Corrosion protection for suspension systems

In a world where economies of scale often demand that a particular component, such as a lower control arm, be produced in one facility and then shipped half way around the world to be assembled into a vehicle or sub assembly, corrosion is often the cancer that costs companies dearly.

The costs are not only in terms of rework or scrap costs, but with stringent quality management systems in place, several thousand parts per million could impact on the prospects of being awarded future business.

It’s against this backdrop that suppliers and manufacturers come together to fight the scourge of corrosion.

Types of corrosion protection

When considering corrosion protection at a manufacturing level there are two basic categories:

Permanent protection systems such as:

• Paint
• Metallic coatings

Temporary protection systems such as:

• Protective oil or wax coatings
• Volatile Corrosion Inhibitors (VCI) – also known as vapor phase corrosion inhibitors
• Desiccants
• Barrier foils

Permanent systems are well known and easily identifiable, but in recent years much study has gone into selfhealing materials that mimic the human body to repair minor damages. This is invaluable in instances where vehicles regularly encounter minor road damage, such as stone-chip, that eventually leads to irreversible corrosion.

Self-healing protective coatings mimic human skin

Funded by the European Union, the main objective of the multi-level protection of materials for vehicles by 'smart' nanocontainers (MUST) project has been the development of effective environmentally-friendly, multilevel active protection systems for structural metallic materials used in future vehicles. The MUST group research investigated a combination of advanced polymeric matrixes and active nanocontainers that had the potential to provide healing effects.

After several years of development the team has managed to produce novel functional nanocontainers capable of storing active agents which are released by specific triggers such as pH, temperature, mechanical impact, water and chlorides. The “healing” properties of these agents can repair damages in coatings and protect underlying metallic substrate.

The research results indicate that the addition of functional nanocontainers and nanotraps to automotive pretreatments and primers significantly improve the long-term corrosion protection and coating adhesion properties of permanent corrosion protection systems.

The most successful products originating from MUST are:

• Automotive self-healing pre-treatments with nanocontainers of corrosion inhibitors
• Active anti-corrosion primer with nanotraps for automotive applications

The MUST technologies have been patented and are currently being commercialized for future mass production.

While permanent corrosion protection is widely used for highly vulnerable visible areas such as Body In White and certain driveline components, temporary protection of components during manufacture, transport and assembly is just as important.

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Each of the temporary systems mentioned above has a place in the fight against corrosion, but in this study we’ll focus on the VCI packaging system as typically applied to automotive manufacturing: In particular multimetal applications commonly used to protect suspension systems such as lower control arms which are typically made up out of a forged aluminum control arm and a steel sleeved inner rubber bush.

VCI corrosion protection systems

VCI dates back to the 1940s when Shell developed a paper-borne vapor phase inhibitor to protect ships’ cannons.

The system works by creating an inert environment around the component to be protected, as such it doesn’t try to control humidity, such as a desiccant, or deposit a coating which needs to be removed before the part can be used.

Rather, the VCI is exuded as a gas which eventually reaches an equilibrium within the (most often) sealed packaging, resulting in a temporary deposit of the active chemical on all surfaces of the component. This deposit is continually replenished until the VCI contained within the carrier is depleted, or the part is removed from the packaging at which stage the coating “flashes off” therefore not requiring any mechanical or chemical cleaning.

This characteristic is very compelling for manufacturers as no application equipment is needed and, unlike oils and waxes, leaves no residue that needs to be disposed of. This makes the system cost effective and environmentally friendly. VCI is also not hazardous when being handled.

There are, however, also some limitations to the system:

• VCI cannot reverse corrosion that initiated prior to packaging
• VCI has to be applied very selectively: using the incorrect grade / composition of VCI can damage certain materials – in particular certain grades of aluminum, copper, brass, gold and silver and of particular interest to suspension component manufacturers, sub components in multi-metal/ multi-material composite assemblies.

Should the incorrect VCI be selected, incompatible components may exhibit a discoloration (painted, plastic or rubber components) or oxidation in the case of non-ferrous materials such as aluminum.

However, these limitations are relatively easy to overcome:

• By controlling the manufacturing process and ensuring parts are correctly handled at all times, the risk of corrosion during production is reduced. It’s also important that unprotected parts are introduced to the VCI environment as soon as possible to further reduce the risk of corrosion.
• The supplier of the VCI should be consulted for the correct grade / type of VCI, especially if the component is made up of multi-metal/multi-materials. Initial trials should also be conducted to confirm the correct type and volume of VCI is being introduced into the system.

VCI holds several advantages over other corrosion protection systems:

• The VCI can, in certain cases, be used for packaging, thereby reducing costs, and in the case of polyethylene-VCI the material can also act as a barrier material
• When used to protect multi-material/multi-metal parts a single system can be used
• There is no special application or removal equipment required The material is not hazardous
• Once the system has been verified it is completely repeatable – changes to humidity will not affect the protection
• Depending on the type of VCI and the packaging system parts can safely be stored for up to three years (Longer if specialized barrier materials are used in the packaging)

For more than 70 years manufacturers have been using VCI and other corrosion protection systems to safeguard expensive components against corrosion, but now these could be replaced by 'smart' nanocontainers such as those developed by MUST: Not necessarily to protect against corrosion, but more likely to guard against structural damage if carbon fiber replaces steel and multi-metal systems as the material of choice.