Schaffhausen, December 2009
Plastics don`t rust
Electrochemical corrosion – the most frequent type of corrosion in metals – is practically unknown in plastics. Nevertheless, changes in the molecular structure of plastics can occur if aggressive media are used or if there is mechanical stress. This degradation can be prevented by means of stabilizers and the right material selection.
The term corrosion stems from the Latin “corrodere”, which means “to gnaw away at”. The first scientific definition of corrosion was given by the alchemist and universal scholar Andreas Libavius (1555–1616) in his “Alchemia” of 1597: “‘Corrosion’ is a calcination with ‘corroding medicines’, the acridity (acrimonia) of which penetrates and dissolves the structure into the smallest of parts (secundum minima).” This definition obviously refers to metals, since plastics had not even been invented then. The DIN 50 900 definition for the Corrosion of Metals, Part 1 (1982) also pertains to
metals: “1.1 Corrosion: the reaction of a metallic material to its environment, which effects measurable change in the material and can lead to impairment of the function of a metal component or of an entire system. In most cases, this reaction is of electrochemical nature; in some instances it can however also be of chemical (not electrochemical) or metal physical nature.” But once again, we must reiterate that this is a metal norm. In the meantime, the term corrosion has become established for plastics as well. Most of the corrosion types which occur in metals are also found in plastics. A significant difference is, however, that electrochemical corrosion, which is
the most frequent type of corrosion in metals, the result of good conductivity, is practically unknown in plastics. In acids and alkalis, as well as in saline solutions or saline environments, where metal pipes rust through (corrode) very quickly, plastic pipes have a nearly unlimited lifetime.
Why do plastics corrode?
Plastics are composed of long molecular chains (macromolecules). In order to improve their characteristics (stability, fire retardation, colour, etc.), additives are mixed in. Weak cross-linking of macromolecules produces elastomers (seals, diaphragms), strong cross-linking duroplastics. When plastics are damaged, the bonds in the macromolecular chains are broken or the macromolecular chains are separated from one another. Additives can slow down these processes (stabilizers) or accelerate them. Splitting macromolecules or separating macromolecular chains depends on the structure and the atoms involved in the bonding. There are atom bonds which are easy to fission and others which are more difficult to fission.
Chemicals can cause such fissures. The temperature plays an important role, as does the pressure. Rule of thumb: An increase in temperature of 10°C doubles the medium attack on a material. Other factors have an effect on plastics and materials in general:
Mechanical stress
- static
- dynamic
- cyclical
- impact
Environment (air, humidity, water)
- chemical media (oil, tensides, etc.)
- radiation (light, UV, etc.)
- microbiology
All these factors can cause plastics to fail. Some, such as mechanical stress, are calculable and some, such as temperature or environmental load, are controllable through materials selection. Yet, failures will happen because the component was not designed specifically for plastics or because the loads were not foreseeable. Moreover, plastics are not gas-tight. Many chemicals permeate into the plastic, remain there or pass through it. Aggressive media that are highly diffusive and have surface-active properties, are particularly dangerous.
Cause of damage in plastics
Non-cross-linked plastics (thermoplastics) are swollen and dissolved, while the cross-linked plastics (elastomers and duroplastics) only swell. If the attacking substance and the plastic are very similar in molecular structure, the swelling and/ or dissolving is promoted. Besides swelling, there are other means of destruction. For example, concentrated nitric acid attacks and destroys the molecular chains, the plastic becomes brittle. Stabilizers slow this process down. Liquids that have no significant
effect on unstressed plastics can cause fissures when the plastic is under stress.
Often media that are stress crack inducing will come into contact with the plastic
components, weakening the bonding of the macromolecules and thus facilitating
local relaxation of the material via micro fissures. This phenomenon is also known
as crazing and is one of the most frequent causes of damage. The interaction of
tension, medium and material leads to embrittlement of the material.
Material recommendations
A piping systems does not only consist of pipes, but also jointing elements, seals, valves and measurement instruments. Besides pressure and temperature, the media transported in a piping system determines which material is suitable. For many applications, especially standard ones, the decision is usually an easy one. More difficult, however, is choosing the right material for the more “exotic” applications or in borderline situations, in which case the behaviour of a system might have to be estimated. If there is any uncertainty, the job may have to be declined or a limited lifetime indicated. The more information is available, the easier it is to make a recommendation. The following parameters and variables are relevant: pressure, temperature, exact description of the medium, ambient conditions, diameter of the pipeline, etc. A useful tool is also the chemical resistance list, although it only gives an indication of the possible resistance of the material and should not be viewed
as absolute, especially for applications in the borderline range. Resistance lists frequently pertain to pure media or were created in laboratories under ideal conditions. Experience and results from field tests or studies flow continually into the media lists of GF Piping Systems. Reduction factors, susceptibility for stress cracking and jointing technology information are other aspects of the resistance list. If there is any uncertainty, such as in untypical applications, we recommend contacting the specialists at GF Piping Systems.
Plastics don't rust
Rust is the corrosion product that results from the reaction of iron or steel with oxygen in the presence of water. The good conductivity of metals plays an important role in this process. Plastics are nonconducting, in other words they are insulating and do not conduct any electricity (or heat). Nonconducting, however, also means that no galvanic elements can build up. Plastics have a major advantage, particularly in contact with water or chemicals diluted with water. Water, saline solutions, acids and alkalis have nothing on plastics. In water treatment plants, in electroplating systems, for pickling in the steel industry, in waste water treatment, in storage tanks, etc., plastics can be used for decades without the slightest sign of corrosion.
Author:
Hans Jörg Sommer, Head of Chemical/Mechanical Advices and Claims, GF Piping Systems, Schaffhausen, Switzerland
Georg Fischer – Adding Quality to People`s Lives
GF Piping Systems is one of the three core businesses of the Georg Fischer Corporation and a leading supplier of piping systems in plastic and metal with global market presence. Connecting technology, fittings, valves, measuring devices and pipes are used for water conveyance and treatment as well as the transport of liquids and gases for industrial purposes. GF Piping Systems provides innovative, engineered solutions for the segments building technology, chemical process industry, food & beverage, microelectronics, ship building, water and gas utilities and water treatment. Sales companies in more than 25 countries and representatives in another 80 countries ensure customer support 24 hours a day. Production sites in Europe, Asia and the US are near the customers and meet local requirements. The Georg Fischer headquarters is based since its foundation in 1802 in Schaffhausen, Switzerland.
Key figures GF Piping Systems 2008
More than 4200 employees worldwide
1066 MCHF sales
80 MCHF EBIT
Further information is available at www.piping.georgfischer.com