PVDF and stainless steel couplings compared

Stainless steel is often the automatic choice in industries where chemical resistance and reliability are critical. But for a large number of stainless steel applications, PVDF is also an option worth considering. For example, in the food industry, chemical industry, semiconductor industry and medical industry. PVDF often offers the same or better performance - at lower costs (up to about ¼ of the stainless steel price). Delivery times are often slightly better and it has a favourable assembly time.

Pressure and temperature

Of course, the application remains the leading factor when answering the question of which material is the most suitable. It is important to understand the properties of the materials. Firstly, what is the pressure in the system? It remains a fact that plastics and therefore PVDF can withstand less pressure than stainless steel. But in practice it appears that the maximum pressure of PVDF is underestimated and in most applications we handle is more than sufficient with a safe margin. As an indication: the maximum pressure of a PVDF coupling is often around 10 Bar. Then of course the hoses and other components in the system also need to be considered.

Another important consideration is the temperature of the medium. Not everyone realises that PVDF can withstand temperatures of -40°C to 100°C to 10 bar with a threefold degree of certainty. To give an indication: 80% of all applications fall within these criteria.

Photo: This specification list compares the chemical resistance of a number of materials in which components are available.

Chemical resistance

When it comes to the chemical properties of the medium, PVDF has a reputation for being very chemically resistant. This is true, but there are also limitations. The illustration shows that there is no direct winner between stainless steel and PVDF. For fluorine (F2) stainless steel is better, but for hydrofluoric acid (HF) PVDF is better. Reality is therefore nuanced and chemical compatibility must be considered for each situation.

Pharmaceutical applications and food industry

Stainless steel is potentially subject to unwanted chemical reactions. The places where this happens can be breeding grounds for bacteria. This sometimes occurs in the pharmaceutical and food industries where companies must demonstrate to the FDA that they have a 100% cleaning process.

Ultrapure water (UPW) applications

In ultrapure water (UPW) applications, PVDF provides superior performance. Stainless steel often cannot be used in such applications because the ionized water draws ions from the steel, forming iron oxide, hydroxide or carbonate. Plastic is immune to this type of corrosion.

Cooling systems

Stainless steel sometimes causes problems with galvanic corrosion in cooling systems, especially in combination with aluminium. And because aluminium has excellent heat transfer properties it is often used in cooling systems. For example, in the cooling of electronics. An alternative can be plastic, but if mechanically higher demands are made by, for example, vibrations in mobile cooling systems, then aluminium fittings can also be an option. These are then made of a special type of aluminium.

Mechanical vibrations

As soon as mechanical stress, such as vibrations, is involved, stainless steel is quickly preferred. If mechanical stress is expected it is always a good idea to have our mechanical engineers take a look. But as said: the application determines which material is preferred.

Production and assembly aspects of PVDF

For an application that is compatible with both PVDF and stainless steel from all angles, PVDF can offer considerable savings. On cost price, delivery time and assembly time. If other components in the system are also made in PVDF, a few additional benefits apply. Manufactured parts are available more quickly because it is easier to form and manufacture in various sizes and shapes. All with less effort and cost than when working with stainless steel.

Why PVDF?

• Chemical resistance (in many cases)
• Electrical isolation: no galvanic corrosion
• Shorter delivery times
• Low price

Why stainless steel?

• Mechanically superior
• Chemical resistance (in some cases)
• High pressure
• High temperature
• Glossy appearance (for espresso machines for example)