How much extra thickness measurement does substantial corrosion add?

One trigger, one deadline, everywhere

This is the most consistent rule in the chapter. Wherever a reading lands in the substantial-corrosion band — annual, intermediate or special survey, any ship type, any society — two things follow, and they follow in the same words.

First, the extent increases until the boundary of the affected area is established. The purpose is stated as finding out how far it runs, not as taking more readings for their own sake, which is worth remembering when the number is being argued about on board.

Second, the additional work is completed before the survey is credited. Not carried as a recommendation, not deferred to the next attendance. In UR Z10.2 the escalation is written three separate times — at the special survey, at the annual survey and at the intermediate survey — each with its own crediting deadline attached. UR Z10.2 §2.4.2

Alongside the widened readings sit two standing obligations that carry between surveys: areas recorded as suspect are examined, and areas previously found substantially corroded are gauged again. Those follow the ship, not the survey.

Rule text on an ESP ship, guidance on everything else

Here is the division that matters, and it is not a division between societies — it runs through IACS's own requirements.

On a ship under enhanced survey — the Z10 series, covering bulk carriers, tankers and chemical carriers — the pattern table is the provision. The clause names it as the requirement for the extended measurement, together with whatever the survey programme adds. There is nothing discretionary about the grid. Z10 series §2.4.2

On the general side — the Z7 family, which applies to all self-propelled vessels and is the only requirement for the ships the Z10 series does not name — the obligation splits in two. The escalation itself stays mandatory: where the readings show substantial corrosion, the extent is to be increased. But the table beside it is offered as something that may be used as guidance for the additional readings. The duty is flat; the method is not. UR Z7 §2.2.11

Two class rules reproduce the same split in their own drafting. BV carries a table whose caption describes itself as guidance, while the clause that sends you to it makes the widening compulsory. ABS writes the requirement as prose rather than a grid: the Surveyor specifies the additional measurements, the count of readings goes up, and the measurements spread wider. Different sentences, identical structure — a hard obligation wrapped around a soft pattern. BV [2.6.1] · ABS 7-3-2/7

The working consequence: on an ESP ship you can price the escalation from the table before you leave the office. On a non-ESP ship you can price the trigger but not the response, and the sensible move is to agree the pattern with the attending surveyor at the opening meeting rather than after the first thin reading.

What the widened pattern actually is

The non-ESP version is two lines, and every document that has one has the same two:

• Plating — the suspect area together with the plates beside it, each read on a five-point pattern laid over a square metre.
• Stiffeners — the suspect area, three readings in a line across the web and three the same way across the flange.

Note what the stiffener line does not say. Plating widens to neighbouring plates; the stiffener extent stays at the suspect area itself. BV's table makes that explicit, and the same shape is in the IACS and ABS versions. There is also no line at all for a girder, a floor, a bracket or a web frame — so on a non-ESP ship those members are covered by the surveyor's judgement and nothing else. BV Tab 4 · UR Z7 Table 2 · ABS 7-3-2/7.1

The ESP version is a different document altogether — four or five printed sheets, member by member. On a single-side-skin bulk carrier the sheets cover the shell, the cargo hold transverse bulkheads, the deck and topside tanks, the double bottom and hopper, and then a fifth sheet that carries one member only: the side shell frames in the holds. That last sheet is the one a bulk carrier survey reaches for most often. UR Z10.2 Table VIII

Three figures from those sheets are worth carrying, because they are the ones that get guessed: shell plating widens to the suspect plate and four plates around it; inner and double bottom widen to the suspect plate and every adjacent plate; and a cargo hold transverse bulkhead is read on two bands — one at about mid height, one next to the upper deck or beneath the upper stool shelf plate where upper stools are fitted.

The statutory version carries the identical member set, printed as untitled blocks rather than numbered sheets and written in the recommendatory voice — the widened measurement should be finished before the survey is credited, in an instrument that is mandatory through SOLAS. It is the same split of force and content that runs through the whole ESP Code. A.1049(27) annex 10

The CSR clause runs both ways

Ships built to the Common Structural Rules get a special provision on substantial corrosion. Most people know it as a relief. It is not always one, and the difference falls along ship type.

The same construction standard therefore buys a bulk carrier a way out of the widened grid and hands a double-hull tanker additional work at every intervening survey. Anyone who has learned the CSR provision on bulk carriers and carries it across to a tanker has it exactly backwards. Z10.2 · Z10.5 · Z10.4

On the statutory side the relief arrived late and narrowly. The 2011 Code as adopted carries none; it entered through the MSC.525(106) amendment, and only into a new clause covering the void spaces of a double-side-skin bulk carrier. Reading the base text alone will not show it. MSC.525(106)

Two things ABS says that no IACS requirement does

The twenty per cent deck rule. On a chemical carrier under enhanced survey, where substantial corrosion covers more than a fifth of the deck surface of a tank, the whole deck structure above that tank — longitudinals and web frames included — is gauged to the same patterns. It converts a local finding into a structural one at a stated threshold, and no IACS requirement in this set carries anything equivalent. ABS 7-3-2/7.5.2(f)

And a clarification worth having in writing. ABS states that substantial corrosion is not of itself a condition of class — but that an area of substantial corrosion does bring additional survey and examination. That is the correct answer to a question owners ask constantly, and it is one of the few places any document in this set answers it directly. ABS 7-A1-4/17.1

Four traps in the printed tables

These are not interpretation questions. They are things the documents do that make a careful reader think they have the wrong page.

• UR Z10.5 shows two table numbers at once. Every sheet header prints the old number struck through and the new one underlined. Cite it as Table IV and expect the printed page to look wrong. Z10.5 Table IV
• ABS gives the same member two different areas. The upper two thirds of a hopper transverse bulkhead is one square metre in the tanker section and two square metres in the double-skin bulk carrier section. Both are current; the ship type decides which applies. ABS 7.7.1 · 7.9.1(f)
• BV drops a word in one footnote. One footnote says protected by coating where two footnotes earlier the same document says hard protective coating. Whether the omission is deliberate is not stated, and the safe reading is the fuller phrase. BV Tab 12–16 notes
• Two IACS requirements omit the deadline in one place and state it in another. In Z7.1 and Z7.2 the special survey clause carries no completion deadline at all, while the annual survey clauses require the extra work before crediting. The deadline is real; it is simply not written in every clause that needs it. Z7.1 · Z7.2

What to do with it

Price the escalation before the survey, not during it. On an ESP ship the widened grid is knowable from the sheets, so the cost of the first substantial-corrosion finding can be worked out in advance. Doing that turns a mid-survey surprise into a decision that was already made.

Find the boundary, then stop. The clause asks for enough readings to establish where the affected area ends. Readings taken well inside a known area answer a question nobody asked; readings taken outward from the edge answer the one the rule poses.

Record the finding knowing what it commits. An area entered as substantially corroded is gauged again at every subsequent survey for the rest of the cycle. That is the right entry when the readings support it — and it is a reason to be sure they do.

Check the ship type before applying the CSR clause. It is the one provision on this page that means opposite things on two ships that look similar on paper.

Sources

1. UR Z10.2 Rev.37IACS — Hull surveys of bulk carriers, Feb 2023index entry →
2. UR Z10.4 Rev.18IACS — Double hull oil tankers, hull surveysofficial text ↗
3. UR Z10.5 Rev.20IACS — Double-skin bulk carriers, Feb 2023official text ↗
4. UR Z7 Rev.29IACS — Hull classification surveys, ships other than ESPofficial text ↗
5. A.1049(27)IMO Res. A.1049(27) — 2011 ESP Codeofficial text ↗
6. ABS Part 7ABS — Rules for Survey After Construction, Jul 2026official text ↗
7. BV NR467 Part ABureau Veritas — Rules for the Classification of Steel Ships, Part A, Jul 2026official text ↗

Class-society and IMO documents are indexed and linked, never re-hosted. Where a class rule is stricter than the IACS baseline, the class rule governs.

Related questions

• What counts as substantial corrosion in each class?K5.2
• Extent of TM at the annual survey, bulk carrierK6.8
• How many gauging points per plate?K6.2
• Extent of TM at Special Survey No.4 and subsequent, bulk carrierK6.6