Smart rust killer

New material for clamp bodies delays crevice corrosion on stainless steel piping

Many attempts to find a solution for the reduction or prevention of crevice corrosion at sea or near to the coast have failed, but now a material has been developed for clamp bodies. It claims to delay the formation of crevice corrosion and lengthen maintenance intervals, particularly for on and offshore installations. It's all made possible by a corrosion inhibitor, which is dispensed into the gap between the clamp body and the stainless steel pipe.

Corrosion is a topic that still keeps countless employees in design, research and development busy, in practically all areas of industry and probably every country worldwide. Corrosion is understood to be the reaction of a metal with its surroundings, which brings about a measurable change to the material and affects the operation of a mechanical component or of an entire system. This reaction is usually of an electro-chemical nature, but sometimes it can also be of a chemical or metal-physical nature. Be it one of the now frequently common variations such as pitting or surface corrosion (generally known as 'rusting') or the contact or stress corrosion cracking which occur under certain conditions - the gradual destruction of metal surfaces can have catastrophic effects on the components in question.

Corrosion is now an everyday problem in the chemical industry, in power stations, in the oil and gas production industry or on the construction site. The same increasingly also applies to process technology, because new developments are increasing the demands placed on materials: Higher pressure ranges, higher temperatures and flow rates of the media involved, as well as more corrosive contaminants, require prompt, application-friendly corrosion protection.

One problem that occurs particularly on oil and gas drilling rigs, and other applications near to the coast or at sea, is known as crevice corrosion. All metallic materials that form oxidic protective layers can be affected by it. Even stainless steel, which is generally considered to be corrosion resistant, is not immune to this type of surface destruction, depending on quality, processing and appropriate subsequent chemical treatment.

In the case of installations at sea, a particular effect sets in as a result of adverse environmental conditions: an aggressive medium, in this case seawater with a high salt content and simultaneously high air humidity, collects in inaccessible crevices, corners and cavities, reacts with the surface of the material and becomes depleted in oxygen. The corrosion products hydrolyse, the pH value falls and anions can migrate. The oxidic protection layer can no longer be maintained, which leads to corrosion inside this crevice.

A particular point of attack for this type of corrosion is in pipe systems, as these are usually installed on the outside of the installation, and hence directly exposed to the harsh environment. In fact the pipes, usually made of SS 304 or SS 316 stainless steel (V2A and V4A or A2 and A4), are really corrosion resistant under normal circumstances, and the clamp bodies made of polyamide (PA) or polypropylene (PP) are thoroughly resistant to seawater and even more corrosive media, but the two components are by no means immune to crevice corrosion when in combination and under the circumstances described here. Corrosive seawater penetrates the crevice between the pipe and the clamp body, collects on the underside and reacts there with the surface of the material.

In a similar manner to pitting corrosion, clearly visible pits and indentations are formed in the metal, which may well actually lead to holes and cracks in the pipe after long periods of use, amplified by increased tensile stresses and inherent vibrations of the entire system, and hence to leakage. Studies carried out on offshore installations off the British coast showed that after just five years there was evidence of crevice corrosion under two out of three clamp bodies made of PA and PP. In the same period, initial cracks and holes had already formed, leading to leakage.

In the course of increasing worldwide industrialisation and growing demand for oil and gas, the first boom in the oil industry took place in Central and Northern Europe in the late seventies. Many of the oil deposits still active today were developed in this period; installations for production and processing were planned, constructed and installed at sea. In the early eighties, various attempts on installations in the North Sea and the Atlantic to reduce or prevent crevice corrosion between clamp body and pipe have been developed, trialled and applied on site.

However, as all of them have so far not been universally applicable, it would indicate that the costs are too high or simply not demonstrating the desired effect in preventing crevice corrosion. The operator companies decided to rely very largely on standard components, but to restrict the cycles of use to a predetermined period, in order to replace all the affected parts of the system in the course of a general overhaul. With the considerable extent of the pipework and the difficult maintenance conditions on site, this is tremendously time consuming and expensive. As a result of the temporary loss of the pipes while they are being replaced, there is a risk that the actual productivity of the installations will be forfeited.

Anti-corrosion material

In close cooperation with international clients in the oil and gas industry, the Stauff Group companies working in England and Scotland have developed the PP-AC material for plastic clamp bodies. It is claimed to delay the formation of crevice corrosion significantly. In the case described here, it lengthens the maintenance intervals on installations in the onshore and offshore industry, and it minimises servicing requirements and costs, delivering potential savings.

The conventional PP-based material PP-AC (PP: polypropylene, AC: anti-corrosion) delays the formation of corrosion by depositing a special corrosion inhibitor in the gap between clamp body and stainless steel pipe. This property was first successfully tested and confirmed in experiments in the salt-spray mist chamber in accordance with DIN 50021 or ASTM B117, and in in-house SO₂ test runs. All the tests showed that clamp bodies made of PP-AC represent a good alternative for protection against crevice corrosion on pipes.

The material is available as standard for sizes 1A, 2 and 3 of the Standard Series according to DIN 3015, Part 1, and for sizes 1D, 2D and 3D of the Twin Series according to DIN 3015, Part 3. The usual pipe diameters in the offshore sector, between 6 mm (1/4") and 25.4 mm (1"), are all covered. The dimensions of a PP-AC clamp body correspond with those of a normal clamp according to DIN 3015. It is therefore possible to make use of the entire range of installation components also available as standard in stainless steel grades A2 and A4 (cover plates, bolts, weld plates, rail nuts etc) for installation on weld plates or mounting rails, or multi-level installation. As the material can be machined in a variety of ways, other diameters and entirely different clamp designs not conforming to DIN 3015 are entirely conceivable, as is its use in other branches of industry.

Background information: corrosion consumes billions

As long ago as 1991, the direct and indirect costs arising as a result of corrosion were estimated at over DM 110 billion (around EUR 56 billion) for the Federal Republic of Germany alone. Amounting to nearly 5% of gross domestic product, this figure shows that corrosion damage had already assumed immense proportions at that time. Even though considerable sums have already been invested in recent decades in the development of new methods and effective materials, experts estimate that even last year an average of 4% of the GDP of a western economy were "annihilated" in the truest sense of the word, by corrosion. With an estimated GDP of EUR 2.9 trillion in the Federal Republic of Germany in 2005, this would mean a loss of around EUR 120 billion. About 25% of this could be avoided by the consistent application of solutions that have already been developed.