Chapter IV: Biofouling And Corrosion Study

Introduction

Until the joint National Park Service (NPS) and Navy mapping survey in 1983, no major assessment of the USS ARIZONA had occurred since salvage activities after the ship's sinking in 1941. The five-year project contained many facets in its search for information relevant to long-term management of submerged components of the Pearl Harbor National Historic Landmark, especially the USS ARIZONA and the USS UTAH. The July 1986 study examined, among others, one particular aspect of the ship: the role of biofouling in the overall corrosion process. This chapter discusses the methodology and presents the results of that biofouling and corrosion study. Although some comparative data was taken from the USS UTAH, this research focused almost exclusively on the USS ARIZONA.

Our objectives were to: 1) develop a baseline inventory of biological communities and sedimentation extant on structural remains of the ship, and 2) obtain quantifiable measurements of the present state of deterioration of the ship's metal structural elements at various locations (ref. 1). The data gathered would then be used to assess the causes and rates of deterioration of the ship components, and would subsequently provide a scientific basis for making informed decisions regarding the ARIZONA's management and preservation. These objectives do not imply a commitment by federal managers to undertake stabilization of deterioration or in-place preservation (ref. 1).

Several working hypotheses pertaining to the ship's biofouling and corrosion status were formulated prior to the survey to define specific data requirements.

Hypotheses related to vertical ship surfaces were:

1. A layer of hard fouling growth creates anoxic (low oxygen) conditions at the metal surface and reduces the corrosion rate to far below what would occur on a nonfouled surface.

2. The fouling growth (dead and alive) is stable and dense, and forms a relatively homogenous layer over nearly all vertical areas.

3. Thickness and density of fouling growth is negatively correlated with corrosion rate of the underlying surface.

4. Interior hull and wall surfaces are expected to have lesser areal coverage and thickness of fouling growth, and thus may be subjected to higher corrosion rates than exterior surfaces.

5. Hull components buried in silt exhibit a very low corrosion rate.

Hypotheses related to horizontal (wood and steel) ship surfaces were:

1. A layer of sediment creates anoxic conditions on horizontal surfaces and reduces corrosion and decomposition rates far below those that would occur on uncovered surfaces.

2. The sediment layer is stable and covers nearly all horizontal areas.

3. Corrosion and deterioration of horizontal surfaces are correlated with sediment thickness and porosity.

Two other hypotheses were formulated concerning possible long-term effects on biofouling communities:

1. Nutrient, pollutant and plankton levels will slowly decline in Pearl Harbor as a result of ongoing pollution abatement.

2. Biofouling communities will decrease because of their dependence on the above factors.

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