Titanium Chemical Cleaning: The Annual Wash Is the Harshest Service of the Year

Titanium Chemical Cleaning: The Annual Wash Is the Harshest Service of the Year

A titanium heat exchanger is normally bought against a process stream it will see for 8,760 hours a year, and then once a year somebody circulates a cleaning fluid through it that never appeared on the corrosion review. That two to eight hour wash is frequently the most aggressive chemistry the metal ever meets: concentrated, heated, reducing, inhibited with a package designed for carbon steel, and left standing in dead legs that the circulation map forgot. Published titanium corrosion data put unalloyed titanium above 100 mpy (2.54 mm per year) in boiling 0.7 per cent sulphamic acid, while the same acid is routinely marketed as a gentle descaler for heat transfer surfaces. The gap between those two statements is where equipment is lost. This guide covers the layer between material selection and the next start-up: what scale actually does to titanium, why a factory pickling recipe cannot be pasted into a shutdown work order, which cleaning agents published sources disagree about, how to write a recipe for a mixed-metal circuit, and what evidence proves the wash did more good than harm. Our titanium agitator maintenance guide gives one sensible line in its FAQ: dilute nitric acid at no more than 5 per cent for scale, then a clean-water rinse, with hydrofluoric and hydrochloric acid prohibited. Our five real tube bundle failure cases explains why bundles fail, and our brine lithium extraction exchanger page already insists that the wash recipe belongs on the RFQ. None of them is a cleaning procedure. This article starts where those three stop, and our HCl corrosion limits by grade guide and pickling and free-iron guide remain the references for the process medium and for the factory surface route respectively.

At a Glance: Ten Decisions That Settle a Titanium Cleaning Order

Read rows two, four, six and ten first. The recurring failure is not a badly cleaned exchanger; it is a shutdown work order copied from a steel plant, applied to a machine whose passive film is a few nanometres thick and whose corrosion resistance depends on oxidising conditions that the cleaning fluid may not provide. Every number in this table has a governing document or a published dataset behind it, and several of them contradict each other. Where sources conflict we say so, name the conflict, and give the verification action rather than printing one false-precision value. Treat the table as a list of decisions the plant owner and the cleaning contractor must settle before a supplier can be asked to quote replacement parts. A mill can certify a grade, a wall thickness and a surface state; it cannot decide what an installed circuit will be washed with next year. That division of responsibility is the difference between a defensible cleaning record and a signature on a job card.

The "It Survived the Process, So It Will Survive the Wash" Trap

Titanium earns its specification in oxidising or neutral chloride service, where a self-healing TiO2 film rebuilds in air and in oxygenated water within moments of being damaged. A cleaning bath is often the exact opposite: a hot, concentrated, poorly aerated reducing solution whose job is to dissolve inorganic solids. The alloy was chosen because it resists the process; nothing about that choice qualifies it for the cleaning agent. Two further differences make the wash worse than the duty. First, temperature: CIP and acid circulation commonly run at 50 to 80 C, while the process stream may be ambient, and published data place the crevice corrosion threshold of titanium in chloride deposits dropping sharply above roughly 70 C. Second, stagnation: a cleaning loop has dead legs, blind bores, tube-to-tubesheet gaps and gasket crevices where fresh inhibitor does not reach and spent acid sits for hours after the pump stops. Our process equipment specification guide covers how to order the vessel; it does not and should not specify the maintenance chemistry. That belongs in a separate plant document.

Why the steel-plant recipe is the default, and why that is the problem

Most industrial chemical cleaning practice was written for boilers, carbon steel piping and copper-alloy condensers. Hydrochloric acid is the cheap, fast carbonate dissolver for steel systems, and a family of broad-spectrum inhibitors was developed around it. One Chinese cleaning reference states plainly that a widely used broad-spectrum inhibitor in that family is unsuitable for titanium and can in some formulations increase corrosion rather than reduce it. Another reference notes that hydrochloric or dilute sulphuric acid can be made to work on titanium by adding ferric or cupric salts as oxidising inhibitors, but warns that iron or copper then plates onto the titanium surface, which is precisely the free-iron contamination that our free-iron pickling guide exists to remove. So the workaround for one failure mode creates another. Our grade-by-grade acid limits guide sets out where Gr2, Gr7 and Gr12 sit in reducing acids; those windows are process concentration and temperature windows, not cleaning-bath permissions. A buyer who reads a 5 per cent HCl limit as permission to circulate inhibited HCl at 70 C for six hours has moved outside every assumption behind the number.

The four-way disagreement on hydrochloric acid

It is worth being explicit about how unsettled this is. One titanium cleaning reference lists HCl among the acceptable agents for carbonate scale and iron scale. Several reactor-maintenance references ban concentrated HCl, phosphoric acid and HF outright and recommend nitric or citric acid instead. A third group allows HCl or sulphuric acid with oxidising inhibitors but flags metal deposition and subsequent localised attack. A fourth group, describing field practice, reports 10 to 15 per cent industrial HCl being used on titanium tube bundles and calls it a dangerous mistake. Our own brine exchanger page adds a fifth nuance: Gr7 and Gr12 tolerate reducing HCl and H2SO4 washes inside defined concentration and temperature windows, while Gr2 does not, and the wash recipe must be on the RFQ. These statements are not all reconcilable, because they describe different grades, concentrations, temperatures, inhibitor packages and exposure times. We will not average them into a single number. The executable rule is the one the Chinese cleaning reference actually gives: analyse the deposit, run a dissolution trial on several candidate agents, then run a corrosion and hydrogen-uptake trial on coupons of the actual grade with and without the proposed inhibitor, and only then fix the recipe. Our C276 versus C22 guide uses the same discipline for a family where the filler metal and the redox behaviour cannot be swapped casually; cleaning chemistry deserves no less.

Practical definition: a titanium-safe cleaner is not a product name. It is a specific formulation at a specific concentration, temperature, circulation velocity and contact time, on a specific deposit, qualified by a coupon test on the specific alloy, with a documented hydrogen increment and a defined neutralisation and rinse endpoint. Anything shorter is a guess with a pump attached.

The commercial consequence is straightforward. When a cleaning contractor proposes a product by brand and says it is suitable for titanium, the correct response is to request the full formulation including the inhibitor and surfactant package, the qualification data on the grade installed, and the maximum fluoride and chloride content. A supplier who cannot produce those three items is quoting a steel job. On a plant where the exchanger represents a large share of the capital cost and a shutdown window costs far more than the cleaning contract, that is not a risk worth taking to save a day of laboratory work.

Why the Deposit Is Not the Corrosion Problem — the Gap Under It Is

Scale is an insulating layer; it raises energy consumption and it hides the metal, but it does not by itself attack titanium. The damage comes from the thin stagnant electrolyte trapped between the deposit and the wall. Oxygen inside that gap is consumed by cathodic reaction and cannot be replenished because the overlying sediment blocks exchange. Titanium then dissolves at the anodic sites, its ions hydrolyse and release hydrogen ions, and chloride migrates inward to maintain charge balance. Published failure analyses of exchanger tubes report the pH inside such an occluded cell falling below 1 while the bulk liquid outside remains near neutral, and describe the process as self-catalytic once nucleation occurs. External descriptions of tight sediment deposits put the sharp drop in the crevice threshold at roughly 70 C and note that damage may take months or years to become visible, with the first symptom a small leak after a long stable run.

Why this is a cleaning article and not a materials article

Because the same mechanism sets both the cleaning interval and the cleaning acceptance criterion. If the plant waits until the thermal duty has visibly collapsed, the deposit has already been in place long enough for the occluded cell to nucleate. Cleaning on a fixed interval derived from fouling rate, rather than on a performance trigger, is what keeps the wall out of the incubation period. And when the deposit is finally removed, the exposed surface must be read correctly: dark etched spots and shallow pits revealed by the wash are corrosion damage, not residual staining. Field accounts describe operators polishing these marks or dismissing them as discolouration and returning the unit to service, after which the pits continue to grow. Eddy current testing is the appropriate tool for subsurface pitting beneath remaining scale, because ordinary wall thickness measurement can land on a clean patch and underestimate the risk.

Grade selection changes the margin but not the mechanism. Our Gr2 versus Gr7 seawater comparison and Gr7 versus Gr12 guide explain where palladium or molybdenum-nickel additions widen the crevice window, and our 316L to 254SMO upgrade ladder sets out why critical pitting temperature and critical crevice temperature are different tests with the crevice value always the lower one. That last distinction matters here: a deposit creates an artificial crevice, so the CCT logic, not the free-surface CPT logic, is the one governing the fouled wall. Our heat exchanger material selection guide compares tube grades; this article is about what happens between two planned cleanings. Where the tubes enter a tubesheet, the geometry creates a second, permanent crevice that a circulation loop may never flush, which is why our tube-to-tubesheet welding and expansion detail should be read alongside any cleaning plan.

Hydrogen is generated inside the same gap

Published failure analysis of seamless titanium exchanger tubes reports that crevice corrosion is often accompanied by hydrogen absorption, with needle-shaped hydride phases visible in the corrosion layer under metallographic examination, and hydrogen diffusing inward to become a crack source. So the under-deposit cell attacks the wall twice: it dissolves metal locally and it charges the surrounding metal with hydrogen. Our hydrogen embrittlement and E1447 guide covers the mechanism and the measurement method; the practical point for a cleaning specification is that removing the scale does not undo the charging that already happened, and a reducing wash can add to it. Any cleaning plan for a heavily fouled chloride-duty bundle should therefore include a surface examination after descaling and, where the service is critical, a hydrogen check rather than a simple visual sign-off.

The B600 Myth: Why a Factory Pickle Recipe Does Not Transfer to a Running Plant

This is the most consequential section in the article. ASTM B600 describes cleaning and descaling titanium at the fabricator, and its workhorse final pickle is 10 to 30 volume per cent nitric acid plus 1 to 3 volume per cent hydrofluoric acid at about 49 C, held at ten parts nitric to one part HF. The standard is unusually blunt that the ratio matters more than either concentration, because the nitric acid is the oxidising component that keeps titanium passive while HF does the dissolving; let HF drift upward and the same bath starts charging the part with hydrogen. That recipe is legitimate in a works. It is not legitimate as a shutdown instruction, and the reason is not the chemistry: it is the three controls that exist in a workshop and vanish on a plant.

The three controls an installed vessel does not have

First, coupons and bath analysis. GB/T 23602 requires the cleaning system to be monitored by chemical analysis of hydrogen on coupons through the process and states that if hydrogen rises more than a set increment above the pre-cleaning result the acid must be changed or adjusted; Chinese commentary on B600 describes that permitted increment as having been tightened in the 2022 revision, with the exact clause confirmed in the edition named in the contract. Our B600 pickling guide sets out the bath ratios, the coupon rule and the free-iron acceptance test in detail. Second, a quantitative hydrogen test on the finished part, by vacuum hot extraction per ASTM E1447, in a laboratory with calibrated ppm standards. Third, a fallback: B600 and GB/T 23602 both allow excess hydrogen to be removed by vacuum annealing of the cleaned workpiece. An installed reactor, a welded tube bundle or a 20 metre coil cannot be put into a vacuum furnace, cannot be sectioned for coupons at representative locations, and cannot be re-annealed without destroying its dimensional and metallurgical state. Our titanium coiled tubing page illustrates the geometry problem on its own: a long continuous coil has no accessible coupon location and no practical annealing route.

Field experience matches the logic. A reactor maintenance reference describes a chemical plant that added a hydrofluoric-acid-containing cleaner to a titanium reactor to improve silica removal and suffered severe inner-wall pitting in a short time. A Chinese cleaning reference records a PTA oxidation reactor condenser whose titanium tubes were immersed in 20 per cent HNO3 plus 2 per cent HF and corroded severely, then offers a much tighter in-service practice: HF no more than 0.25 mass per cent with HNO3 at 10 to 15 mass per cent, contact under one hour, and an explicit warning that the nitric-to-HF mass ratio must not be allowed to reach 30 to 1. Other references ban fluoride altogether for in-service cleaning. Note that the workshop figure is a 10 to 1 volume ratio and the field figure is a mass-ratio ceiling; they are not the same number and must not be quoted interchangeably. We are not adjudicating between these sources. The point is that a document written for a dip tank in a factory is being used to justify a pumped circuit through an installed machine, and the two are not equivalent operations.

Welded equipment raises the stakes further. Our WPS and NDT qualification guide explains why joint efficiency and the governing acceptance code are separate decisions, and our GTAW shielding guide covers the oxygen and nitrogen pickup that occurs when hot titanium is not fully protected. A weld and its heat-affected zone are the first places a reducing bath attacks, and a cleaning-induced defect there becomes a code and repair question, not a housekeeping one. Where the material certificate is the only record tying the wall to its heat number, our EN 10204 3.1 mill certificate page explains what that document does and does not prove: it says nothing about what a maintenance contractor circulated through the machine three years later.

Rule of thumb: if a proposed in-service cleaning recipe contains hydrofluoric acid or any free fluoride, treat the B600 citation as a red flag rather than a justification. Ask which of the three workshop controls will be reproduced on site, in writing, and who signs the hydrogen result. If the answer is none, the recipe belongs back in the factory.

There is one more subtlety. Silica-rich scale on seawater-cooled titanium tubes is genuinely difficult, and one reference recommends a nitric-HF mixture for it while another recommends EDTA, DTPA or NTA chelants for sulphate scale and explicitly warns that oxalic acid severely corrodes titanium. So the honest answer to a difficult deposit is not to ban an entire class of chemistry and hope; it is to run the dissolution trial and the coupon test. Our hydrogen uptake guide gives the measurement route if the plant elects a reducing bath, and the point at which a coupon hydrogen increment should force a change of agent rather than a longer circulation time.

The Cleaning Agent Table Nobody Can Agree On

Below is a map of what published references recommend by deposit type, with the conflicts left visible. concentrations are given as published and are not a formulation for use; every one of them needs qualification on the installed grade. The recurring pattern is that an agent described as mild in a product brochure has a hard concentration, temperature or pH ceiling in the corrosion literature, and the ceiling is the part that gets dropped when the recipe is copied into a work order.

Two conclusions follow from the table. First, there is no universal titanium-safe cleaner. There is only a qualified recipe for a named deposit on a named alloy. Second, a product data sheet saying suitable for titanium is not a qualification: it usually means the concentrate does not contain free fluoride, and says nothing about the installed temperature, the dead legs, the mixed metals or the contact time. Our passivated titanium fastener page makes the same point from the fastener side, where a surface treatment is an identification or cleaning state rather than a protective coating; a cleaning product is no different. When the agent is chosen, write the prohibited-components line into the work order explicitly: free fluoride, uninhibited reducing mineral acid, chlorinated solvent residue, oxalic acid, and carbon steel contact tools. Then write the maximum temperature and the maximum contact time as separate lines, because a contractor who is given only a concentration will optimise for speed.

One final data point deserves its own sentence because it is so often inverted. A reference on titanium in corrosive media states that unalloyed titanium corrodes at a very low rate in boiling 0.3 per cent sulphamic acid and at over 100 mpy in boiling 0.7 per cent, and that adding 0.375 g per litre ferric chloride to the 0.7 per cent solution brings the rate to 1.2 mpy. That is a two-order-of-magnitude swing from a small change in concentration and a trace addition of oxidiser. Anyone who specifies sulphamic acid by product name without a concentration, a temperature, a pH floor and an inhibitor is not specifying a cleaner; they are specifying a variable.

One Circuit, Several Metals: Write the Recipe for the Weakest Material

A cleaning contractor works on the whole circulating loop, not on the material named in the equipment title. A large surface condenser may combine a carbon steel shell, a titanium tube bundle, a copper-alloy lined waterbox, cast steel sacrificial anodes, stainless instrument taps and rubber or graphite gaskets. One Chinese reference describes exactly this configuration and concludes that only a nitric-acid-based cleaner is usable, and that nitrite plus ammonia passivation is unacceptable because of the copper. Another reference states the general rule for mixed systems directly: the formulation must be chosen against the tolerance of every material present, not the primary one. Our titanium to steel galvanic guide explains why the titanium bundle is normally the cathode and the steel and anodes are consumed, and why wetting area ratio governs severity. During a wash the electrolyte is deliberately made more conductive and often hotter, so the same couple runs harder for the duration of the job.

The temporary loop is part of the specification

The pump, hoses, tank, strainers, heaters and thermowells brought in for the job are frequently carbon steel, because that is what a cleaning contractor owns. Two independent problems follow. Dissolved iron from the loop can plate or deposit on titanium surfaces, and steel components coupled to a large wetted titanium area become small anodes against a large cathode. Our free-iron contamination guide documents the roughly half-volt potential difference and the fact that a rust stain can disappear while the pit keeps growing; our galvanic guide explains why coating only one side must mean coating the cathode. Specify the temporary loop materials in the method statement, require a free-iron test after the wash, and require the loop to be flushed and drained to a defined endpoint. Where gasketed joints are opened for the job, our flange, bolt and gasket three-piece guide covers the pressure-class and sealing-face synchronisation; a cleaning job adds a fourth constraint, which is that the new gasket and any exposed crevice must tolerate the cleaning chemistry and be flushed afterwards.

Dead legs decide where the acid stays

A circulation map drawn for heat transfer is not a circulation map for cleaning. Instrument stubs, drain legs, bypasses, the shell side of a partially drained exchanger, a spare nozzle with a blind, and the annular gap at a tubesheet joint can hold spent acid long after the main circuit has been neutralised. On shell-and-tube and jacketed equipment the geometry matters: our titanium reactor page, storage tank page, custom reactor and exchanger page and coiled tube page all show configurations where a low point or a long coil cannot be positively drained by gravity. Require a written drain and flush map with every dead leg identified, a flush sequence with sample points at the worst locations, and a verification that the final rinse at those points meets the same pH and conductivity criterion as the main return line. Our condenser tube supply page is relevant where a retube or a partial bundle replacement is being planned alongside the clean, because a new tube surface does not need the same aggression as a fouled one.

Mechanical cleaning is chemistry by another route

The TiO2 film is only a few nanometres thick. Reactor maintenance guidance prohibits metal tools and steel wool inside a titanium vessel and prescribes PTFE scrapers, wooden or plastic shovels and high-pressure nylon brushes; a separate cleaning reference allows soft-bristle brushes, controlled-pressure water jets and non-metallic scrapers, and prohibits abrasive tools and steel brushes. One field reference quotes high-pressure water jetting at 60 to 80 MPa as the first option before chemistry. Those numbers disagree and are not offered here as a setting, but the direction is consistent across sources: mechanical removal first where it is feasible, because it avoids adding a reducing bath to the hydrogen budget. What mechanical cleaning does add is free iron and scratches if the tool is wrong, plus the risk of embedding abrasive in a soft titanium surface. Dedicated non-metallic tools, a documented final free-iron check and a borescope inspection of the cleaned wall belong in the same work order. Where the tube bore is accessible, sponge-ball or soft-plug cleaning can remove soft deposits continuously and reduce the number of chemical washes per year altogether.

Hydrogen Budget: Every Wash Spends What You Cannot Anneal Back

Our hydrogen embrittlement guide makes one distinction that governs this whole chapter: the 150 ppm figure commonly seen on a mill certificate is a factory compliance ceiling, not an in-service safety limit. The certificate describes the state at dispatch. Hydrogen is then added by almost every downstream operation: an overheated or HF-rich pickle, cathodic over-protection, hot concentrated alkali, reducing conditions where the passive film cannot be maintained, and crevice corrosion itself. A chemical wash with a reducing acid at 70 C is a concentrated dose of one of those sources, delivered directly to the entire wetted surface at once.

Why the controls that work in the factory do not work here

In the workshop the remedy is well defined. GB/T 23602 asks for coupon monitoring of hydrogen through the cleaning process and a bath change when the increment exceeds the stated threshold; ASTM B600 and the same Chinese standard allow vacuum annealing to remove excess hydrogen; ASTM E1447 provides the quantitative measurement. An installed vessel cannot be sectioned for representative coupons, cannot be put into a vacuum furnace, and in most cases cannot even be sampled for bulk hydrogen without cutting out a piece of the pressure boundary. Our weld qualification and NDT guide explains how a weld repair on an in-service component becomes a code question with an authorised inspector involved; the same logic applies to any proposal to cut a coupon from a live vessel. Consequently the only defensible strategy is prevention plus in-process evidence: qualify the recipe on a separate coupon of the same grade in the laboratory, hold the plant to the qualified temperature and time envelope, and log those parameters continuously during the actual circulation.

The accumulation is the real issue. A single well-controlled wash may add an acceptable increment. Ten annual washes, several of them extended because the deposit was harder than expected, plus a period of cathodic over-protection and one hot caustic CIP, are cumulative and cannot be reset. Published guidance on alkaline media puts hydrogen pickup and possible embrittlement above roughly 77 C at pH 12 or higher, while plant practice references recommend keeping alkaline cleaning below 60 to 70 C. Our GTAW shielding guide describes oxygen and nitrogen pickup above 400 C during welding; hydrogen is the lower-temperature analogue that arrives without any colour change to warn the operator. For critical equipment, keep a cleaning log that records the agent, concentration, temperature, duration and any hydrogen result for every event, and treat the accumulated history as part of the fitness-for-service review rather than a stack of job cards. Our custom equipment page is where that history is most valuable, because a one-off fabricated vessel has no fleet of identical units to compare against.

Procurement consequence: ask a cleaning contractor for the hydrogen increment measured on a coupon of your grade, not for a statement that the product is non-corrosive to titanium. The first is a number with a method; the second is a marketing sentence. If E1447 measurement falls outside the laboratory normal working range for the value being claimed, ask for the method validation before accepting the report.

There is also a positive reason to raise this in a commercial conversation. A plant that understands its hydrogen budget will plan more mechanical cleaning, better feedwater pretreatment, online velocity control and fewer aggressive washes, and will therefore buy fewer replacement bundles. That is not a lost sale for a stockholder; it is a customer who specifies the correct grade, wall and surface state on the next order and stops blaming the alloy for a maintenance problem. Our failure-mode tube ordering guide and ASTM B338 tube guide are the natural places to start that specification.

How to Prove the Wash Worked: Thermal, Chemical, Surface and Hydrogen Evidence

Cleaning without verification is a cost with no measurement. Four independent lines of evidence are needed, and none substitutes for another. A restored heat transfer coefficient proves the deposit is gone but says nothing about wall loss; a neutral final rinse proves the bulk acid is gone but not that a blind leg was flushed; a white-cloth wipe proves loose residue is gone but not that the passive film is intact; and a coupon corrosion rate proves the recipe was mild in a beaker but not that the plant held the temperature.

Thermal and hydraulic recovery

Record the approach temperature, overall heat transfer coefficient or pressure drop at a defined load before and after the clean, at the same flow and inlet temperature. A plant that cannot produce a before reading has no way to justify the next interval. Heat-transfer monitoring references suggest tracking the power required to hold a set temperature or the heating time at a constant setting as a practical fouling indicator, with a gradual drift under stable conditions signalling deposit growth. This is also the cheapest way to move from reactive cleaning to planned cleaning, which is what keeps the wall out of the under-deposit incubation period described earlier.

Rinse chemistry and drying

Reactor cleaning guidance calls for multiple high-flow deionised water rinses until pH is neutral and conductivity qualifies, with one reference quoting below 10 microS per cm and another below 1, and a third giving a pH band of 6.5 to 7.5. A separate reference requires drying immediately with clean dry nitrogen or oil-free compressed air. These numbers conflict, so name the criterion in the method statement, sample at the worst dead leg as well as at the main return, and record the drying step with a time and a dew point or a visual and wipe confirmation. Where the equipment will be opened for inspection, our three-step equipment arrival acceptance guide describes the documentary discipline for MTC, dimensional and NDT records; the cleaning file should be built to the same standard and kept with the vessel history. Note that a neutral pH at the main return says nothing about a spent acid pocket in an instrument stub; that is why the drain map from the previous chapter is an acceptance document, not a sketch.

Surface, wall and hydrogen evidence

Use borescope or endoscopic inspection at welds, flange connections, behind agitator blades and at tubesheet joints; use eddy current where pitting beneath any remaining deposit is suspected; record ultrasonic wall thickness at marked locations so the next clean has a comparable baseline; and run a free-iron spot test after any steel tool or carbon steel loop has been present. Where the grade and wall were originally ordered against a corrosion allowance or a specific surface condition, our ASTM B265 plate guide and mill certificate page explain what the original documents establish, so a post-clean thickness reading can be interpreted against the correct baseline rather than against the nominal catalogue value. If the recipe involved a reducing bath, file the coupon corrosion rate and the hydrogen increment alongside the plant temperature and time log. Four signatures are better than one: cleaning contractor, plant operations, inspection, and the owner engineer who releases the equipment back to service.

The 14-Line Cleaning Method Statement Buyers Can Copy

The single most useful commercial output of this article is a document that turns a shutdown job into a specified process. One of our own product pages already states the principle in a single clause: the wash recipe must be on the RFQ. These 14 lines are that principle written out. They apply whether the work is done by a plant team or an outside contractor, and they are written so that a supplier of replacement tube, plate or a complete bundle can quote against the same assumptions. Our brine exchanger page and process equipment guide cover the ordering half; our five-case bundle failure article explains what happens when cleaning and operating discipline are left out of the equipment decision.

Line 4 and line 6 are the ones most often omitted, and they are the two that separate a titanium job from a steel job. Line 9 is the one that fails quietly on site, because a circulation drawing is not a drain drawing. Line 13 is the one that pays for the paperwork: a plant with a before-and-after duty record and a lifetime wash count can defend a fitness-for-service decision and can order the correct replacement wall before an unplanned shutdown. When an enquiry reaches us with these 14 lines filled in, we can quote tube, plate, a coil or a complete bundle against the real service and the real maintenance regime, rather than guessing from a grade name. When it arrives as a request for titanium tube with no cleaning history, the responsible answer is to send the missing lines back first.

Limitations and Failure Conditions: Eight Ways an Acid Wash Ends a Service Life

Each condition below either moves the enquiry to a different cleaning route, sends it back for missing specification fields, or turns it into a fitness-for-service question rather than a maintenance task. Several of them are places where titanium itself was never the problem, and we would rather say so here than in a failure investigation.

Hydrofluoric acid or free fluoride anywhere in the cleaning formulation

This is the hard stop. Titanium owes its corrosion resistance to a TiO2 film that fluoride dissolves aggressively. Reactor maintenance references describe a plant that added an HF-containing cleaner to improve silica removal and suffered severe inner-wall pitting and a prolonged shutdown; another records PTA condenser titanium tubes severely corroded after immersion in 20 per cent HNO3 plus 2 per cent HF. A factory B600 pickle can use HF because coupons, hydrogen monitoring and vacuum annealing are available; an installed vessel has none of them. If a supplier cites B600 to justify a fluoride-containing CIP recipe, reject the citation. Where silica scale genuinely demands it, the exception must be owner-approved with a tight concentration, ratio, temperature and time envelope, a coupon hydrogen result, and a named signatory. Otherwise specify the chelant or mechanical route.

Circulating inhibited hydrochloric acid borrowed from a steel or copper-alloy job

Hydrochloric acid is the default carbonate dissolver on steel plant work, and published sources genuinely disagree about titanium: one lists it for carbonate and iron scale, several ban concentrated HCl outright, a third allows it with ferric or cupric oxidising inhibitors while warning that iron or copper then plates out on the titanium, and a field reference calls 10 to 15 per cent industrial HCl on titanium bundles a dangerous mistake. Broad-spectrum steel inhibitors are explicitly described as unsuitable for titanium in one reference and as capable of increasing corrosion. The failure here is procedural, not chemical: a recipe was inherited rather than qualified. Require the grade-specific coupon result, the complete inhibitor formulation and the post-wash free-iron test, or use the oxidising nitric or chelant route instead.

Sulphamic acid specified by product name with no concentration, temperature or pH floor

Sulphamic acid is sold as a mild, low-fume descaler, which is why it is so often accepted without qualification. Titanium corrosion data report a very low rate in boiling 0.3 per cent sulphamic acid, over 100 mpy in boiling 0.7 per cent, and about 1.2 mpy in the same 0.7 per cent solution once 0.375 g per litre ferric chloride is added; the same source warns that the pH must not be allowed below 1.0 and advises extreme care when descaling titanium exchangers. Additional references report hydrolysis above 60 C generating a more aggressive ammonium bisulphate. A work order that says 5 per cent sulphamic acid, circulate until clean has no defensible boundary at all. Write the concentration, the inhibitor, the pH floor, the temperature ceiling, the contact time and the stop condition.

Hot alkaline CIP above the hydrogen-uptake threshold

Titanium is genuinely resistant to alkali, with published corrosion rates at or below about 5 mpy in sodium, potassium, calcium and ammonium hydroxide, which is why hot caustic cleaning is often assumed harmless. The same corrosion reference states that hydrogen pickup and possible embrittlement can occur above roughly 77 C when pH is 12 or higher; other cleaning references recommend holding alkaline cleaning below 60 to 70 C and warn that chloride or chelant additives in alkaline formulations may be more damaging to the passive film than the alkali itself. A hot 1 to 5 per cent NaOH CIP on a jacketed reactor can therefore sit exactly at the boundary. Fix both temperature and pH ceilings, log them continuously, and include the event in the lifetime hydrogen and wash-count record.

Carbon steel pumps, hoses, wire brushes or scaffolding in contact with the cleaned surface

A temporary cleaning loop is usually built from whatever the contractor owns, and that is normally carbon steel. Dissolved iron deposits on titanium, steel tools leave smears and scratches in a passive film only a few nanometres thick, and steel components coupled to a large wetted titanium area become small anodes against a large cathode. The rust stain may disappear after the first rinse while the pit keeps growing, which is the exact mechanism described in our free-iron and galvanic guides. Specify non-metallic or compatible loop materials, PTFE, wood, plastic and nylon tools only, dedicated non-ferrous tooling downstream of the clean, and a final free-iron spot test as a separate acceptance line.

Spent acid left in dead legs, blind bores and tubesheet crevices after the main circuit is neutralised

The main return line can read neutral while a pocket of hot reducing acid sits in an instrument stub, a drain leg, a start-up bypass, a partially drained shell side or the annular gap at a tube-to-tubesheet joint. Those locations are also the tight crevices where oxygen depletion, chloride concentration and a pH below 1 create a self-catalytic cell. A cleaning job makes this worse by filling them with a hot, conductive, inhibited-but-finite fluid. Require a written drain and flush map with every dead leg identified, sample points at the worst locations, a defined flush sequence, and evidence that the worst-location rinse meets the same pH and conductivity criterion as the main return.

Treating dark etched pits revealed by descaling as residual staining

When scale is removed, the newly exposed surface may show dark etched spots and shallow pits. Field descriptions of under-deposit corrosion note that operators frequently mistake these marks for harmless staining and return the unit to service, after which pitting continues beneath newly formed deposits. Eddy current testing is the appropriate method where subsurface pits are suspected, because a wall thickness probe landing on a clean patch underestimates the risk. Make the post-clean borescope and eddy current review an explicit hold point, define what findings require a fitness-for-service assessment, and never allow a cosmetic polish to substitute for that assessment.

A cleaning job with no coupon, no hydrogen result, no temperature log and no release signature

Every other failure on this list is a chemistry or geometry mistake. This one is the governance mistake that lets them recur. Workshop cleaning standards assume coupon monitoring, a hydrogen increment threshold, a quantitative E1447 method and vacuum annealing as a fallback; an installed vessel has none of these, so the only substitutes are qualification in the laboratory, continuous parameter logging on site, and a four-party release record. A contractor who reports that the unit looks clean has reported nothing measurable. Require the procedure revision, agent batch, temperature and time log, coupon corrosion and hydrogen report, rinse results, inspection records and the owner signature before the equipment returns to service.

Summary: Choose Chemical Cleaning if, Choose Mechanical or Redesign if, Escalate if

Choose a specified chemical clean if:

• The deposit has been sampled and analysed, and a dissolution trial has selected an agent that works on that deposit rather than on the sales literature.
• A coupon test on the installed grade, including welded material, has produced a corrosion rate and a hydrogen increment with and without the proposed inhibitor.
• The full formulation, concentration, temperature ceiling, pH limit, contact time, circulation velocity and prohibi