Stainless Steel and Corrosion – The Speedway Cases

Speedway : what are they?

About half of the insurance claims are related to water damages[1]. Most of these damages are due to household plumbing system failures. The cracking of flexible supply tubes, frequently called speedway, are one of the most common causes of loss.

These tubes supply cold and hot water to kitchen and washroom faucets. They are also used for toilet water supply. Sometimes they supply water to the dishwashers and refrigerators. Their use has literally exploded in the last ten years, despite a higher amount of cracking and a rise in insurance claims.

The speedway is essentially a rubbery hose covered with a metallic sheath made of braided stainless-steel jacket. Metallic tips are inserted at the extremities to allow the installation.

While the rubbery hose gives speedway flexibility, the metallic sheath is used for reinforcement. In fact, the rubbery hose cannot withstand, on its own, the normal water pressure. When the physical integrity of the metallic sheath is affected, the normal water pressure transported by the speedway causes cracking of the rubbery hose underneath.

The main cause of integrity loss in speedway’s metallic sheath is the cracking caused by a stress cracking corrosion phenomenon.

 

Stress cracking Corrosion Phenomenon

The stress cracking corrosion is a typical degradation of stainless steel. The simultaneous presence of three factors is required for this phenomenon to occur. The phenomenon will not occur if one of the factors is lacking.

  1. Material Susceptible to This Type of Deterioration

It is generally known that stainless steel owes its great resistance to corrosion to its particular chemical composition. In fact, stainless-steel contains a minimum of 12% of chrome.

Chrome participates in the creation, on the metal surface, a layer of chromium oxide which assures a protection against corrosive actions in the environment. This protective layer, also called passivation layer, is invisible to the naked eye. It forms naturally when metal is exposed to the environmental air and owns a remarkable ‟healing” property when in contact with a great number of aggressive substances.

Nevertheless, some substances have a devastating effect on the integrity of the passive film created on the surface of the stainless steel. These substances are chlorine and its compounds. So, the stainless steel is susceptible to corrosion when these compounds are present.

The alloy 304 stainless steel is recommended and often adopted by the manufacturers for the manufacturing of speedway type flexible tubes. Although the cases investigated by our engineers recently showed that 200 series stainless-steel are often used to replace Grade 304.

The grades of the 200 series are less resistant to corrosion than Grade 304, therefore the risk of stress cracking corrosion increases in the presence of a chlorine-rich environment.

  1. Corrosive Environment 

One of the most important sources of chlorine in a home comes from commonly used cleaning products. Usually chlorine is added for its disinfecting actions.

Most of the time, cleaning products are stored in cabinets underneath sinks. This causes the formation, in confined and poorly ventilated spaces, a corrosive vapour that will then condense on the existing metallic surfaces. The condensation phenomenon, by its repetitive nature, increases the concentration of corrosive products on metallic surfaces that will end up corroding.

  1. The Presence of Stress of Tension

Typically, the stress responsible for the stress cracking corrosion is a combination of several types of stress:

  • Residual stress results from the manufacturing process. Indeed, in the case of flexible supply tubes, the manufacturing process of the wires constituting the metallic sheath, called wire drawing, consisting in a sequence of plastic cold working. This process causes residual stress of tension that persists in the final product, based on the duration of use. This stress adds up to normal operating conditions and increases the stress cracking corrosion risk on the sheath.
  • Normal operating conditions stress. In the case of flexible supply lines, the water pressure, that is always present in plumbing pipes, is the normal operating conditions stress.
  • Stress induced by the installation. In the case of flexible supply lines, the bending property of the tube for easy installation in the narrow spaces, may bring to excessive deformation; this will cause additional stress.

 

Responsability and good practice

In conclusion, considering the complexity of the stress cracking corrosion phenomenon, it is often very difficult to determine the only responsibility for the risked damages during the failure of a flexible supply tube.

Actually, the responsibility is, occasionally, shared between the manufacturer, installer and user. In other case, one of the three key factors – materials, environments, and stresses – has a distinct contribution more significant than the other three, and, consequently, the responsibility may be attributed to one of the three involved parts.

So, to reduce failure risks of the flexible supply tubes with metallic sheaths and for a prolonged lifetime, we may, as a user, follow a few simple steps:

  1. Never store chlorine-based cleaning products in cabinets underneath the sinks. Please note, except for special stainless-steel products, all cleaning products contain chlorine in different proportions.
  2. Run your hand across the tube. If small wires from the sheath sting your hand, it is time to consider replacing the tube as the stress cracking corrosion has probably started.
  3. Visually examine the tubes for traces of rust. If rust is present, the stress cracking corrosion has probably started. Therefore, it is probably time to replace your tubes.
  4. Observe the tube deformation: if the tubes seem to be bent or deformed, softly straighten it and put it back in a position to avoid all pronounced distortion.

 

By Marina Banuta, P.Eng., Ph.D. – Head of Department, Mechanical, Electrical and Materials

[1] Source: Water Damage Risk and Canadian Property Insurance Pricing, research paper from the Canadian Institute of Actuaries.

 

By |2019-06-14T16:43:17+00:00April 5th, 2019|News|0 Comments