Plant Structure Corrosion Monitoring

Once a plant is in operation it is important to monitor the progress of any corrosion which might be taking place. The four approaches described below vary in sophistication and cost.

The most appropriate for any plant is determined by a number of factors, including the mechanisms of corrosion which are anticipated and the implications of catastrophic or unexpected failure. Key areas of the plant require closer monitoring than readily replaceable items.

The measures described below do not replace the mandatory inspections of pressure vessels, etc. for insurance purposes. The overall philosophy of corrosion monitoring is to improve the economics of the plant’s operation by allowing the use of cheaper materials and generally reducing the over-design that goes into plant to combat corrosion.

1. Physical examination

Full records of all constructional materials that are used in the plant should be maintained and updated when repairs are undertaken.

The exteriors of all parts of the plant should be subjected to frequent visual examination and the results reported and stored for future reference. This maximizes the warning time before corrosion failures occur, since the majority of failure mechanisms cause leaks before bursting.

Key items of plant, those in which some degree of corrosion is anticipated and those which might suffer catastrophic failure should be examined in greater detail. Internal visual inspection during shutdowns is sufficient to identify most corrosion effects.

Cracking can usually be seen with the naked eye but where cracking is considered to be a possible mechanism an appropriate non-destructive test method should be employed. In items of plant which are shut down only infrequently (relative to the timescale of possible corrosion or cracking failure) external non-destructive testing is often possible.

Candidate non-destructive test methods include:

a. Ultrasonic techniques:

Wall thickness can be mea- sured to monitor the progress of general corrosion, cracks can be detected and hydrogen blisters identified. Certain construction materials such as cast iron cannot be examined by ultrasound. Skilled operators and spe- cialist equipment is required. Plant can be examined in situ except when it is above 80°C.

b. Magnetic particle inspection:

Surface emergent and some sub-surface cracking can be detected in ferro- magnetic materials. The technique must be used on the side of the material in contact with the corrodent.

c. Dye penetration inspection:

This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks.

2. Exposure coupons and electrical resistance probes

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise) concerning the corrosion behaviour of the construction materials,  it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behaviour.

This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 × 50mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion.

Electrical resistance probes comprise short strands of the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. Instrumentation in the box senses the electrical resistance of the probe.

The probe’s resistance rises as its cross-sectional area is lost through corrosion. The materials should be in the appropriate form, i.e. cast/wrought/welded, heat treatment and surface condi- tion.

Metal coupons should be electrically isolated from any other metallic material in the system. They should be securely attached to prevent their being dislodged and causing damage downstream.

Simple coupons and probes cannot replicate the corrosion effects due to heat transfer but otherwise provide very useful information. It should be noted that any corrosion they have suffered represents the integrated corrosion rate over the exposure time.

Corrosion rates often diminish with time as scaling or filming takes place, thus short-term exposures can give values higher than the true corrosion rate.

3. Electrochemical corrosion monitoring

A number of corrosion-monitoring techniques, based on electrochemical principles, are available. These give an indication of the instantaneous corrosion rate, which is of use when changing process conditions create a variety of corrosion effects at different times in a plant. Some techniques monitor continuously, others take a finite time to make a measurement.

1. Polarization resistance: The current-potential behaviour of a metal, externally polarized around its corrosion potential, provides a good indication of its corrosion

The technique has the advantage of being well established and hence reliable when used within certain limitations.

This technique can only be used for certain metals, to give general corrosion rate date in electrolytes. It cannot be employed to monitor localized corrosion such as pitting, crevice corrosion or stress corrosion cracking, nor used in low-conductivity environments such as concrete, timber, soil and poor electrolytes (e.g. clean water and non-ionic solvents). Equipment is available commercially but professional advice should be sought for system design and location of probes.

2. Impedance spectroscopy: This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed

The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes which are taking place

3. Electrochemical noise: A variety of related techniques are now available to monitor localized No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analysed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking are obtained.

4. Thin-layer activation

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream.

A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected.

The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries.

Design implication for corrosion behaviour

The design of a plant has significant implications for its subsequent corrosion behaviour. Good design minimizes corrosion risks whereas bad design promotes or exacer- bates corrosion.

1. Shape

The shape of a vessel determines how well it drains (Figure 1). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive.

This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils.

Flat-bottomed storage tanks tend to suffer pitting corrosion beneath deposits or sediments which settle out. Storage tanks may be emptied infrequently and may not experience sufficient agitation or flow to remove such deposits.

Flange face areas experience stagnant conditions. Additionally, some gasket materials, such as asbestos fibre, contain leachable chloride ions.

This creates crevice and stress corrosion cracking problems on sealing surfaces. Where necessary, flange faces which are at risk can be overlaid with nickel-based alloys. Alternatively, compressed asbestos fibre gaskets shrouded in PTFE may be used.

Graphite gaskets can cause crevice corrosion of stainless steel flanges. Bends and tee-pieces in pipework often create locally turbulent flow. This enhances the corrosivity of the process liquid. These effects should be minimized by the use of flow straighteners, swept tees and gentle bends.

Flow- induced corrosion downstream of control valves, orifice plates, etc. is sometimes so serious that pipework requires lining with resistant material for some twelve pipe diameters beyond the valve.

Fig.1. Details of design creating corrosion problems

2. Stress

The presence of tensile stress in a metal surface ren- ders that surface more susceptible to many kinds of corrosion than the same material in a non-stressed condition.

Similarly, the presence of compressive stress in the surface layer can be beneficial for corrosion, and especially stress corrosion cracking, behaviour.

Tensile stresses can be residual, from a forming or welding operation, or operational from heating-cooling, filling-emptying or pressurizing-depressurizing cycles. The presence of a tensile stress from whatever origin places some materials at risk from stress corrosion cracking.

Some items of plant can be stress-relieved by suitable heat treatment, but this cannot prevent operational stress arising. Cyclic stresses can also give rise to fatigue or corrosion fatigue problems.

Information relating to the fatigue life of the material in the service environment is required, together with the anticipated number of stress cycles to be experienced by the item over its operational life. The fatigue life (the number of cycles to failure) or the fatigue strength (the stress level below which it does not exhibit fatigue problems) is then used in the design.

The presence of stress raisers, including sharp comers and imperfect welds, produces locally high stress levels. These should be avoided where possible or taken into account when designing the materials for use in environments in which they are susceptible to stress corrosion cracking or corrosion fatigue.

3. Fabrication techniques

Most fabricational techniques have implications for corrosion performance. Riveted and folded seam construction creates crevices as shown in Figure 2.

Those materials which are susceptible to crevice corrosion should be fabricated using alternative techniques (e.g. welding). Care should be taken to avoid lack of penetration or lack of fusion, since these are sites for crevice corrosion to initiate.

Welding should be continuous, employing fillets where possible, since tack welds create locally high stresses and leave crevice sites. Welding consumables should be chosen to create weld metals of similar corrosion resistance to the parent material.

This often requires the use of a slightly over-alloyed consumable, to allow for loss of volatile alloying elements during the welding process and to compensate for the inherently poorer corrosion resistance of the weld metal structure.

Strongly over-alloyed weld consumables can create galvanic corrosion problems if the weld metal is significantly more noble than the parent material.

In all welds the heat-affected zone is at risk. The new structure which forms as a consequence of the thermal cycle can be of lower corrosion resistance, in addition to the often poorer mechanical properties, than parent material.

Austenitic steels such as type 304 and 316 are also susceptible to sensitization effects in the heat-affected zone. In these materials carbide precipitation during the welding thermal cycle denudes the parent material of chromium.

This creates areas of significantly diminished corrosion resistance, resulting in knife-line attack in many corrosive environments. This is avoided by the use of the low-carbon equivalents (304L, 316L, etc.) or grades such as type 321 or 347 which are stabilized against sensitization.

With correct welding techniques, however, this should be necessary only with thick sections (5 mm for 304 and 8 mm for 316). Some materials, particularly certain aluminium alloys, duplex stainless steels in certain reducing environments and most steel plate, are susceptible to end-grain attack.

Penetration along the end grain can be very rapid, with corrosion exploiting the potential differences that exist between inclusions and ferrite crystals in steel and between austenitic and ferrite grains in duplex stainless steel.

Where end-grain attack is significant this should not be exposed to the corrosive environment. It can be covered by a fillet ‘buttering’ weld if necessary.

Fig.2. Details of jointing processes creating additional corrosion risks (crevices and stress concentrations)

4. Design for inspection

Unseen corrosion can be the most damaging type of attack. Items should be designed to permit periodic inspection.

This involves the provision of sufficiently large manways, the installation of inspection pits, the placing of fiat-bottomed vessels on beams instead of directly onto concrete bases and the facility for removal of thermal insulation from vessel walls.