Hand-considered hypotheses, 2201–2300

Corrosion science and protection: passivity, localised attack, coatings, cathodic protection, environmental cracking

Written one at a time, not composed. The logical form is printed with every entry because the forms, not the subjects, are what vary: several claim a quantity is misattributed, several assert a bound or a threshold, several invert a standard practice, and a few are no-go statements that would delete a family of measurements if true. None has been tested. Numbers followed by “(estimate)” are order-of-magnitude expectations, not measurements; there are no citations here, and nothing in this file is a datum.

Passivity and the growing film

Misattribution
H2201. The current decay after a potentiostatic step into the passive region is read as film growth, but on alloys whose cation is only sparingly soluble much of the charge is dissolution followed by reprecipitation of an outer hydroxide layer, so the film thickens while the barrier layer barely changes. A quartz-crystal mass signal set against the charge passed at the same potential should register more mass than the barrier thickness can account for, and ellipsometry on the same specimen should find the inner layer growing far more slowly.
Hidden term
H2202. The passive current density measured in aerated chloride solution is the sum of film formation and the reduction of whatever oxidant the cell contains. Trace ferric iron from the salt and oxygen leaking past the seal both contribute, so the quoted value is partly a property of the cell rather than of the alloy. Holding potential and temperature fixed while the oxygen partial pressure is stepped from about 1 ppm to air should leave a true passive current almost unchanged and move an oxidant-dominated one in proportion (estimate).
Identity
H2203. The two time constants recovered when a passive film is stepped anodically, one assigned to double-layer charging and one to film growth, may be a single process: a growing film changes the interfacial capacitance as it thickens, and a distributed time constant fits as two. A step series of increasing amplitude should then show the two constants migrating together, whereas a genuine double layer would leave the fast one fixed.
Bound
H2204. Passive current cannot be pushed below a floor set by point-defect migration through the film, which is nearly independent of thickness once the film exceeds a few nanometres (estimate), so thickening past that point buys nothing. Anodising a thickness series at fixed potential and comparing current with ellipsometric thickness should flatten the current while the thickness keeps rising.
Threshold
H2205. The passivation potential taken from a sweep is a scan-rate artefact, because the apparent knee is where film formation first outruns chemical dissolution on that sweep's timescale. Potentiostatic steps held until the current plateaus should place the knee at a potential that does not move when the sweep rate is changed by a decade, and any knee that moves is a kinetic boundary rather than a thermodynamic one.
Reversal
H2206. On some nickel-base alloys the film thins before transpassive oxygen evolution begins. The higher-valence cation is soluble and leaves as fast as it forms, so current climbs while material is lost from the film. Simultaneous ellipsometry and current measurement across the transpassive region would place a falling thickness inside the rising-current interval.
Count
H2207. A point-defect description of passive film growth carries at most two independent kinetic parameters, a field and a rate constant, and fits that report five are trading one against another. Refitting a thickness series with each parameter fixed in turn should leave the residuals unchanged wherever the extra parameters are redundant, and the covariance matrix should expose the trade directly.
Artefact mimicking effect
H2208. Part of the passivity gained by polishing is geometric: a rough surface has more real area than the coupon's nominal area, and normalising current to the nominal figure inflates the apparent improvement. Capacitance-based roughness factors measured before and after each finish should absorb most of the difference, leaving a residual that is the true effect of surface preparation.
Scale argument
H2209. Below roughly four nanometres (estimate) a passive film is only a few unit cells thick, and the Pilling–Bedworth ratio loses its meaning. There is no bulk lattice for the film to expand against, so the stress that cracks it is set by the interface rather than by the volume misfit. Growing films to the same nominal thickness on a flat and on a strongly curved substrate separates the two, because curvature changes the interface term while leaving the misfit unchanged.
Inversion of standard practice
H2210. Holding a specimen at a high potential to stabilise its passive film is the wrong move when the cation is mobile in the film, since the same field that thickens the film also drives vacancies toward the metal interface and builds the precursor of a breakdown site. Two specimens — one taken to a high potential and then returned, one grown at the lower potential throughout — should differ in breakdown potential in the direction the usual practice forbids, with the high-potential specimen the weaker.
Selection effect
H2211. Reports of bistable passive films come from specimens that survived the experiment; the ones that broke down were dropped from the analysis, so the two-state behaviour is a survivorship artefact of the data set rather than a property of the film. Pre-registering every specimen, including those that fail, and plotting the potential at which each leaves the passive state should produce a single population rather than two.
No-go claim
H2212. Capacitance alone cannot separate a passive film's thickness from its permittivity: one measurement, two unknowns, and every thickness quoted from capacitance is really a thickness multiplied by the ratio of assumed to true permittivity. The same film measured by ellipsometry and by capacitance across a thickness series would leave the implied permittivity drifting if the separation is impossible, and leave it constant if it is not.

Localised attack

Misattribution
H2213. Metastable pit transients are integrated to give a pit volume, but on a filmed electrode the transient's shape is set by the film resistance charging the double layer, so a compact event and a slow one can carry the same charge and mean different things. Comparing each transient's rise time with the measured film resistance times capacitance should collapse the shapes onto one curve, and any transient that refuses to collapse is a genuine growth event.
Threshold
H2214. Crevice attack needs a gap-depth product large enough that the ohmic drop along the gap exceeds a set fraction of the polarisation driving the cathodic reaction. Below that geometry the crevice cannot acidify, however long it is exposed. A wedge crevice with a measured gap profile should begin to attack at the position where the computed IR drop crosses that fraction, and the position should move when the bulk conductivity is changed.
Count
H2215. The number of stable pits per unit area saturates because neighbouring pits compete for the same current, so the saturation density is set by the total current available divided by what one pit demands at that potential. Potentiostatic holds at increasing potential should raise the density and then flatten it while individual pit currents keep growing, which is the signature of competition rather than of a finite supply of sites.
Scale argument
H2216. The hemispherical diffusion field that justifies a pit's cubic-root growth law only holds once the concentration boundary layer is larger than the pit mouth. Below that size transport is one-dimensional and the growth exponent is different, so the law and the data diverge at small pits. Pit size distributions after short and long exposures should place the exponent change at a size that scales with the diffusion layer thickness, and the transition should shift when the solution is stirred.
Reversal
H2217. Adding chloride to a crevice that is already occluded can reduce attack, because the added conductivity spreads current toward the mouth and lowers the local acidification instead of deepening it. Stepping chloride concentration up after occlusion, while measuring the potential drop along the gap, would make that drop shrink rather than grow wherever conduction outweighs the chloride's aggressive role.
Artefact mimicking effect
H2218. Pits counted after a standard coupon test include a population initiated at grinding scratches, which is a preparation artefact and not a material response. Coupons from one heat finished by grinding, by diamond polishing and by electropolishing should give three different pit densities at the same potential, and the ground specimen's excess should vanish when the scratches are removed without changing the alloy.
Identity
H2219. Intergranular and transgranular stress corrosion in one alloy may be a single mechanism seen at different grain-boundary misorientations. The crossover would be an angle rather than a change of chemistry at the crack tip. Orientation mapping of crack paths against measured boundary character should place that crossover at a consistent misorientation, and heat treatments that alter the boundary population should change the proportion of each path without changing crack velocity.
Unit trap
H2220. A pitting potential quoted against a calomel electrode in a cell with a chloride gradient across its junction carries a liquid-junction potential of tens of millivolts (estimate), which is the size of the alloy-to-alloy differences being reported. Repeating the measurement with a bridge matched to the bulk composition should shift the number by that amount, and the alloy ranking survives only if the junction was not doing the ranking.
Selection effect
H2221. The deepest pit on a coupon is reported as the coupon's pitting resistance, but it is the survivor of a population whose largest member grows with the area searched, so a small coupon cannot express the deepest members. Coupons of several areas exposed identically should find the deepest pit rising with area at fixed exposure time, and only the depth normalised to that trend is a material number.
Hidden term
H2222. The induction time before pitting contains both the time to break the film and the time for a pit to grow to the detection limit. Only the first is a material property. Two techniques of different sensitivity, optical and electrochemical, run on identical specimens should give induction times differing by a constant offset that scales with the detection threshold rather than with the alloy.
Bound
H2223. A pit in a thin sheet cannot deepen beyond the point where the resistance of its own solution column chokes the current. Maximum depth is therefore bounded by sheet thickness and conductivity rather than by the alloy's resistance to dissolution. Foils of several thicknesses held at the same potential would drive maximum depth toward a ceiling that falls with thickness, and the ceiling should rise when the solution is made more conductive.
Inversion of standard practice
H2224. Finer grinding is assumed to improve pitting resistance, but on alloys whose film grows faster on deformed metal the residual scratch deformation can be protective at low chloride activity and harmful at high. A finish series run at two chloride activities would make the ranking of finishes cross over, with the coarse finish better in the dilute solution and worse in the concentrated one.
Threshold
H2225. The critical chloride level for breakdown tracks chloride activity relative to water activity rather than concentration, so a brine concentrated by evaporation breaks down at a different concentration from the same brine diluted back. Holding chloride molality fixed while water activity is lowered with a second, non-aggressive salt should move the breakdown potential even though the chloride number on the label has not changed.
Null claim
H2226. At fixed potential and temperature the number of initiated pits is not a material constant. It is set by the size distribution of inclusions intersecting the surface, which varies from heat to heat at the same composition. Two heats certified to one specification but carrying different inclusion size distributions should give pit counts differing by more than replicate scatter, and those counts should track the inclusion census rather than the chemistry.
Symmetry claim
H2227. Pit growth is symmetric only when the current distribution around the mouth is symmetric, so a pit that has broken into a crevice grows asymmetrically and the direction of that asymmetry identifies which end is being cathodically protected. Serial sectioning after exposure should find the deepest penetration next to the mouth rather than at the tip whenever current is drawn from outside the crevice.
Causal ordering
H2228. In crevice corrosion the acidification is usually said to follow chloride ingress. The order may be reversed: chloride migrates to preserve electroneutrality as metal ions accumulate, so pH falls first and the chloride follows. Sampling pH and chloride with microelectrodes at one position on a time series settles which arrives first, and the answer should flip if the crevice is pre-filled with chloride.

Kinetics, impedance and the meaning of a fit

Misattribution
H2229. The charge-transfer resistance extracted from an impedance spectrum sits in series with the film's ionic resistance, so a Tafel slope computed from it is too steep by the ratio of the two, and the error grows as the film thickens. A thickness series in which the charge-transfer term should stay constant would leave the extracted slope drifting with thickness, which no genuine kinetic change can produce.
Hidden term
H2230. The ohmic correction taken from a Luggin capillary omits the resistance of a thick porous film. An IR-corrected potential is then still shifted by the part of the path the capillary never sampled. Moving the capillary from one distance to two and extrapolating to zero should leave a residual offset whose size tracks film thickness, and that residual is the missing term.
Identity
H2231. The low-frequency Warburg response of a corroding electrode is the diffusion tail of the anodic partial reaction, not a separate process, and a ring-disc electrode can show that the flux leaving the disc equals the flux inferred from the impedance. Collection efficiency measured at the ring should match the impedance-derived flux across a range of rotation rates if the two are the same thing.
Derived quantity
H2232. Exchange current density is a derived quantity twice over, current divided by an area and area inferred from a roughness factor, so a porous electrode's value can be wrong by one to two orders of magnitude without any single measurement being wrong (estimate). Measuring roughness independently by double-layer capacitance and by gas adsorption should shift both the area and the derived density, and the two corrections should account for the discrepancy between laboratories.
Bound
H2233. A reaction with a chemical step before electron transfer cannot show a Tafel slope steeper than about 120 mV per decade at room temperature. The preceding step caps the sensitivity of rate to potential. Anything steeper is uncompensated resistance or a change of mechanism, and an impedance-derived ohmic correction applied to the same data should bring the slope back inside the bound.
Count
H2234. An impedance spectrum of N usable points supports at most about N/5 independent parameters (estimate), so a fit with more is underdetermined even when its residuals look small. Fitting synthetic spectra with known parameter counts and asking how often the fit recovers them would collapse at exactly that ratio, whatever the model.
Reversal
H2235. A galvanic couple's potential can move opposite to the direction expected from the more noble partner when the cathodic reaction is under diffusion control. The couple then shifts the anodic curve instead of the cathodic one, so the usual polarity rule inverts. Rotating the cathode to raise the diffusion-limited current should drive the coupled potential the way the naive prediction forbids, and stopping the rotation should reverse it.
Artefact mimicking effect
H2236. An inductive loop at low frequency in a corroding system is often the potentiostat's own control loop interacting with a high-impedance reference, not an adsorbed intermediate relaxing. Repeating the measurement with a low-impedance reference and in galvanostatic mode should remove the loop, and an intermediate that is real would survive both changes.
Scale argument
H2237. At high current density the diffusion layer thickness is set by the electrode's size relative to the natural convection boundary layer. Microelectrode data therefore cannot be scaled to plant geometry by current density alone. Electrodes of several diameters at matched current density should give different apparent kinetics until the diameters exceed the boundary layer thickness, after which the values should converge.
Selection effect
H2238. Published impedance fits are the ones that converged; spectra that would not fit were set aside, so the literature's distribution of equivalent circuits is a distribution of successful fits rather than of systems. A round robin in which every spectrum is shared, including the ones that refused to converge, would find the discarded fraction carrying information the accepted ones cannot.
Threshold
H2239. Below a critical current density the corrosion potential stops responding to the cathodic reaction's rate, because the mixed potential is then pinned by the slope of the anodic curve. Varying the oxidant supply at several current densities would collapse the potential sensitivity below that density, and the collapse point should move with the anodic Tafel slope.
Hidden term
H2240. The temperature coefficient of a corrosion rate in an aerated system is dominated by oxygen solubility and diffusivity, both of which fall as temperature rises. An Arrhenius plot can therefore yield a negative apparent activation energy. Running the same series deaerated and aerated should separate the two, the deaerated branch showing the true activated step and the aerated one showing the transport term.
Symmetry claim
H2241. The anodic and cathodic transfer coefficients of a one-step reaction sum to the number of electrons in the transition state, so they are not independent and cannot both be fitted freely. Imposing the constraint on both branches of a polarisation curve should give the same fit quality as freeing them, and any improvement from freeing them is a sign that the reaction is not one step.
Causal ordering
H2242. In a pitting event the current transient precedes the potential transient by the double-layer time constant, and which one leads identifies whether the event is a breakdown or a repassivation. Recording current and potential simultaneously at microsecond resolution gives the order directly, and the order should invert between an event that starts a pit and one that closes it.

Coatings and inhibitors

Misattribution
H2243. The barrier performance of a coating is usually attributed to low permeability. Much of the measured impedance is actually the ionic resistance of the coating plus that of the oxide beneath it, and a coating-free control shows how much belongs to the substrate. The same coating applied over two differently oxidised substrates should give impedances that differ by the oxide's share, which permeability alone cannot explain.
Hidden term
H2244. Water uptake inferred from coating capacitance includes the coating's own dielectric relaxation, so the water volume fraction is overestimated, most severely in the first hours when the relaxation is largest. Gravimetric uptake on a free film and capacitance on the same formulation should diverge early and converge later, and the divergence is the relaxation term.
Threshold
H2245. Coating impedance is set by defects below a threshold thickness and by the film above it, so the thickness dependence flattens at the thin end and grows only logarithmically past it. A thickness series at constant defect density should locate the knee, and seeding the same number of pinholes into each thickness should move the knee upward.
Artefact mimicking effect
H2246. Salt-spray ranking and service ranking disagree because the spray's impinging droplets generate a defect population service never sees. A condensing cabinet generates a different defect population. The same coating set ranked by spray, by cyclic condensation and by field exposure should find spray agreeing with service only for coatings whose failure mode happens to be droplet-driven.
Reversal
H2247. A more hydrophobic coating can do worse under wet–dry cycling, because water that enters at a defect cannot leave and the contact time at the interface, not the uptake, sets the osmotic driving force. Hydrophobic and hydrophilic coatings of otherwise identical formulation cycled identically should leave the hydrophobic one blistering more, and the ranking should invert under continuous immersion.
Count
H2248. A multilayer coating has at most as many independent diffusion barriers as it has chemically distinct layers, so extra coats of the same material add thickness rather than barriers. One thick coat and several thin coats of the same total thickness should give the same impedance if that count is right, and any advantage of the multilayer is then an interface effect rather than a barrier effect.
Unit trap
H2249. Inhibitor efficiency expressed as a percentage is nonlinear near 100. Values of 99 and 99.9 per cent differ by a factor of ten in corrosion rate while looking like adjacent numbers. Reporting rates alongside efficiencies across a concentration series would expose the compression, and the concentration at which the rate halves should be read from the rate, not the percentage.
Bound
H2250. The efficiency ceiling for an inhibitor at fixed concentration is set by transport to the surface rather than by its adsorption constant in slow-flow systems. Rotating a disc electrode through a range of speeds should raise efficiency to a plateau whose value is independent of the isotherm fit, and the plateau is the transport bound.
Reversal
H2251. Anodic inhibitors accelerate attack below a critical concentration because they passivate most of the surface and concentrate the current on what remains, so the dose that looks almost sufficient is the most dangerous. A concentration series bracketing the critical value, run at two flow rates, should place the deepest penetration on the low-flow side of the threshold, with the threshold itself moving with transport.
Scale argument
H2252. At micrometre resolution a coating is discontinuous, since pigments, voids and interfacial roughness are all of the same order as the film. The continuum permeability used in lifetime models has no meaning there. Local impedance mapping at that resolution would find variations larger than the bulk value, and the bulk value should equal the area-weighted mean only when the map is coarse enough to average the discontinuities.
Selection effect
H2253. Coatings qualified on small coupons carry fewer defects than the same coating on a structure, so qualification selects for the defect-free case and the reported performance is a best case. Applying one formulation to coupons and to a full panel and counting defects on both would put the panel's count above the coupon's by more than area alone predicts.
Identity
H2254. Self-healing in a chromate-free coating and inhibition by a soluble inhibitor may be one sequence, release then transport then adsorption on exposed metal, at different length scales. Only the release kinetics would differ. A coating with and without an inhibitor reservoir, adjusted to the same leach rate, should give the same protection at a scribe, and the reservoir should matter only where the leach rate is the limiting step.
Null claim
H2255. Below the critical pigment volume concentration, changing pigment loading does not change barrier performance, because the particles remain isolated in the binder and the diffusion path is unbroken. A loading series bracketing that concentration should keep impedance flat below it and drop it steeply above, with the knee at the percolation threshold rather than at any chosen loading.
Hidden term
H2256. Pull-off adhesion strength includes the adhesive's own cohesive failure. The reported number is therefore a lower bound on interfacial strength and can be changed by changing the glue rather than the interface. Inspecting the failure locus after pull-off should find adhesive left on the coating wherever the true interfacial strength exceeds the glue's cohesive strength, and only specimens failing cleanly at the interface give an interfacial number.
Causal ordering
H2257. Blistering is said to follow water ingress, but the osmotic driving force can be established first by soluble salts left on the substrate. In that case the blister's location is fixed before the coating is applied. Coating salt-contaminated and clean substrates identically should nucleate blisters at the contamination sites from the first hours, with the clean specimen blistering later or not at all.
Threshold
H2258. Cathodic disbondment has a critical interfacial pH above which adhesion is lost, so the same cathodic current does nothing below it and strips the coating above it. A buffered catholyte series at constant current should make disbondment distance rise sharply over a narrow pH interval, and the interval should be the same for two different coatings if pH rather than current is the variable.

Cathodic protection and galvanic coupling

Misattribution
H2259. The hundred-millivolt criterion is applied to the measured potential shift, which includes the IR drop, so the true polarisation is smaller than the number being compared with the criterion. Current-interruption measurement of the IR-free shift on the same structure would find the criterion met less often than the uncorrected survey suggested, and the shortfall should scale with soil resistivity.
Hidden term
H2260. A coated structure under cathodic protection draws a current that includes electronic conduction through the coating itself. That fraction is not a defect current and does not indicate damage. A coating of known electronic conductivity on an inert substrate should draw current in the same range with no holiday present, and the fraction should be subtracted before the current is read as a damage indicator.
Bound
H2261. Protection potential is bounded below by hydrogen evolution at the local pH, and going further negative adds no protection while causing disbondment and hydrogen uptake. Stepping potential below the hydrogen line while measuring disbondment and permeation should raise disbondment and permeation together while the corrosion rate stays at zero, so the extra polarisation buys only damage.
Threshold
H2262. Along a protected line, potential uniformity holds only above a critical current density, because the attenuation length set by coating conductance and soil resistivity is shorter than the line. Measuring the potential profile at several current levels should flatten the profile abruptly once current exceeds that value, and the value should track the coating's conductance.
Reversal
H2263. Raising an anode bed's resistance can improve current distribution on a poorly coated line by suppressing the drain-point short. Lower resistance is therefore not always better. A line of known coating conductance run against two anode beds of different resistance should give the higher-resistance bed the more uniform profile, which the usual preference for lower resistance forbids.
Count
H2264. Independently controllable potentials along a protected line number no more than the attenuation length allows, so reference electrodes spaced more closely than that oversample the same degree of freedom. An eigenvalue count of the distributed line model should match the number of distinct potentials actually observed, and adding electrodes beyond that count should add no information.
Scale argument
H2265. In low-conductivity water the potential at a reference electrode is dominated by the solution IR, so the true polarisation cannot be inferred without a Luggin or an interruption. The error grows with the distance between reference and structure. Two references at different distances from the same point should read potentials whose difference is the IR drop, leaving polarisation as the intercept.
Artefact mimicking effect
H2266. The off-potential read immediately after current interruption still includes depolarisation that occurred during the interruption, so the IR-free value depends on how long the interruption lasted. Varying the interruption length over a decade and extrapolating to zero time should give a more negative potential than the standard reading, and the gap should be larger for coatings of higher capacitance.
Identity
H2267. Stray-current corrosion and galvanic corrosion may be one phenomenon seen from two frames, a potential difference driving current through an electrolyte. The distinction is bookkeeping about where the source sits. A cell in which the same current is supplied first by a sacrificial anode and then by an impressed source should corrode identically at the cathode, which makes the two categories a naming convention rather than a mechanism.
Inversion of standard practice
H2268. Coating plus cathodic protection is the standard combination, but a coating of high dielectric strength can concentrate protection current at its holidays and accelerate disbondment there, so bare steel in some soils is protected more uniformly. Coated and bare coupons under the same protection current should leave the coated one losing adhesion at its defects while the bare one merely polarises.
Selection effect
H2269. Field protection surveys report potentials at accessible test posts, and the inaccessible sections, under roads, in casings and at river crossings, are precisely the ones that fail. Instrumented sections with remote monitoring should record potentials outside the survey's distribution, and the discrepancy should be largest where access is hardest.
Unit trap
H2270. Protection current density referenced to coated area and to bare area differ by the coating's porosity factor. One structure can therefore be reported as adequately or grossly under-protected depending on which area was used. Reporting both numbers for one structure should expose the factor, and the criterion should be restated against the area it was derived from.
Null claim
H2271. For a coating with no defects, cathodic protection provides no benefit until a defect exists, because the current path is the defect and an intact coating is an insulator. A defect-free coating under protection should draw only the electronic leakage current, and its protection potential should be unchanged when the current is switched off.
Causal ordering
H2272. Disbondment follows the interfacial pH rise rather than the current, and the current is only the means of producing the shift. A pH-sensitive probe under the coating during protection should catch pH rising before the disbondment front moves, and buffering the interface should stop the front while leaving the current unchanged.

Environmental cracking and hydrogen

Misattribution
H2273. Cracking in a system with pre-existing surface defects is often called stress corrosion when it is hydrogen-assisted, and the two separate by their potential dependence: dissolution cracking accelerates anodically, hydrogen cracking cathodically. Slow strain rate tests at potentials on both sides of the free-corrosion potential should put the ductility minimum on the cathodic side if hydrogen is the agent.
Hidden term
H2274. The crack-tip strain rate in a constant-load test contains the global creep of the specimen plus the local creep at the tip. Only the local rate controls film rupture, so the loading mode silently sets the answer. Constant-load and constant-extension-rate tests at matched initial stress intensity should give different crack velocities, and the difference should vanish once the global creep is subtracted.
Threshold
H2275. A crack cannot keep growing by film rupture below a critical tip strain rate, because repassivation outruns the exposure of bare metal. A strain-rate series with repassivation kinetics measured separately should place the cessation of growth where the repassivation time equals the interval between ruptures, and the threshold should move with the alloy's repassivation rate rather than with its yield strength.
Reversal
H2276. Cathodic protection can accelerate cracking by generating hydrogen at the tip, so the safe condition is a window between underprotection and overprotection rather than a single potential. Slow strain rate tests across a range spanning both should recover ductility and then lose it again, with the second fall coinciding with the onset of hydrogen uptake.
Count
H2277. Crack growth has two regimes, one dependent on stress intensity and one plateau, and a third threshold regime is usually a transient of the test rather than of the material. Constant-intensity tests held long enough for the transient to decay should erase the apparent threshold regime and leave only the two.
Bound
H2278. The plateau crack velocity is bounded by hydrogen transport to the process zone rather than by stress intensity. Its height should scale with diffusivity and trap density and not with applied load. A temperature series and a charging-current series at fixed stress intensity should move the plateau along the transport prediction, and the plateau should be flat in stress intensity over the same range.
Artefact mimicking effect
H2279. Part of the apparent acceleration of fatigue crack growth in a corrosive environment is the loss of oxide-induced closure rather than a change in crack-tip damage, and closure correction removes most of the difference. Closure-corrected growth rates in air and in solution at the same effective stress intensity should nearly coincide, leaving a residual that is the true environmental contribution.
Identity
H2280. Sulfide stress cracking and hydrogen embrittlement are one mechanism with a poison in the environment, and the sulfide's role is to raise the fraction of cathodic hydrogen that enters the metal. Permeation and cracking measured on one steel at matched charging current, with and without sulfide, should tie the cracking threshold to the permeation flux rather than to the sulfide concentration.
Unit trap
H2281. A threshold stress intensity quoted in MPa root m depends on specimen thickness, and if plane strain is not achieved the number is a specimen property rather than a material one. A thickness series at constant ligament should bring the threshold down to a plateau, and only the plateau value is comparable with published data.
Selection effect
H2282. Stress corrosion tests are run on alloys that survived fabrication, so heats that cracked during forming were removed before the laboratory saw them and the tested population is the survivors. Testing the failed heats alongside the survivors should leave the survivors' threshold above the service failures, which is the selection effect made visible.
Scale argument
H2283. Solution chemistry at a crack tip is confined to nanometres, where a bulk pH and a bulk potential have no meaning. The relevant variable there is the potential of zero charge at that scale. Coupling an atomistic tip model to a measured crack-tip potential should predict a local pH differing from the bulk by more than the bulk's own variation, and the prediction should change sign when the tip's surface charge does.
Null claim
H2284. In a dissolution crack the tip pH is not the bulk pH, so the bare-surface current density measured on a scratched coupon is not the tip current and the number is a laboratory construct. A scratch test set against an independent tip measurement should bracket the tip value only when the scratch's local chemistry is matched, which it usually is not.
Hysteresis
H2285. The potential at which cracking stops is not the potential at which it starts, because the tip's own chemistry holds the crack open below the initiation threshold. Stepping potential down after growth is established should give an arrest potential below the initiation potential measured on a smooth specimen, and the gap should widen with crack depth.
Conservation claim
H2286. Hydrogen entering the metal plus hydrogen evolved as gas must equal hydrogen produced cathodically. A permeation efficiency below one is therefore not an inefficiency but a measurement of the recombination kinetics. Simultaneous permeation and volumetric gas measurement on one specimen should close the balance, and the missing fraction should vary with the poison's surface coverage.
Symmetry claim
H2287. Static-load and cyclic-load crack growth at the same maximum stress intensity are not comparable, because the cycle adds a ratcheting mechanism a static hold cannot reproduce. A static hold and a cyclic test at matched maximum intensity should give different growth rates, and the difference should shrink as the load ratio approaches one.
Threshold
H2288. Embrittlement has a critical hydrogen concentration set by trap density rather than lattice solubility, so the same total hydrogen can be harmless or fatal depending on the microstructure's trap states. Thermal desorption and cracking on the same specimens should correlate the cracking threshold with the strongly trapped fraction rather than with the total.

Monitoring, standards and inference

Misattribution
H2289. Corrosion rates from linear polarisation use the Stern–Geary constant as though it were constant, but it is a function of the Tafel slopes and therefore of the mechanism, so a rate can be wrong by a factor of two with no measurement error at all. Comparing linear-polarisation rates with weight loss across systems of different mechanism should place the discrepancy in the slopes rather than in the instrumentation.
Hidden term
H2290. An electrical resistance probe's reading contains the temperature coefficient mismatch between its reference element and its test element. That mismatch appears as corrosion whenever the temperature moves. A probe in a non-corrosive fluid taken through a temperature cycle should register apparent metal loss, and the size of that loss should track the mismatch rather than the fluid.
Bound
H2291. Ultrasonic thickness resolution on a hot wall is bounded by the sound velocity's temperature dependence rather than by the transducer's frequency, because the velocity change across the wall's gradient blurs the echo. A hot calibration block and a cold one measured with the same probe should give apparent thicknesses differing by more than the probe's specified resolution.
Threshold
H2292. Below a critical wall loss the remaining life is set by the load and the flaw's geometry rather than by the corrosion rate, so rate-based inspection intervals are irrelevant in that regime. A fitness-for-service assessment and a rate extrapolation run on the same wall-thickness data should give different next-inspection dates, and the difference should grow as the wall thins.
Inversion of standard practice
H2293. Inspection intervals set from corrosion rate are the wrong way round. The interval should be set by the probability of detecting the smallest critical flaw, a property of the method rather than of the metal. Probability-of-detection curves for the chosen technique should give an interval shorter than the rate-based one when the critical flaw is small and longer when it is large.
Unit trap
H2294. mpy and millimetres per year differ by a factor that includes density, so a rate quoted in mpy for one alloy and in millimetres per year for another can reverse their ranking. Converting both to one unit before comparing should change the order whenever the densities differ by more than the rates.
Count
H2295. A single coupon can reveal only one corrosion mechanism, because the specimen has one surface history and one exposure, so claims of several mechanisms from one specimen require several coupons prepared differently. Replicate coupons prepared in different ways and exposed together should put each mechanism on its own coupon rather than on all of them.
Artefact mimicking effect
H2296. The slow drift of a corrosion potential over days is often the reference electrode's own drift rather than the steel's, and the two are indistinguishable from one channel. A second reference in the same cell, one new and one aged, should put the drift on the aged channel alone, which makes the electrode the source.
Scale argument
H2297. Laboratory coupons and field structures differ in area and in time by factors that do not scale linearly. Initiation and propagation have different exponents in each, so the ratio of their rates drifts with exposure. Coupons and a full structure exposed together, measured the same way, should make the ratio of their rates change with exposure time, and the change should follow the initiation exponent.
Selection effect
H2298. A standard's precision statement is written from round-robin data that survived the round robin, so laboratories that failed are absent and the quoted reproducibility is optimistic. Publishing the excluded laboratories' results alongside the included ones should widen the stated precision, and the widening should be largest for the technique with the most demanding procedure.
Null claim
H2299. One potential measurement cannot establish cathodic protection. Protection is a polarisation state, and a potential without a reference to the free-corrosion value is ambiguous. Measuring free and polarised potentials on the same structure would leave the same absolute potential meaning protection on one coating and none on another.
No-go claim
H2300. No electrochemical technique can measure the corrosion rate of a passive metal without perturbing the film, and the perturbation scales with the technique's sensitivity, so a more sensitive measurement is a more damaging one. Comparing rates from techniques of different perturbation amplitude on the same passive specimen would have the most sensitive technique reporting the highest rate, which is the perturbation made visible.

Status

100 of 1,000. Every entry was written on its own and given its own logical form; the label sits above the claim so a reader can see that the entries differ structurally and not only in subject.

Corrosion rewards this treatment because its standard measurements each embed an assumption that is rarely stated: that a current is a rate, that a potential is a state, that a coupon represents a structure. Several of the entries above attack one of those assumptions directly. None has been tested; a hypothesis list is worth the experiments it provokes, and nothing more.

If one of these deserves a bench, it is H2220 — that a quoted pitting potential can be moved by tens of millivolts simply by matching the salt bridge to the bulk, which is a morning's work with two cells and one electrode.

Incoprea Research · hand-considered hypotheses 2201–2300 · 3 November 2026 · ~/Desktop/Nov3/