Corrosion in gaseous environments – part II

In this second part, we describe Airecs experience of corrosion in Brazed Plate Heat Exchangers (BPHE) used for heat recovery from hot flue gases. As mentioned in the previous part, the types of corrosion found in such applications are limited. Two of the most common types of corrosion can be excluded due to the material used in Airecs BPHE’s, i.e. stainless steel brazed with either nearly pure copper or a nickel-chromium alloy named MBF-51. These two are galvanic corrosion and intergranular corrosion. Stainless steel, in Airecs case SS 316L (1.4404), owes (as most metals do) its corrosion resistance to the thin oxide layer which forms on its surface when the metal is exposed to oxygen in e.g. air. The metal has actually been alloyed to enhance the performance of this oxide layer. If this oxide layer becomes depleted or removed from the metals surface the potential difference should, according to theory, between the “naked” metal surface and the brazing material be sufficient for a galvanic cell to form. Airec has never experienced cases where this type of corrosion has caused damages to the BPHE. This is most likely a consequence of the close symbiosis between the materials (the metals tend to form an intermediate form in their boundary layer) created in the brazing process.

Airec uses a low carbon version of the stainless steel to avoid intergranular issues, so this type of corrosion has been addressed already in the heat exchanger design phase. Intergranular corrosion in such a stainless steel is highly unlikely.

Let us look at the types of corrosion which have been experienced in Airecs BPHE.

General / uniform corrosion

Description: A uniform thinning of a metal without any localised attack, i.e. corrosion at an even rate over the whole of the wetted surface. This type of corrosion does not penetrate very deep inside the metal. Most familiar example the corrosion type is the rusting of steel in the air.

The corroded bottom plate of a Cross 30.

General corrosion is always a secondary effect of a leakage in Airecs BPHE. The majority of the few cases where this type of corrosion has appeared are from leakages at or in the coolant connections caused by vibrational material fatigue. For uniform corrosion to appear in the gas channel the gas must contain acid anhydrides e.g. sulphur trioxide, SO₃, or carbon dioxide, CO₂, typically some form of exhaust.

Crevice corrosion

Description: A form of local corrosion which takes place in voids, gaps, or cavities between adjoining surfaces. An important condition for the occurrence of crevice corrosion is the formation of a differential aeration cell at the site of corrosion. This kind of corrosion is diffusion controlled, i.e. all form of transport to and from the site is via diffusion (the net movement of a substance from a region of high concentration to a region of low concentration). Stainless steel tends to create an environment inside a crevice, on its own, which is significantly more corrosive than outside the crevice, i.e. the environment inside the crevice becomes autocatalytic. Since the corrosion is diffusion controlled, the process is highly favoured by stagnant flow in the surrounding electrolyte. The geometry of the crevice is crucial for the severity of the corrosion. Depth and width of the crevice govern the severity, being the worst case a crevice with large depth and narrow gap.

Crevice corrosion requires a great deal of water/electrolyte and is therefore seldom a problem in gaseous applications. The damages caused by this type of corrosion are very similar in appearance to those caused by pitting corrosion (see next section). Airec has only seen one case where the damages clearly have been caused by crevice corrosion.

A brazing joint between two dimples. Notice that corrosion (in this case crevice corrosion) has nearly penetrated the metal on the left side of the upper dimple. Image captured with a digital optical microscope.

The exhaust heat from a biogas driven CHP was recovered with nickel brazed Compact 25 unit. BPHE’s brazed with nickel are generally regarded as having a stronger corrosion resistance than a copper brazed. It was therefore quite surprising that the customer reported the heat exchanger leaking after just a few months. One of the first observations made during the investigation of the leaking unit was that the gas channels were filled with precipitate, in fact were some of the channels still clogged after ca. half an hour of flushing with pressurised water and leakage tests underwater, which was noticed when the unit was pre-cut to fit into a wire-EDM (microscopic investigations of surfaces demands clean cutting surfaces).

Gas channels clogged with precipitate even after long time of flushing with pressurised water.

Exhausts from untreated biogases are known to contain a small amount of silica, Si, which tend to precipitate on cold surfaces, the customer has therefore installed more than one gas filter upstream from the heat exchanger. Obviously, these have malfunctioned, and the compact unit has become more of a sieve than a heat exchanger.

Small gaps between the deposit and the heat exchanging plate have provided the cavities for the corrosion attack. The large amount of precipitate has provided the stagnant flow conditions for the condensate. The severity of the attack has most likely been increased by a high sulphur, S, content in the exhaust which increases the acidity of the condensate.

Pitting Corrosion

Description: A form of localized corrosion which is very like crevice corrosion. The difference is that while crevice corrosion is caused by deposits on a metallic surface or by the existence of voids, gaps, and cavities between adjoining surfaces, pitting corrosion takes place on the “free/exposed surfaces”. The attack is confined to a small fixed area of the metal surface leading to the formation of cavities or pits, and there the bulk of the surface remains un-attacked. Pitting occurs due to a localized breakdown of the passive film (the protective layer of metal oxides which provides the corrosion resistance to most metals), hence metals, which forms passive films, are more susceptible to this form of corrosion. The presence of ions such as chloride, Cl^-, bromide, Br^-, and iodine, I^- in appreciable concentrations tends to cause pitting of steel. Thiosulfate, S₂O₃^2- (mainly), sulphate, SO₄^2-, nitric oxide, NO₃^- and perchlorate, ClO₄^-, also induce pitting on steel. The pitting process is often considered to consist of the following stages: 1) Local breakdown of passivity (pit nucleation), 2) early pit growth, 3) late (stable) pit growth and (possibly) 4) re-passivation. One or more of the pitting-causing ions, mentioned above, must be present in sufficient concentrations. Pitting corrosion is promoted if the electrolyte has a stagnant flow (the pit inducing ions will have sufficient time to create a stable pit).

Air bubbles emanating from a hole in the plate material created by pitting corrosion during a leakage test. Image captured underwater. 

Air bubbles emanating from a hole in the plate material created by pitting corrosion during a leakage test. Image captured under water.

Historically, pitting corrosion has been the most common type of corrosion in Airec heat exchangers, mainly due to instalment which promotes stagnant flow conditions for the condensate. The rule of thumb is that the heat exchanger should be installed in such manner that the gas (and thereby the condensate) flows from the top down. If the condensate is highly corrosive the condensate outlet connection should be placed at the lowest point.

Still, there has been at least one case per year the latest years where the heat exchanger leaks due to pitting corrosion. All these cases have had some form of deposit on the heat exchanging surfaces which have provided the stagnant flow condition which promotes the formation of a stable pit. Airec advice against usage of our products in halogen-rich environment, therefore the most likely pitting inducing ion is sulphate. One way to prevent this to happen is to regularly clean the gas side surfaces, either by connecting a cleaning in place, CIP, to the condensate connection and block the gas connections (Compact) or by incorporating an inspection hatchet, large enough to use a pressure washer, in the casing (Cross 30). Since the corrosion rate can be quite high when a stable pit has formed, the interval between the cleaning should be kept to a minimum.

A base of a dimple in a Cross 30 unit which has been attacked by pitting corrosion.

Stress Corrosion Cracking (SCC)

Description: SCC is defined as the delayed failure of alloys by cracking when exposed to certain environments in the presence of static tensile stress. It is a phenomenon associated with a combination of static tensile stress, environment and in some systems, a metallurgical condition which leads to component failure due to the initiation and propagation of a hairline crack. The tensile stresses may originate from the external load, centrifugal forces or temperature changes, or they may be internal stresses induced by cold working, welding or heat treatment. The cracks are mainly formed in planes normal to the tensile stresses and propagate intergranularly or transgranularly, more or less branched. If they are not detected in time, they will cause fast, unstable fracture. The environment causing stress corrosion is specific to each metal.

The stress level in the material after brazing has been investigated by Airec and found to be low (in our products), therefore SCC is usually not a problem to be considered for Airec heat exchangers.

Airec has only experienced a solitary case where the suspected corrosion type was SCC, and that failure was in a connection pipe between two Cross 30 units. Airec purchases these pipes and according to our supplier, the steel used in the pipe is a steel of higher grade than 316L. The connection pipe in question was part of a large process installation where the exterior of the pipes was exposed to the gas (the connection pipes are usually situated on the external side of the casing). Unfortunately, the operational conditions in this process vary rather much on an hourly basis. The failure has puzzled both Airec and the pipe supplier. The operational environment, at least not the one designed for, has not been reported as having a risk for SCC.

It is still unclear if it was an impure coolant or the gas composition that caused the corrosion failure.

Microscopic image of the suspected SSC. The crack has been enlarged by vibrations during the cutting of the pipe with an angle grinder.

General tips on corrosion prevention

The best general tips are to install the heat exchanger with the recommended gas flow (top-down) and to perform regular maintenance to ensure surfaces free from deposits.

There is a strong correlation between stagnant flow conditions for the condensate caused by a deposit and faulty heat exchanger due to corrosion in Airecs heat exchangers. If the condensation rate in the application is high, the heat exchanger should, in theory, be self-cleaning if the operation is continuous. Unfortunately, this is not the reality, precipitates can adhere strongly to the surfaces and there are few processes which operate 24-7. As mentioned earlier, to avoid a build-up of deposits on the surfaces CIP cleaning of the gas side is recommended for the Compact models and incorporation of a maintenance/inspection hatch into the casing for the Cross 30 in order to make it possible to clean surfaces with a pressure washer.

Of course, it would be best to avoid any possibility for precipitation or other deposit in the first place, but Airecs experience is that it is nearly impossible to predict the occurrence of these.

Material combability with the fluids

Besides acidic (pH < 3) or strongly basic (pH > 12) environments, there are three substances which should be handled with extra attention in applications with condensing gases.

Ammonia, NH₃, and related substances

Ammonia and related substances, e.g. amides, urea, etc. are NOT compatible with copper. The metal deteriorates rapidly in such environments. The preferable option for such gases is to use nickel brazed heat exchangers.

Biogases do in most cases contain a certain amount of ammonia (or ammonia related substances), Airec has seen cases where the brazing joints in a heat exchanger have been more or less depleted within few weeks of service.

Brazing joints in a Compact 36 which has operated in exhausts from a biogas combustion. The brazing joint in the gas channel (encircled in red) has nearly vanished, whereas the joint in the coolant channel (encircled in blue) is intact.

Chlorides and other halogens

As mentioned before, chloride (but also fluoride, bromide and iodine) is the most common instigator of pitting corrosion. This implies that substances that could provide these elements as a breakdown product, e.g. most refrigerants, can cause corrosion issues. It is a question of concentration (should be low) and flow conditions (high flow is better).

Sulphur and sulphur containing substances

Any gas which contains sulphur oxides should be regarded as acidic and can cause corrosion. Sulphur has a deteriorating effect on the passive oxide layer on a stainless steel. Sulphur belongs to oxide column in the periodic table, this means that it reacts in a similar fashion as oxygen. In some cases, it even reacts faster than oxygen. One such case is of iron, Fe. The sizes of the atoms (and therefore the bond length) are better suited and the resulting molecule is energetically favoured. In fact, one of the commonly found iron minerals is pyrite, commonly known as fool’s gold. Therefore, a damaged passive layer (mechanical or chemical) will often be replaced with a sulphur layer if the substance is present in the operational environment. Fortunately, this is counteracted if the stainless steel (as 316L does) contains molybdenum, Mo, which prefers to react with oxygen.

Further, sulphur is suspected to promote the formation of a stable pit and is therefore regarded as a pitting promotor. Airec recommends keeping the sulphur content in the gas to a minimum to avoid corrosion issues.