At downhole temperatures exceeding 90°C, corrosion control in 15 wt% HCl stimulation fluids shifts from conventional amine-alkynol blends to higher-boiling acetylenic alcohols because the tertiary propargylic alcohol head of 2,6-dimethylhept-5-en-3-yn-2-ol provides water dispersibility while the enyne moiety supplies π-electron density for chemisorption on chloride-covered low-carbon steel. In batch autoclave screening conducted to NACE TM0193-2016 with N80 coupons, the compound is evaluated at 0.2–1.5 wt% active in 15 wt% hydrochloric acid containing 0.5–1.0 wt% nonylphenol ethoxylate and 0.05–0.2 wt% potassium iodide as an intensifier. At 90°C and 6 h exposure, the class-typical weight-loss response remains below the 0.05 lb/ft² per 24 h field acceptance criterion when the active concentration is maintained above 0.3 wt%; below 0.1 wt%, visual pitting initiates at machined coupon edges. Above 2.0 wt%, the branched alkenyl tail causes phase separation and a viscous interfacial sludge in spent acid, which has been observed in field returns as reduced injectivity and longer flowback clearing times.

The inhibition mechanism is not purely electrochemical: the acetylenic alcohol undergoes acid-catalyzed rearrangement to α,β-unsaturated ketone species that can oligomerize on the steel surface, creating a hydrocarbon-rich film whose thickness and cohesion depend on dissolved iron concentration. Soluble Fe(II) above 3,000 mg/L compresses the polymer film and requires supplemental film-repairing additions of the enynol in staged increments of 0.2 wt% per 6 h rather than a single front-loaded dose. Above 150°C, the molecule undergoes rapid acid-catalyzed dehydration to hydrocarbon oligomers that plate out on the coupon surface and complicate corrosion-rate measurement. Screening under NACE TM0193-2016 uses pre-weighed N80 coupons degreased to ASTM G1-03, immersed in glass cells at ambient pressure for temperatures below 93°C; evaluations above 120°C are run in Hastelloy C-276 autoclaves at 1,000–1,500 psi (6.9–10.3 MPa) because the acid vapor pressure exceeds safe glass-cell limits. The compound is compatible with low-carbon J55 and Q125 tubular steels but is not recommended for martensitic stainless steel in reducing acid brines when chloride exceeds 50,000 mg/L, because the adsorbed film can be undermined by localized chloride attack at weld toes and upset threads. Published data for this specific enynol configuration in sour brines containing H2S above 10 psi (69 kPa) is limited; field qualification therefore requires autoclave exposure with H2S-charged acid and post-exposure electron microscopy before demulsifier selection.

What Restricts Low-Current-Density Ductility in Bright Nickel Plating When This Acetylenic Alcohol Is Used as a Class II Leveler?

In a Watts nickel bath maintained at 50–60°C with nickel sulfate hexahydrate at 240–300 g/L, nickel chloride hexahydrate at 30–60 g/L, and boric acid at 35–40 g/L, 2,6-dimethylhept-5-en-3-yn-2-ol functions as a Class II brightener and leveler at 0.01–0.15 g/L, complementing saccharin at 0.5–2.0 g/L as the Class I carrier. The internal C≡C bond and the tertiary alcohol group adsorb selectively on high cathode-potential sites, suppressing outward growth of nickel crystallites and shifting the deposit grain size from the micrometer range into the submicrometer range; this is measured by Hull cell panels plated at 2 A for 5 min in a 267 mL cell, where the bright range extends from 0.5 A/dm² to 8 A/dm² when the leveler concentration is near 0.05 g/L. At concentrations exceeding 0.2 g/L, the low-current-density region darkens and the deposit becomes brittle, because excessive adsorption blocks nickel reduction and increases hydrogen co-deposition; ductility measured by bend testing or tensile elongation drops below 1.5%, which fails the decorative nickel requirements in ASTM B456-17 for components subjected to post-plate forming. At below 0.005 g/L, leveling is lost and the deposit follows substrate scratches rather than filling them, producing a dull appearance under 20× stereomicroscope inspection. The bath must be filtered continuously through 1–5 µm polypropylene cartridges and agitated by air or eductor flow to prevent concentration gradients that create patchy brightness on complex geometries. The enynol addition is made as a 1 wt% stock solution in deionized water or dilute isopropanol; direct injection of the neat liquid can exceed local solubility and form oily droplets that are dragged into the cathode boundary layer and cause pitting. Carbonyl-free activated carbon treatment is applied at 0.5–2.0 g/L weekly to remove organic breakdown products, but over-treatment strips the leveler and reduces bright range. Published data for this specific enynol configuration in high-chloride sulfamate nickel baths is limited; the operating window above applies to standard Watts formulations and may require a 0.02 g/L upward shift when the bath is run at 60°C and 8 A/dm² with high agitation.

Closed-loop perchloroethylene dry-cleaning machines with copper sight glasses and steel condenser coils incorporate the enynol into stabilizer packages at 25–150 ppm to control acidic decomposition products generated by thermal cracking and light-catalyzed oxidation. The functional test is total acid acceptance under ASTM D2942-96(2018), which measures the milliequivalents of hydrogen chloride neutralized per liter of solvent before the solvent pH falls below the threshold for metal attack. A copper stability test under ASTM D3316-20 is used to verify that the acetylenic alcohol prevents copper dissolution and redeposition on steel in the distillation loop. The compound is added with epoxide co-stabilizers such as butylene oxide at 0.05–0.2 wt% and is not effective when used alone in aluminum-finisher solvent systems, because the enol-ether rearrangement product can form aluminum soaps that plate out on heating coils. In trichloroethylene vapour degreasing, the operating stabilizer level is kept near 100 ppm, and solvent pH is maintained above 6.5; below this pH, acid attack at the solvent-air interface produces visible copper chloride staining on condenser fins.

Meyer–Schuster Conversion to Phorone and Catalytic Hydrogenation to Diisobutyl Ketone

Under acidic conditions, 2,6-dimethylhept-5-en-3-yn-2-ol undergoes Meyer–Schuster rearrangement to phorone, 2,6-dimethylhepta-2,5-dien-4-one. Protonation of the tertiary hydroxyl group is followed by water loss to an allenyl carbocation; hydration of the allene at the central carbon and keto-enol tautomerization give the conjugated enone. The rearrangement is run in dilute sulfuric acid at 30–60°C or over a sulfonic acid ion-exchange resin in a fixed bed, with the product phorone separated by phase cut at 55–65°C under 2–5 kPa vacuum to avoid oligomerization. Phorone is then hydrogenated to diisobutyl ketone over Raney nickel or palladium-on-alumina at 80–120°C and 2–4 MPa hydrogen pressure in a fixed-bed reactor; the two C=C double bonds are saturated without measurable hydrogenolysis of the carbonyl group when the sulfur content of the feed is below 1 ppm. The resulting diisobutyl ketone is a high-boiling solvent with a distillation range of 168–169°C at 101.3 kPa, a relative evaporation rate of 0.18 relative to n-butyl acetate, and a Hansen solubility parameter that places it in the nitrocellulose and vinyl resin solvency window. This solvent is used in coil-coating primer formulations and in high-solids polyurethane systems where slow evaporation prevents dry spray and improves flow-out. The catalytic hydrogenation step requires strict water removal; feed water above 0.2 wt% promotes aldol condensation of phorone to higher molecular weight by-products that deactivate the nickel surface and reduce selectivity to diisobutyl ketone below 95%. Published data for this specific enynol-to-phorone step are limited to batch reactor studies, while the phorone hydrogenation step is well documented in ketone solvent process literature. Analytical release of the solvent is performed by gas chromatography for purity and by ASTM D1078-11 for distillation range; color is controlled to ≤20 Hazen by ASTM D1209-05(2019).

When Acid-Bath Ferrous Iron Exceeds 120 g/L, Replenishment of the Enynol Must Be Staged

Continuous steel strip pickling lines running 10–15 wt% hydrochloric acid at 70–95°C use the enynol at 0.5–1.0 g/L to suppress base-metal loss without delaying scale removal. The dose is tied to dissolved iron by ASTM G31-21 coupon weight-loss and is trimmed when Fe(II) exceeds 120 g/L, because spent acid with high ferrous chloride accelerates inhibitor decomposition and requires daily replenishment at 0.1–0.2 g/L per 8 h shift. The inhibitor is injected into the recirculation line downstream of the plate-and-frame filter press to avoid localized overheating in the steam sparger section, where steam temperatures above 140°C can cause premature rearrangement of the acetylenic alcohol and formation of solvent-insoluble oligomers. In sulfuric acid pickling at 5–10 wt% and 60–80°C, the same active is reduced to 0.3–0.5 g/L because sulfate scale on the steel surface competes with inhibitor adsorption and requires higher free inhibitor in the bulk bath. Published data for this exact configuration in continuous strip lines is limited, so the concentration boundaries should be validated against ASTM G31-21 weight-loss coupons pulled from the recirculation loop at startup and after each 8 h production shift.