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The High Cost of Corrosion: Intelligent Inspection Well Worth the Money

Dr. Angela Wensley has attended literally hundreds of pulp and paper mill shutdowns in her 27 years as a materials and corrosion engineer. From 1977 until 1992 she was based in Vancouver with MacMillan Bloedel Research where she balanced field inspections with running a corrosion research laboratory. Since setting up her own company, Angela Wensley Engineering, in 1992 she has obtained a much more global perspective. Her work has expanded to include North America, South America, Australasia, and Scandinavia. On the job she works closely with the nondestructive testing companies whose responsibility is to detect the problems that, if left unrepaired, could lead to equipment failure. PPC found her in one of those rare moments when she was back in her office in White Rock, BC.

PPC: Why do we need to inspect our equipment?

AW: Inspection allows us to find and repair problems before they develop into more serious situations. I have seen my share of catastrophic failures, including a continuous digester failure due to caustic stress corrosion cracking and a deaerator storage tank failure due to corrosion fatigue cracking. The prior inspections in both these cases were not of the quality that we have today even though inspection methods were available that were capable of detecting the cracks before they propagated to failure. Since then, we inspect these and other pressure vessels with great diligence — perhaps too much diligence. We now know what to look for and where to look for it, yet spend as much time on more routine inspections in areas where the risk of failure is low or the consequences of failure are not serious. Time spent on less-important inspections actually detracts from the time that could be spent investigating potentially serious problems. I create inspection plans that prioritize the inspections. But don’t get caught up too much on formality — there is no substitute for a good set of eyes combined with a curious mind since sometimes the problems don’t occur where they are “supposed” to be. While cases of cracking of continuous digesters continue to occur, the problem of corrosion thinning has also become prominent over the past decade. We know where continuous digesters experience thinning and can tailor our inspections to focus on these areas. It is important to get good thickness data as very expensive decisions are based on these numbers. For example, will a mill spend a million dollars on applying stainless steel weld overlay or on installing an anodic protection system? The inspection dollars have to be well spent if the decisions resulting from them are to be well made.

PPC: The digester and deaerator failures occurred over 20 years ago and we hope we have learned from them. Can you give us a more recent example?

AW: The cracking of composite tubes at primary air port openings. In 1998, I came upon these cracks in a recovery boiler in the southern US almost by accident when we were doing some penetrant testing of tubes after some weld repairs. A week later I was inspecting a recovery boiler in Canada and found the same thing. Some of these cracks propagated through the stainless steel tube cladding and into the underlying carbon steel — a dangerous situation. A tube leak in the lower furnace is unacceptable since a smelt-water explosion can have catastrophic consequences including loss of life as well as loss of the boiler for many months until it is either repaired or replaced. Prior to the “discovery” of these cracks there was no protocol for their detection and repair or replacement and no measures had been taken to modify boiler operation to prevent the cracking. Today we understand that corrosion fatigue is the most dangerous form of composite tube cracking and that thermal cycling provides the stresses to support the growth of the cracks. Efforts to keep the corrosion environment away from the tubes by not spraying liquor onto the walls and to reduce the degree of thermal cycling by balancing air flows are showing promise in preventing cracking. It is also important to realize that the pulp and paper industry would rather change the materials of construction to suit the process rather than change the process to suit the materials of construction. In the case of cracking in recovery boilers it may be better to replace the type 304L-clad composite tubes with a material such as Alloy 825 that has greater resistance to cracking.

PPC: Is it ever possible to wait until a problem occurs before dealing with it?

AW: That’s the “leak before rupture” position. In theory, a pressure vessel with a crack growing through the wall should develop a leak before the crack has grown large enough for a catastrophic failure to occur. One problem with this is our inability to detect a leak, especially when the vessel is covered with thermal insulation. In the case of cracks in composite tubes in the hearth of a recovery boiler, even a leak is unacceptable. I am a firm believer in putting as much money as possible into proper material selection and inspections before the equipment ever goes into service. When I was with Macmillan Bloedel we purchased a portable “alloy analyzer,” a device that would tell us what metals or alloys the equipment was made from. As part of what is called “positive materials identification” (PMI) we checked every single component of a new paper machine that was being built for us and found one part in twenty was the wrong material. Not only that, the “mix-up” always involved the substitution of a less-corrosion resistant alloy for the material that was actually specified. A large oil sands company heard about our success with PMI and called me to ask about the analyzer. When I told them about the price of the instrument — $40,000 in the 1980’s — they decided they didn’t really need one. A year later they had a failure of a pipe that resulted in an entire plant being destroyed by a fire. The failure was found to be due to a material mix-up, the use of carbon steel instead of a chromium-molybdenum alloy. They purchased ten alloy analyzers to inspect their other remaining plant. That’s a classic case of never having enough money for inspection before a failure and all the money in the world after a failure.

PPC: What do you see as helping us avoid excessive inspection costs?

AW: Proper materials selection is the key. When we replace vessels because of corrosion or cracking we should think twice about replacing the equipment in kind. Just because the old vessel/tank/whatever lasted 20 years doesn’t mean that the new one will. Thanks to recycle that has increased process temperatures and the concentrations of corrosive species such as chlorides, modern pulp and paper mill environments are much more aggressive than they used to be. We push our machines to run faster, increasing the stresses. Also, thanks to the miracle of modern steelmaking, stainless steels are not as corrosion resistant as their older counterparts. Type 316L stainless steel used to typically have 2.8% molybdenum, element that confers pitting resistance in chloride-containing environments. Today, type 316L stainless steel invariably has 2.01% molybdenum, at the bottom of its allowable range. It may be better to select a duplex stainless steel instead. And this is a good a point to get into extolling the virtues of duplex stainless steels. These steels are called “duplex” because their microstructure consists of two phases — austenite and ferrite — in roughly equal proportions. Think of these phases as strengthening the steel in much the same was as a graphite fibre reinforced hockey stick. Duplex stainless steel are not only much stronger than conventional stainless steels, they have superior corrosion resistance, cracking resistance, and wear resistance. If I had my way, all new batch and continuous digesters would be constructed using duplex stainless steels, along with their ancillary vessels such as flash tanks, steaming vessels, level tanks, sand separators, and liquor storage tanks. Also, oxygen reactors and suction roll shells and much more. It has been demonstr
ated that new duplex stainless steel digesters are no more costly than new digesters built using carbon steel. Duplex stainless steels resist external stress corrosion cracking beneath wet thermal insulation, a problem that is plaguing stainless steel tanks, pipes, and vessels constructed using conventional stainless steel such as types 304L and 316L. The great thing is that equipment constructed using duplex needs far less inspection and maintenance since in most applications duplex stainless steels just don’t experience appreciable corrosion or cracking in service. Money spent up front on quality assurance inspections during manufacture will be very well spent indeed.

PPC: So are duplex stainless steels the answer to everything?

AW: Not quite. Duplex stainless steels experience embrittlement at the temperatures characteristic of recovery boilers so we won’t see a composite boiler tube with a cladding of duplex stainless steel. And there are cases where duplex stainless steels are not resistant to corrosion or cracking. Bottom scrapers in continuous digesters are a good example. Although a non-pressure part, the failure of a bottom scraper arm can cause real misery and take the digester out of commission for days. Several years ago it was not commonplace to nondestructively test the bottom scrapers. They were usually constructed from carbon steel and experienced corrosion in service. Stainless steel linings intended to protect the arms experienced cracking. Replacement in solid type 304L stainless steel was thought to be the answer but many of the solid stainless steel scrapers developed such extensive stress corrosion cracking that they had to be scrapped after only one year of service. Construction using duplex stainless steel has not succeeded in preventing stress corrosion cracking. Here is an example of where the better solution may be to revert to carbon steel while we attempt to understand why duplex stainless steels are cracking in an environment where they aren’t supposed to crack.

PPC: Where does research come to the aid of inspections?

AW: To understand the many materials and corrosion problems including bottom scraper cracking, it is necessary to spend some money on research. This is the best way to find the optimum solution to it. Following the catastrophic failure of the digester a Digester Cracking Research Committee was established to investigate the problem and to come up with solutions. I served on that committee along with my friend Andy Garner of Paprican. The cause of the failure was confirmed to be caustic stress corrosion cracking. Research found that the maximum rate of cracking was 10 mm per year. This is why we inspect digesters annually and remove all cracks that we find since a growth of 10 cm would be greater than the corrosion allowance of 6 mm that most digesters have.

Years ago when I worked for a research organization I spent most of my time as a consultant. Today I also spend most of my time in consulting, although I still have a soft spot for research. I have set up a corrosion laboratory at a local engineering firm where I continue to investigate the phenomena of corrosion in alkaline pulping liquors.

PPC: So where are we headed with inspections of our existing equipment?

AW: More intelligent inspections. There is no need for a shotgun approach to inspections when we know where the problems are, what risks they may pose, and how to look for them. Intelligent inspections never cost too much.