Midland Corrosion Services Ltd.

Corrosion of Copper Alloys

Copper alloys such as brasses (Cu/Zn alloys), bronzes (Cu/Sn alloys) and Copper Nickel alloys are widely used in industrial and domestic applications.

Brasses are generally immune to general corrosion but can suffer from stress corrosion cracking in ammonia containing water. Brasses containing 20-40% zinc are highly susceptible to SCC. Alloys containing more than 15% zinc may also undergo dezincification in which the zinc rich β phase is dissolved leaving a weak, porous structure behind. The dezincification can be of the plug or layer type. Dezincification resistance can be improved by increasing the copper content or adding small amounts of arsenic, antimony or phosphorous. Specifying a DZR brass or gun-metal (a copper-tin-zinc-lead alloy) will mean that the risk of dezincification is exceedingly low.

Tin bronzes, alloys of tin and copper with possibly minor lead, zinc and nickel additions, are used primarily in the cast form for pumps valves, gears etc. They tend to have limited corrosion resistance in seawater. Phosphor bronzes are much less susceptible to SCC than brasses and have good corrosion resistance in seawater, although their resistance to hydrochloric acid is limited. Aluminium bronzes containing 5-12% Al have excellent resistance to impingement attack (erosion corrosion) and high temperature oxidation. Al bronzes can suffer from SCC in moist ammonia.

Copper nickel alloys offer perhaps the best general resistance to aqueous corrosion of all the copper alloys. They have excellent resistance to corrosion in acids, sea water corrosion, SCC and impingement attack. They have been used widely in condenser and heat exchanger tubes and as pipes to transport seawater in boats.

Tip 20 - Corrosion of Aluminium I

Aluminium is a very active metal, which is why it takes a lot of energy to reduce it from its ore. However, like stainless steel, it is protected against corrosion by a surface passive film, in this case aluminium oxide. And like stainless steel, it can suffer from pitting corrosion in chloride-containing water.

Whereas most metals, however, have higher corrosion resistance in alkaline solutions, this is not the case for aluminium. The Al2O3 surface oxide is removed in both acidic and alkaline solutions and therefore aluminium should not be used in waters more acidic than pH 5 and more alkaline than pH 8.5.

The corrosion protection of aluminium can be increased by anodising and this is typically used in aluminium window frames. Either a hard or a soft anodised layer can be applied in an anodising bath. Decorative colours may also be applied by anodising.

There are many aluminium alloys in use, which have enhanced mechanical properties over pure aluminium. Typical alloying additions are magnesium, silicon and copper. Because of the relatively high strength:weight ratio, these alloys are often used in aerospace applications. However, the corrosion resistance is generally poorer than for commercially pure aluminium.

Tip 19 - Coatings – Introduction

Surface coatings on metals fulfil a number of functions but are usually added for either decorative, wear resistance or corrosion resistance purposes or a combination of all 3. Coatings for corrosion protection can be inorganic (e.g. phosphate on steel); organic (paints, bitumen) or metallic (e.g. chrome, tin, galvanising). Coatings provide a barrier layer between the metal and the environment, usually water. However, the coating may also provide active corrosion protection due to other processes such as galvanic protection and the use of corrosion inhibitors.

Paints consist of a dispersion of pigments and other additives in a binding medium. This binding medium consists of an organic resin dissolved or dispersed in a solvent. Most modern paints also contain several additives to modify the consistency, drying times or provide additional corrosion prevention. What is generally termed emulsion paint is a dispersion of a solid pigment in a liquid resin and water.

Chrome provides a hard shiny corrosion resistant coating on numerous everyday objects such as taps and bicycle frames and wheels. The metallic object is electroplated usually first with nickel and then chromium. Your baked bean can is made of steel plate, which has a thin surface layer of tin applied to provide an inexpensive corrosion protection without contaminating the food.

Galvanising involves coating a steel or iron substrate with zinc. It can be applied by hot dipping the part in a molten zinc bath or by electroplating. The advantage of galvanising over some other coatings is that if the underlying steel is exposed to the environment, it is still protected against corrosion since zinc is more electrochemically active than steel. This is termed galvanic protection.

Whatever coating method is used, good surface preparation is vital to provide long-term protection. The Institute of Corrosion provides the ICATS scheme for training painters in proper surface preparation.

Tip 18 - Preventing Corrosion by Effective Design

It is probably true that corrosion is not top of the agenda when engineers design anything. They are usually much more concerned with mechanical properties, weight, cost and availability. However, by neglecting to consider corrosion, many structures or systems will either, not function as designed, fail completely or look aesthetically bad within a few years of being installed. A lot of the problems due to corrosion can be prevented or minimised by some simple steps. These are:

• Consider the corrosion resistance of a metal or alloy in the particular environment before specifying (This includes resistance to general corrosion and all forms of localised corrosion). Mild steel, which has very limited general corrosion resistance is often used because of its low cost and ready availability in many forms, will nearly always have to be protected by a suitable coating.
• Preventing bimetallic couples between dissimilar metals especially where a large cathode (more noble metal) is coupled to a small anode (more active metal). Where dissimilar metals need to be connected, consider using an insulating gasket or washer between them.
• In water systems, design to flow rates so that they are not excessive and may cause erosion corrosion or impingement attack. However, do not introduce dead-legs where water can stagnate.
• Prevent narrow crevices and joining techniques, which introduce high stresses, wherever possible. Consider post weld annealing to eliminate residual stresses caused by welding.
• Prevent or eliminate poor surface conditions, which can lead to the onset of localised corrosion. E.g. dark oxide films, weld spatter and deposits on stainless steels should be removed after welding.
• Lag cold water pipework (in addition to hot water pipes) to prevent condensation on the outer surfaces. Also consider situations which favour wicking of water to form a continuous electrolyte.
• Design for ease of access for inspection and regular maintenance. Inspections and maintenance are often neglected when assess in extremely restricted.

Tip 17 - Erosion Corrosion of Copper

Erosion corrosion of copper pipes is not uncommon in domestic (potable) water systems. It is not found, however, in central heating systems or closed recirculating cooling systems due to the low levels of dissolved oxygen in the water in these systems. It occurs due to turbulent flow in pipes, which releases air bubbles from the water and which remove the protective corrosion product layers from the copper surface. Corrosion, which then occurs in these localised regions, takes the form of grooves or horseshoes in the surface (with the ‘horse’ appearing to have walked upstream). In severe cases, wall perforation can develop within a matter of weeks.

Erosion corrosion is most often found just downstream of elbows or changes in pipe section, where turbulent flow is more likely. However, there is a critical flow velocity below which erosion corrosion should not occur. This is often quoted as below 3m/s for cold water pipes and 2m/s for hot water pipes. However, in soft water or base-exchange softened water, erosion corrosion is possible at flow velocities below these values. Other factors, such as burrs left on cut end s of copper pipes and sharp changes in sections increase the likelihood of erosion corrosion. To prevent the risk of erosion corrosion, flow rates of <2m/s for cold water and <1m/s for hot water systems should be met.

In summary, to prevent erosion corrosion:

• Design the system with pipe sizes and pump settings to reduce flow rates as low as possible whilst still meeting the demands of hot and cold water supply.
• Try to avoid sharp changes in section or overly complicated pipe sections with numerous elbows and tees.
• If using base-exchange softened water, do not over soften. If necessary blend softened water with the mains supply to retain at least 60 mg/L of total hardness (CaCO3).
• Remove all burrs and solder deposits from around joints.

Tip 16- Corrosion of Copper I

Copper pipes have been used to transport water since Roman times. This is due to copper’s availability, formability and inherent corrosion resistance. Compared to other construction materials, such as iron and aluminium, copper is quite inert with a much more noble corrosion potential. It is thermodynamically impossible that hydrogen ion reduction can take place on the surface. Therefore, corrosion can only take place if oxygen is present in water.

Corrosion in practice is reduced by the formation of protective corrosion product layers. These layers consisting of cuprous oxide overlaid with thicker layers of basic copper carbonate/sulphate and calcium carbonate can be clearly seen with the naked eye. They are therefore not truly passive layers like those formed on stainless steels or aluminium. Nevertheless, corrosion rates and leaching of copper ions into water fall substantially during the first few weeks of immersion as these layers grow. It is only when these layers either do not form properly or become disrupted during service that problems of corrosion with copper can occur.

General corrosion of copper can occur in acidic waters with pH <7 due to the non-formation of these protective scales. It is generally only a problem in some very soft surface waters with high levels of peat acids. More common in the UK, however, is localised corrosion, which may lead to pinholing of copper pipes. This occurs usually when there is combination of adverse circumstances related to design, installation, commissioning, water quality, pipe quality and operational parameters.

Localised corrosion can be due to either erosion corrosion or several forms of pitting corrosion. These will be considered in the next corrosion tips.

Tip 15 - Corrosion of Austenitic Stainless Steels in Water

The resistance of stainless steels to pitting is given by their pitting resistance equivalent numbers (PREN). This is a theoretical way of comparing the pitting resistance of various types of stainless steels based on their chemical compositions. Basically, the higher the PREN, the higher is the resistance to pitting. PREN’s range from 10 for some of the ferritic grades to around 20 for 304 and 25 for 316 to over 40 for some of the super duplex grades, e.g. 1.4410 (SAF 2507) and 1.4501 (Zeron 100).

Types 304 and 316 are most commonly used for potable and waste water supply. There is a risk of pitting or crevice corrosion depending on the chloride content and SCC may occur in hot water where there are tensile stresses, either residual due to manufacturing processes or applied because of design and operational factors.

The UK Drinking Water Inspectorate’s Code of Practice for selection of stainless steels at ambient temperatures is reproduced below. Nevertheless, duplex stainless grades and even 316 have been widely used in seawater, although they are susceptible to pitting and especially crevice corrosion.

For hot water, upper chloride limits of 50mg/l for 304L and 250mg/l for 316 may be appropriate. In narrow crevices, localised corrosion may occur at lower chloride levels

In marine environments, in particular, concentration of chlorides in the splash zone at any water/ air boundary occurs and this region is even more corrosive than areas of total immersion.

It is important to understand the environmental conditions properly, in order to select the correct grade of stainless steel. If too low quality stainless steel is selected, then there will be risks of pitting and crevice corrosion. On the other hand, being over cautious and selecting a higher quality stainless steel than necessary, e.g. super duplex when 316 will do, will lead to greatly increased costs. ‘And don’t forget, in most marine applications, carbon steel used with coatings and cathodic protection usually gives satisfactory life.

Tip 14 - Corrosion of Austenitic Stainless Steels I

In the next few weeks, we are going to consider the corrosion of specific metals and alloys, starting with stainless steels.

It is sometimes thought that stainless steels don't corrode but, while they are generally immune from general corrosion, they can suffer from a whole range of localised forms of corrosion in different environments.

There are four basic classes of stainless steels: ferritic, austenitic, duplex (ferritic/austenitic) and martensitic. All of these contain chromium to enhance corrosion resistance due to the formation of a microscopically thin layer of chromium oxide, which forms on the metal surface.

The cheaper ferritic grades contain only chromium (>11%) as a major alloying addition, which means they have a ferritic structure and are therefore magnetic. They have good formability but limited corrosion resistance. However, they have been used successfully for car exhaust systems.

Austenitic grades also contain nickel, typically 8-10%, which gives them superior corrosion resistance in a wider range of environments and produces a non-magnetic austenite structure. They also have superior ductility compared to the ferritic grades. This class of stainless steels is by far the most commonly used.

Martensitic grades are used where superior strength or hardness is required. This is achieved by increasing the carbon content and heat treatment. The higher chromium grades (e.g. 431 or 1.4057) can also offer superior corrosion resistance to ferritic grades.

Duplex stainless steels contain lower nickel levels than pure austenitic grades and have a structure composed of both austenite and ferrite. This gives the steels a good combination of mechanical properties and corrosion resistance. The super-duplex grades offer the best resistance of all the stainless grades to pitting and crevice corrosion in high chloride environments, e.g. seawater.

Next time, we shall consider the corrosion of austenitic stainless steel in more detail.

Tip 13 - Corrosion of Stainless Steels

As mentioned in tip 13, because of a microscopically thin passive chromium surface layer, stainless steels do not suffer from significant rates of uniform or general corrosion, except in very strong acids. Due to disruption of this passive film, austenitic grades may undergo several types of localised corrosion. These are:

• Pitting Corrosion
• Crevice Corrosion
• Bimetallic (galvanic) Corrosion
• Stress Corrosion Cracking
• Intergranular attack and Weld Decay

Pitting corrosion of austenitic stainless steels can occur in chloride containing waters. Special grades e.g. 316 containing molybdenum have been produced which are more resistant to pitting attack (see next tip).

Stress corrosion cracks in stainless steels may in certain environments and temperatures (e.g. natural waters below 100 deg. C) only initiate from the base of pits. Therefore prevention of pitting in these environments will also prevent SCC occurring. However, in other environments or at higher temperatures SCC may occur without the need for pitting.

Intergranular corrosion of stainless steels can occur if sensitisation due to prolonged heat treatment or welding occurs. This occurs due to precipitation of chromium carbides at the grain boundaries and can be prevented by switching to low carbon or titanium stabilised grades.

Welding of stainless steels may also produce a heat tint, which lowers the corrosion resistance. Therefore, welding of stainless steels is usually carried out using inert argon gas shielding. If a heat tint deeper in colour than pale straw is produced, it should be removed by mechanical grinding or acid pickling followed by passivation.

Finally, the corrosion resistance of stainless steels may be impaired by discolouration, deposits and staining during use. The corrosion properties can be maintained by a proper maintenance and cleaning programme.

Tip 12 - Stress Corrosion Cracking

Stress Corrosion Cracking (SCC) sometimes known as Environmental Assisted Cracking is a particularly dangerous form of corrosion. This is because it can lead to catastrophic failures of plant and equipment, often under high pressures, without any warning. It occurs with alloys, exposed to certain specific environments, when under tensile stresses. These stresses can be either residual, i.e. from manufacturing processes (such as welding or machining) or applied, i.e. operational stresses.

Austenitic stainless steels can undergo SCC in chloride-containing environments at temperatures usually above 60ºC. Alpha-beta brasses undergo SCC in ammonia containing environments. Aluminium alloys also can undergo SCC in chloride-containing environments. These are the most well known examples of alloy/environments giving rise to SCC but in practice SCC can occur in a much wider range of alloy/environment combinations.

Cracks can initiate from a flat surface but often initiation occurs at the base of pits or from notches or deep scratches. Crack propagation then proceeds either trans-granularly or inter-granularly through the metal, or occasionally with a mixture of both modes. Because little metal is lost, it is often difficult to pick up SCC before cracks have grown several mm. Crack detection techniques include ultrasonic testing, the use of dye penetrants and, more recently electrochemical noise techniques.

SCC can often be mistaken for hydrogen embrittlement. The latter also affects many alloys, including high strength carbon steels and can lead as well to catastrophic failure. In contrast to SCC, hydrogen embrittlement can occur in demineralised water and the time to failure decreases with increasing cathodic polarisation. With SCC, on the other hand, because anodic dissolution of the crack tip also occurs, failure times generally increase with cathodic polarisation.