

Image
Figure 1: fixings that could be susceptible to corrosion

The concerning matter is the material selection for the self-drilling screws (or similar fixing) that hold the stainless steel component in place. The reporter says that some manufacturers only provide stainless steel fixings, while others offer a choice between stainless steel and zinc plated carbon steel. The reporter goes on to say that zinc plated carbon steel fixings are, in their experience, not offered with a 50 year warranted life, whereas stainless steel fixings in such circumstances are offered with a life that meets the normally specified 50 year design life for buildings in the UK. The reporter argues that such systems are restraining the full masonry façade of the building and therefore the whole system, including the fixings, must be considered and assessed to meet the required design life of the building.

The reporter says that installations, such as those shown in Figure 1, are gaining in popularity since significant work can be completed before masonry construction starts, with a mason only needing to fit ‘twist and fit’ ties as masonry construction proceeds. Furthermore, as these products are often procured once the masonry subcontractor is appointed, it may be that the products are just considered from a price perspective.

it may be that the products are just considered from a price perspective

The reporter contends that the cavity space cannot be considered a dry environment and that the presence of water in the cavity can cause bimetallic corrosion between stainless steel components and any carbon steel fixings.

The reporter, therefore, urges specifiers to ensure that all fixings for cavity systems are assessed as fit for purpose, including meeting the building's required design life.

Expert Panel Comments

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The reporter rightly raises the issue of corrosion which may occur due to the use of dissimilar metals in damp environments. Cavity and other damp environments have been known to be prone to corrosion problems since failures of masonry ties in coastal environments became apparent.

Dissimilar metal corrosion is something that may occur when different metals are in electrical contact under moist conditions. For this corrosion to occur, the metals in contact must have sufficient difference in their electro potential. The presence of moisture is also required, to form an electrolyte which enables the corrosion reaction to take place. When this situation arises, it will be the less noble of the metals, the carbon steel in this case, which suffers the additional corrosion. Corrosion arising due to this dissimilar metal effect will be localised to the contact area between the dissimilar metals.

Dissimilar metal corrosion is something that may occur when different metals are in electrical contact under moist conditions

Guidance was published in 1979 by British Standards in PD 6484:1979 Commentary on corrosion at bimetallic contacts and its alleviation. This guidance provides an indication of the additional corrosion of carbon and low alloy steel that may result from contact with other metals including stainless steels. Contact between carbon and stainless steels, in rural and industrial/urban atmospheric exposure conditions, may result in slight or moderate additional corrosion. However, under more aggressive exposure conditions, such as in a marine atmosphere, the additional corrosion may be severe and PD 6484 recommends that either protective measures are required, or contact should be avoided. The level of corrosion will depend upon the duration of wetting, the ratio of the exposed areas of carbon steel and stainless steel at the contact, and the nature of the moisture that is present as bimetallic corrosion is sensitive to the presence of constituents in the moisture.

As in the reported case, where stainless steel channels are fixed back to the structural framing with carbon steel fixings, dissimilar metals may come into contact. The channels are generally fixed to the structural framing of the building by self-drilling, self-tapping fixings which pass through a hole in the channel, through the insulation and fix to the structural framing which could be concrete, steel or timber. Where the fixings pass through the stainless steel channel and a mechanically secure connection is made, this is likely also an electrical connection and where the carbon steel could corrode.

Conditions in the cavity

The stainless steel channels and the heads of the fixings are located in the cavity of the cladding system. Since the cavity is vented, daily temperature changes will generate regular airflows. This routine air movement will tend to allow the atmosphere inside the cavity to match the external environment. Therefore, exposure conditions in the cavity will be dependent upon the location and vary from one site to another. Equally, daily temperature changes may lead to condensation from time to time. When condensation occurs on the stainless steel channels it may lead to moisture at fixing positions. Where channels are vertical the condensation may drain to the fixing locations. The geometries under the head of the fixing and between the threads, with very narrow gaps, will lead to moisture ingress to the stainless steel/carbon steel connections, due to capillary action. Once drawn into the narrow gaps, the drying time may be extended due to the small exposed surface area. Levels of condensation will vary, as will the drying times, and therefore, it will generally not be possible to predict metal loss over time due to corrosion.

conditions in the cavity will be dependent upon the location and vary from one site to another

Carbon steel fixings are typically coated with a protective layer of zinc. This zinc coating is very thin and is intended for protection in moderate environments such as inside a building. Thin coatings may not provide protection for extended periods of wetting. Furthermore, the zinc is soft and some of it could possibly be removed during installation of the fixing through the stainless steel channel. Therefore, some of the protection from the zinc in the threads may be lost. Where the zinc is intact, on the head and shank adjacent to the stainless steel, it may be consumed due to the same dissimilar metal effect. The zinc coating adjacent to the dissimilar metal contacts may be consumed more quickly than would be expected for a non-dissimilar metal connection.

Exposure conditions

The atmosphere in cavities of buildings that are located inland may be mild and condensation that forms in them may be low in aggressive constituents. Under these conditions, the additional corrosion due to dissimilar metal connections may be relatively low. In contrast, where buildings are in more aggressive locations, such as marine environments, the atmosphere in the cavities will be more aggressive.  This will be reflected in the nature of the condensation, which may lead to more severe corrosion. Exposure also varies with building height and orientation, so facades higher up on buildings and facing prevailing weather may be subject to greater exposure and risk of corrosion. It is important to note that all building types, not just masonry clad buildings, could be affected.

It is important to note that all building types, not just masonry clad buildings, could be affected

Where minimum design life requirements for the cladding systems are expected to match the building, typically 50-60 years, the potential for dissimilar metal corrosion introduces an element of uncertainty with respect to the stability of the outer skin. Due to the progressive nature of any corrosion, this uncertainty will increase with time. It should also be noted that these fixings are hidden from view, are not readily inspectable and, therefore, should be made robustly.

these fixings are hidden from view, are not readily inspectable and, therefore, should be made robustly

One method of avoiding possible dissimilar metal corrosion while still using carbon steel fixings is to isolate the different metals at the point of connection. However, this is not really feasible where self-tapping connections engage with stainless steel channels.

Specify compatible fixings

Structural designers and other specifiers should take the lead in specifying stainless steel systems and compatible fixings for damp environments where corrosion between dissimilar metals may be a concern, unless other measures to avoid dissimilar metal corrosion are taken. These fixings are more expensive, but their use will completely avoid the problem of dissimilar metal contact inside the cladding system. Designers should specify the whole wall tie system on construction drawings. Arrangements should also be made to check installations on site. Manufacturers should be consulted regarding the life of fixings, taking into account the site exposure conditions. Manufacturers should ensure that the design life for varying exposure conditions for all systems and materials is readily available to specifiers.

There may still be dissimilar metal contact where the stainless steel fixings engage with any structural support system. However, when these connections are inside the building, and there are no condensation issues, there should be no potential for dissimilar metal corrosion.

Finally, as suggested by the reporter, it is easy for later value engineering exercises to propose changes to specified cavity wall tie systems, perhaps because their critical nature is not appreciated. All involved with designing, specifying and building cavity wall systems, of all types, should be mindful of the required life of such systems before changes in specification are proposed.

The National Physical Laboratory has published significant guidance concerning corrosion including Guides to Good Practice in Corrosion Control No. 5 - Bimetallic Corrosion.

CROSS-UK report 811 - Galvanic (bimetallic) corrosion not considered in cladding design, published in 2022, concerned corrosion between stainless steel bolts and aluminium cladding. CROSS-UK report 931 - Masonry panels rock in wind due to missing wall ties, published in 2023, dealt with the critical nature of wall ties generally. This report also referenced the Standing Committee on Structural Safety (SCOSS) Alert - Inquiry into the construction of Edinburgh Schools, published in 2017, which reported upon significant structural defects of external walls at a number of schools.

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